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Selina Concise Physics Class 7 ICSE Solutions – Motion

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Physics for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 2 Motion

• Points to Remember
•  Motion— Motion is the change in position with respect to a set of stationary landmarks.
•  Types of motion are translatory, rotatory and oscillatory.
•  A body is said to move in a translatory motion if the whole body moves through the same distance in the same interval of time.
•  A body is said to be in rotatory motion if it moves about a fixed point without changing the radius of its motion.
•  A body is said to be in oscillatory motion if it moves to and fro about its mean position.
•  A body is said to be in vibratory motion if the body moves to and fro in a particular style.
•  Oscillation of pendulum is one complete to and fro motion.
•  The maximum displacement on either side of a mean position is called amplitude.
•  A motion which repeats itself at regular intervals of time is called periodic motion.
•  A physical quantity which has only magnitude and no direction is called a scalar quantity.
•  A physical quantity which has magnitude as well as direction is called a vector quantity.
•  Speed is the distance travelled per unit time. It is a scalar quantity.
•  The displacement per unit time is known as the velocity of the body. It is a vector quantity.
•  The rate of change of velocity is called acceleration. It is a vector quantity.
•  Frequency is the number of complete oscillations in one second.
•  Unit of frequency is Hertz.
•  Motion is of two types (a) uniform (b) non-uniform motion.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) Two trains going in opposite directions with the same speed are at rest relative to each other.
Answer. False.

(b) A ball is thrown vertically upwards. Its motion is uniform throughout.
Answer. False.

(c) The motion of a train starting from one station and reaching at another station is non-uniform.
Answer. True.

(d) A motion which repeats itself after a fixed interval of time is called periodic motion.
Answer. True.

(e) A ball thrown by a boy from a roof-top has oscillatory motion.
Answer. False.

(f) Mass has both magnitude and direction.
Answer. False.

(g) .Weight always acts vertically downwards.
Answer. True

(h) Mass varies from place to place but weight does not.
Answer. False

2. Fill in the blanks

(a) Two boys cycling on the road with the same speed are at rest relative to each other.
(b) The motion in a straight line is rectilinear motion.
(c) One to and fro motion of a clock pendulum takes time = 2 s
(d) 36 km h^-1 = 10 m s^-1
(e) Total distance travelled = average speed × total time taken.
(f) The weight of a girl is 36 kgf. Her mass will be 36 kg.
(g) The weight of a body is measured using a spring balance.

Activity

3. Match the following

4. Select the correct alternative

(a) A book lying on a table is an example of

1.  a body at rest
2.  a body in motion
3.  a body neither at rest nor in motion
4.  none of these

(b) The motion of a pendulum is

1.  rotatory
2. oscillatory
3.  curvilinear
4.  rectilinear

(c) A car moving on a straight road is an example of

1.  rotatory motion
2.  rectilinear motion
3.  oscillatory motion
4.  periodic motion

(d) A ball falls down vertically. Its motion is

1.  periodic
2.  linear
3.  circular
4.  vibratory

(e) If a body covers equal distance in equal interval of time, the motion is said to be

1.  uniform
2.  non-uniform
3.  oscillatory
4.   rotatory

(f) A boy goes from his house to school by bus at a speed of 20 km h^-1 and returns back through the same route at a speed of 30 km h_1. The average speed of his journey is

1.  24 km h^-1
2.  25 km h^-1
3.  30 km h^-1
4.  20 km h^-1

(g) The earth attracts a body of mass 1 kg with a force of 10 N. The mass of a boy is 50 kg. His weight will be

1.  50 kg
2.  500 N
3.  50 N
4.  5 N

B. Short/Long Answer Questions

Question 1.
Explain the meaning of the terms rest and motion.
Answer:
Rest— A body is said to be at rest if it does not change its position with respect to its immediate surroundings.
Motion— A body is said to be in motion if it changes its position with respect to its immediate surroundings.

Question 2.
Comment on the statement ‘rest and motion are relative terms’. Give an example.
Answer:
Imagine you are sitting inside a moving bus. When you look outside you will observe that you are moving. Now look to the roof of the bus. With respect to the roof of bus, you are at rest. Hence it is concluded that rest and motion are relative terms.

Question 3.
Fill in the blanks using one of the words : at rest, in motion.
(a) A person walking in a compartment of a stationary train is relative to the compartment and is relative to the platform.
(b) A person sitting in a compartment of a moving train is relative to the other person sitting by his side and is relative to the platform.
Answer:
(a) A person walking in a compartment of a stationary train is in motion relative to the compartment and is in motion relative to the platform.
(b) A person sitting in a compartment of a moving train is at rest . relative to the other person sitting by his side and is in motion relative to the platform.

Question 4.
Name five different types of motion you know.
Answer:
The different types of motion are :
1. Translatory motion
2. Rotatory motion
3. Oscillatory motion
4. Vibratory motion
5. Periodic motion
6. Multiple motion
7. Random motion.

Question 5.
What do you mean by translatory motion ? Give one example.
Answer:
If an object like a vehicle, moves in a line in such a way that every point of the object moves through the same distance in the same time, then the motion of the object is called translatory motion.
Example :
The motion of an apple falling from a tree, the motion of a man walking on a road, the motion of a box when pushed from one comer of a room to the other, are all the translatory motion.

Question 6.
Explain the meanings of (i) rectilinear motion, and (ii) curvilinear motion. Give one example of each.
Answer:
(i) Rectilinear motion — If the motion of a body is along a straight line, it is said to be the rectilinear or linear motion. The motion of bullet fired from a gun.
(ii) Curvilinear motion — If the motion of a body is along a curved path, it is said to be the curvilinear motion. For example, the motion of a cycle while taking a turn on the road, a car moving along a curved path, a ball thrown by an athlete are in curvilinear motion.

Question 7.
What is rotatory motion ? Give two examples.
Answer:
Rotatory motion— A body is said to be in a rotatory motion or a circular motion if it moves about a fixed axis without changing the radius of its motion.
Examples : The blades of a fan, a spinning wheel.

Question 8.
What is meant by circular motion ? Give one example.
Answer:
The motion of a body along a circular path is called circular motion.
Example : A girl is whirling a stone tied at the end of a string in a circular path.

Question 9.
How does a rotatory motion differ from the circular motion?
Answer:
(i) In rotatory motion, the axis of rotation passes from a point
in the body itself whereas in circular motion, the axis of revolution passes through a point outside the body. Thus the motion of earth around the sun is the circular motion whereas the motion of earth about its own axis is the rotational motion.
(ii) In the circular and rotatory motions, the distance of a point of a the body from a fixed point always remains same, whereas it is not same in curvilinear motion.

Question 10.
Explain oscillatory motion by giving one example.
Answer:
Oscillatory motion— The to and fro motion of a simple pendulum is an oscillatory motion.
Example : 1. The motion of a swing, 2. Piston of an engine.

Question 11.
What is vibratory motion ? Give one example.
Answer:
In vibratory motion, a part of the body always remains fixed and the rest part moves to and fro about its mean position. During the vibratory motion, the shape and size of the body changes. Example : When we breath, our chest expands and contracts. This motion is vibratory motion.

Question 12.
Differentiate between periodic and non-periodic motions by giving an example of each.
Answer:
Periodic motion : A motion which gets repeated after regular intervals of time is called a periodic motion.
Examples : The earth moving around the sun takes 365 days to complete one revolution and this motion gets repeated after every 365 days.
Non-periodic motion : The motion which does not repeat itself after regular interval of time is called non-periodic motion.
Examples : A footballer running on a field, application of brakes in a moving vehicle, a ball rolling down the ground gradually slows down and finally stops, motion of tides in the sea, etc.

Question 13.
What is random motion. Give one example.
Answer:
Random motion— When an object in a motion has no specific path and which suddenly changes its motion is said to have a random motion.
Example : A flying kite.

Question 14.
Name the type/types of motion being performed by each of the following:
(a) Vehicle on a straight road
(b) Blades of an electric fan in motion
(c) Pendulum of a wall clock
(d) Smoke particles from chimney
(e) Hands of a clock
(f) Earth around the sun
(g) A spinning top.
Answer:
(a) Rectilinear motion
(b) Rotatory motion
(c) Oscillatory motion, periodic motion
(d) Non-periodic motion
(e) Uniform circular and periodic motion
(f) Rotatory motion, circular motion and periodic motion
(g) Rotatory motion

Question 15.
Give two examples to illustrate that a body can have two or more types of motion simultaneously.
Answer:
Sometime a body can have more than one type of motion. Such a motion is called the mixed motion.
Example :
(i) The wheels of a moving train have both the translatory as well as the rotatory motions as it moves from position A to position B while rotating.

(ii) The earth rotates about its axis (rotatory motion) and at the same time it revolves around the sun in a curved path (curvilinear or circular motion) in a fixed time interval (periodic motion).

Question 16.
State the types of motion of the following :
(a) The needle of a sewing machine
(b) The wheel of a bicycle
(c) The drill machine
(d) The carpenter’s saw
Answer:
(a) Periodic motion
(b) Rotatory motion
(c) Mixed = Translatory and Rotatory motion
(d) Mixed = Translatory and Oscillatory motion

Question 17.
Distinguish between uniform and non-uniform motions, giving an example of each.
Answer:

Question 18.
How do you determine the average speed of a body in non-uniform motion ?
Answer:
In a non-uniform motion, the average speed of a body is calculated by dividing the total distance travelled by the body, with the total time of its journey. Thus,
Average speed = Total distance travelled by the body / Total time of joumey

Question 19.
Define the term weight and state its S.I. unit.
Answer:
The weight of a body is the force with which earth attracts the body i.e. the weight of a body is the force of gravity on it. The weight of a body is not constant, but it changes from place to place. It is represented by the symbol W. The S.I. unit of weight is newton (N).

Question 20.
How are the units of weight, kgf and newton related ?
Answer:
1 kg F = 10 N

Question 21.
State three differences between mass and weight.
Answer:
Mass

It is the quantity of matter contained in a body.
Its S.J. unit is kilogram (kg)
It is constant for a body and it does not change by changing the place of the body.
It is measured by a beam balance.

Weight

It is the force with which the earth attracts the body.
Its S.I. unit is newton (N) and other unit is kilogram-force (kgf) where 1 kgf= 10N(nearly).
It is not constant for a body, but it changes from place to place.
It is measured by a spring balance.

Question 22.
Which quantity : mass or weight, does not change by change of place ?
Answer:
The mass of a body is constant and it does not change by changing the position of the body.

Question 23.
State which of the quantities, mass or weight is always directed vertically downwards.
Answer:
Mass is the quantity of matter contained in a body. Weight is the force with which the earth attracts the body. Weight is always directed vertically downwards.

 

C. Numericals

Question 1.
A car covers a distance of 160 km between two cities in 4 h. What is the average speed of the car ?
Answer:
Distance =160 km
Time taken = 4h
Speed = ?
Speed = Distance covered / Time taken
= 160km / 4h = 40 km h^-1

Question 2.
A train travels a distance of 300 km with an average speed of 60 km h_1. How much time does it take to cover the distance?
Answer:
Speed = 60 km h^-1
Distance covered = 300 km

Question 3.
A boy travels with an average speed of 10 m s^-1 for 20 min. How much distance does he travel ?
Answer:
Average speed of boy = 10 m s^-1
Time taken = 20 min
Distance travelled = Speed × Time taken
Convert minutes into seconds
1 minute = 60 sec.
20 minutes = 20 × 60 = 1200 sec.
Distance travelled = 10 m s^-1 × 1200 sec.
= 12000 m Or 12 km

Question 4.
A boy walks a distance 30 m in 1 minute and another 30 m in 1.5 minute. Describe the type of motion of the boy and find his average speed in m s-1.
Answer:
As the speed does not remain constant throughout the journey the motion is non-uniform
Total distance travelled in going and coming back
d = 30 m + 30 m = 60 m
Total time taken in going and coming back

Question 5.
A cyclist travels a distance of 1 km in the first hour, 0.5 km in the second hour and 0.3 km in the third hour. Find the average speed of the cyclist in
(i) km h^-1, (ii) m s^-1.
Answer:
(a) Distance travelled in first hour = 1 km
Distance travelled in second hour = 0.5 km
Distance travelled in third hour = 0.3 km
Total time taken = 3 hr
Total distance travelled = 1 + 0.5 + 0.3 = 1.8 km
(i) Average speed in km h^-1

Average speed in m s^-1
1 km = 1000 m
1.8 km = 1.8 × 1000 m
= 1800 m
1 hour = 3600 seconds
3 hour = 3600 × 3 = 10800 sec.

Question 6.
A car travels with speed 30 km h^-1 for 30 minute and then with speed 40 km h^-1 for one hour. Find :
(a) the total distance travelled by the car
(b) the total time of travel, and
(c) the average speed of car
Answer:
Speed of car for first 30 minutes = 30 km h^-1
Speed of car for next 1 hour = 40 km h^-1
(a) Total distance travelled by the car

Question 7.
On earth the weight of a body of mass 1.0 kg is 10 N. What will be the weight of a boy of mass 37 kg in (a) kgf (b) N ?
Answer:
Weight of a body of mass 1.0 kg body = 10 N
(a) Weight of a boy of mass = 37 kg
(b) Weight of a boy of 37 kg in newton will be 1 kgf = 10N
∴ 37 kgf = 37 × 10 N
= 370 N

Question 8.
The weight of a body of mass 6.0 kg on moon is 10 N. If a boy of mass 30 kg goes from earth to the moon surface, what will be his (a) mass, (b) weight ?
Answer:
(a) Mass remains same it does not change
So mass of boy 30 kg on earth = 30 kg on moon surface
(b) Weight of boy on moon becomes 1 / 6
∴ 30 kg boy will weight 30 x 1/6 = 5kg
1 kg = 10 N ⇒ 5 × 10 N = 50 N
∴ Weight of boy on moon surface = 50 N

 

 

 

Selina Concise Physics Class 7 ICSE Solutions – Heat

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Physics for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 7 Physics ICSE SolutionsChemistryBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 7 Physics Chapter 5 Heat

• Points to Remember
•  Heat is a form of energy that leads to the sensations of hotness or coldness.
•  Temperature is the degree of hotness and coldness of a body.
•  Thermometer is used to measure temperature.
•  The S.I. unit of temperature is °C.
•  The most common liquid for a thermometer is mercury.
•  The main sources of heat are (i) Fire (ii) Sun (iii) Electricity.
•  Those substances which can easily catch fire are called inflammable substances.
•  Those substances which are fire resistant are called non-inflammable substances.
•  The fixed temperature at which freezing of liquid occurs is known as freezing point.
•  The temperature at which vapourisation occurs is known as the boiling point.
•  Substances through which heat is easily conducted are called good conductors e.g. silver, gold, copper etc.
•  Substances through which heat is not easily conducted are called Insultors.
•  Radiation is the process of transfer of heat from a hot body to a cold body without affecting the intermediate medium.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) On touching a lump of ice, we feel cold because some heat passes from our body to the ice.
Answer. True.

(b) Heat flows from a body at a high temperature to a body at a low temperature when they are kept in contact. .
Answer. True.

(c) All solids expand by the same amount when heated to the same rise in temperature.
Answer. False.

(d) Telephone wires are kept tight between the two poles in summer.
Answer. False.

(e) Equal volumes of different liquids expand by different amounts when they are heated to the same rise in temperature.
Answer. True.

(f) Solids expand the least and gases expand the most on being heated.
Answer. True.

(g) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer. True.

(h) Kerosene contracts on heating.
Answer. False.

(i) Water is a bad conductor of heat.
Answer. True.

(j) Medium is necessary for the transfer of heat by radiation.
Answer. False.

(k) Land and sea breezes are convection currents of cold and warm air.
Answer. True.

(l) Liquids are heated by conduction and radiation.
Answer. False.

(m) Black surfaces are the poor absorbers of heat radiations.
Answer. False.

2. Fill in the blanks

(a) Heat is a form of energy.
(b) Temperature determines the degree of hotness or coldness of a body.
(c) On heating a body, its temperature rises.
(d) We use a thermometer for measuring the temperature of a body.
(e) The S.I. unit of temperature is kelvin.
(f) In a thermometer, the commonly used liquid is mercury.
(g) The temperature of a normal human body is 37 °C.
(h) A person is said to have fever if his body temperature is more than 98.6
(i) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from metallic piece to water.
(j) The temperature of boiling water is 100°C.
(k) Liquids expand more than the solids.
(l) Gases expand more than the liquids.
(m) Heat transfer in solids is by conduction.
(n) Heat transfer in liquids and gases is by convection.
(o)Metals are conductors of heat.
(p) Still air is an insulator of heat.
(q) Black and dull surfaces are good absorbers of heat.

3. Match the following

4. Select the correct alternative

(a) If we add a lump of ice to a tumbler containing water,

1. heat flows from water to ice
2.  heat flows from ice to water
3.  heat flows from water to ice if water is more
4.  heat flows from ice to water if ice is more

(b) The temperature of pure melting ice is

1.  0°C
2.  100°C
3.  95°C
4.  98.6°F

(c) A thermometer uses

1.  water
2. mercury
3.  air
4.  none of the above

(d) Which of the statement is correct

1.  Iron rims are cooled before they are placed on cart wheels
2.  A glass stopper gets tight on warming the neck of the bottle
3.  Telephone wires sag in winter, but become tight in summer
4. A little space is left between two rails on a railway track

(e) Heat in a liquid is transferred by

1.  conduction
2. convection
3.  radiation
4.  conduction and radiation

(f) In the process of convection, heat travels

1.  sideways
2.  downwards
3. upwards
4.  in all directions

(g) The vacuum kept in between the walls of a thermos flask reduces the heat transfer by

1.  conduction only
2.  convection only
3.  radiation only
4. conduction and convection

B. Short/Long Answer Questions

Question 1.
What is heat ? State its S.I. unit.
Answer:
Heat is a form of energy which flows. It is the energy of motion of molecules constituting the body.
The unit of heat is same as that of energy, The S.I. unit of heat is joule (abbreviated as J) and other common units of heat are calorie and kilo calorie, where 1 k cal = 1000 cal.

Question 2.
What is meant by the term temperature.
Answer:
Temperature is a quantity which tells the thermal state of a body (i.e. the degree of hotness or coldness). It determines the direction of flow of heat when the two bodies at different temperatures are placed in contact.

Question 3.
State the three units of temperature.
Answer:
The S.I. unit of temperature is kelvin or K. The other most common unit of temperature is degree Celsius (°C) and degree Fahrenheit (°F).

Question 4.
Name the instrument used to measure the temperature of a body.
Answer:
To measure the temperature of a body with the help of a thermometer.

Question 5.
Name two scales of temperature. How are they inter-related?
Answer:
Two scales of temperature are
(i) Celsius (ii) Fahrenheit
Relation:

Question 6.
How is the size of a degree defined on a Celsius scale ?
Answer:
The interval between the ice point and steam point divided by 100 (hundred) equal parts is called a degree on the Celsius scale.

Question 7.
How is the size of a degree defined on a Fahrenheit scale?
Answer:
The interval between the ice point and steam point divided into 180 equal parts is called a degree on the Fahrenheit scale.

Question 8.
State the temperature of (i) ice point and (ii) steam point, on the Celsius scale.
Answer:
(i) Ice point. Is the the mark on Celsius scale at which ice melts. Ice point on the Celsius scale is 0°C.
(ii) Steam point. On the Celsius scale is the mark at which water changes into steam at normal atmospheric pressure. On Celsius scale it is 100°C.

Question 9.
Write down the temperature of (i) lower fixed point, and (ii) upper fixed point, on the Fahrenheit scale.
Answer:
Lower fixed point: On the Falirenheit scale is the mark at which pure ice melts. It is 32°F on Fahrenheit scale.
Upper fixed point: On the Fahrenheit scale is the mark at which water starts changing into steam at normal atmospheric pressure. It is 212°F.

Question 10.
What is the Celsius scale of temperature ?
Answer:
Celsius scale is that which has ice point as 0°C and steam point marked as 100°C.

Question 11.
What is the Fahrenheit scale of temperature ?
Answer:
Fahrenheit scale is that which has ice point as 32°F and the steam point marked as 100°C.

Question 12.
What is the Kelvin scale of temperature ?
Answer:
On Kelvin scale of temperature zero mark is when no molecular motion occurs. Ice point is at 273 and steam point is at 373 K. Thus 0 K = – 273°C and one degree on Kelvin scale is same as one degree on Celsius scale.

Question 13.
The fig. shows a glass tumbler containing hot milk which is placed in a tub of cold water. State the direction in which heat will flow.

Answer:
When we bring two objects of different temperature together, energy will always be transferred from hotter to the cooler object.
Here, also heat will flow from hot milk tumbler to tub of cold water.

Question 14.
Draw a neat labelled diagram of a laboratory thermometer.
Answer:

Question 15.
Write down the body temperature of a healthy person.
Answer:
The temperature of a healthy persons is 98.6 degrees fahrenheit or 37.0 degree Celsius or 310 k.

Question 16.
What do you understand by thermal expansion of a substance ?
Answer:
The expansion of a substance when, heated, is called thermal expansion.
Or
Thermal expansion is the tendency of matter to change .in shape, area and volume in response to a change in temperature.

Question 17.
Name two substances which expand on heating.
Answer:
Mercury and Aluminium wire.

Question 18.
Why do telephone wires sag in summer ?
Answer:
The telephone wires will sag in summer due to expansions and will break in winter due to contraction.
Therefore, while putting up the wires between the poles, care is taken that in summer they are kept slightly loose so that they may not break in winter due to contraction.
While in winter they are kept light so that they may not sag too much in summer due to expansion.

Question 19.
Iron rims are heated before they are fixed on the wooden wheels. Explain the reason.
Answer:
The wooden wheels of a bullock-cart are fitted with iron tyres. To ensure a tight fit, the tyre is made slightly smaller in diameter than the wheel. The tyre is first heated due to which it expands. The heated tyre is then fitted on the wheel. When the tyre cools, it contracts and makes a tight fit on the wheel.

Question 20.
Why are gaps left between successive rails on a railway track ?
Answer:
The rails of railway track are made of steel. While laying the railway track, a small gap is left between the two successive length of rails. The reason is that the rails expand in summer. The gap is provided to allow for this expansion. If no gap is left, the expansion in summer will cause the rails to bend sideways. This may result in a train accidents.

Question 21.
A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle. Explain why ?
Answer:
When hot water is poured over the neck of the bottle, it expands. As a result the stopper gets loosened and can be removed easily.

Question 22.
Why is a cement floor laid in small pieces with gaps in between?
Answer:
The floor is laid in small pieces with gaps in between to allow for the expansion during summer. However glass strips can be placed in the gaps.

Question 23.
One end of a steel girder in a bridge is not fixed, but is kept on roliers. Give the reason.
Answer:
In the construction of a bridge, steel girders are used. One end of the girder is fixed into the concrete or brick pillars and its other end is not fixed, but it is placed on rollers. The reason is that if there is any rise (or fall) in temperature of atmosphere, the girder can freely expand (or contract) without affecting the pillars.

Question 24.
Describe one experiment to show that liquids expand on heating.
Answer:
(i) Take an empty bottle with a tight fitting cork having a hole drilled in its middle, a drinking straw, two bricks, a wire guaze and a burner.
(ii) Fill the bottle completely with water and add few drops of ink in it to make it coloured.
(iii) Fix the cork in the mouth of the bottle and pass the drinking straw through the cork. Put some molten wax around the hole so as to avoid the leakage of water.
(iv) Pour some more water into the drinking straw so that water level in the straw can be seen. Mark the water level in the straw as shown in Figure.

(v) Place the bottle on the wire gauze kept over the two bricks as shown in Figure. Then heat the bottle by means of a burner.
(vi) Look at the level of water in the straw.
You will notice that as the water is heated more and more, the level of water in the drinking straw rises. This shows that water expands on heating.

Question 25.
State one application of thermal expansion of liquids.
Answer:
Mercury is a metal found in liquid state. It expands more and uniformly over a wide range of temperature. So mercury is used as thermometric liquid.

Question 26.
Describe an experiment to show that air expands on heating.
Answer:
(i) Take an empty bottle. Actually the empty bottle contains air. Attach a rubber balloon to its neck as shown in Figure. Initially, the balloon is deflated.
(ii) Place the bottle in a water bath containing boiling water. After some time you will notice that the balloon gets inflated as shown in Figure. The reason is that the air inside the bottle expands on heating and it fills the balloon.

(iii) Take the bottle out of the water bath and 7 allow it to cool by itself. We will notice that the balloon gets deflated and it collapses. This is because the air inside the balloon and the bottle, has contracted on cooling. The air from balloon passes to the bottle, so the balloon gets deflated.

Question 27.
An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into the water. Explain the reason.
Answer:
When the bottle is heated, bubbles of air are seen escaping into the water. This happens because the air present in glass bottle expands on heating and tries to escape out through the tube into the water.

Question 28.
State which expands more, when heated to the same temperature : solid, liquid or gas ?
Answer:
Gases expand much more than the liquids and the solids. Like liquids, the gases do not have a definite shape, so they also have only the cubical expansion.

Question 29.
Name the three modes of transfer of heat.
Answer:
There are three modes of transfer of heat (i) Conduction (ii) Convection (iii) Radiation.
(i) Conduction “is that mode of transfer of heat, when heat travels from hot end to cold end from particle to particle of the medium, without actual movement of particles.”
(ii) Convection. “Is a process of transfer of heat by actual move-ment of the medium particles.”
(iii) Radiation. “Is that mode of transfer of heat in which heat directly passes from one body to the other body without heating the medium.”

Question 30.
Name the mode of transfer of heat in the following :
(a) solid,
(b) liquid,
(c) gas
(d) vacuum
Answer:

Question 31.
What are the good and bad conductors of heat ? Give two examples of each.
Answer:
Good conductors. “The substances through which heat is easily conducted are called good conductors of heat.”
Example : Copper, iron.
Bad conductors. “The substances through which heat is not conducted easily are called bad conductors of heat or poor conductors of heat.”
Example : Wood, cloth.

Question 32.
Name a liquid which is a good conductor of heat.
Answer:
Mercury is good conductor of heat.

Question 33.
Name a solid which is a good conductor of heat.
Answer:
Aluminium is a good conductor of heat.

Question 34.
Select good and bad conductors of heat from the following :
copper, mercury, wood, iron, air, saw-dust, cardboard, silver, plastic, wool.
Answer:
Good conductors — Mercury, copper, silver, iron.
Bad conductors — Wood, air, saw dust, plastic, wool, cardboard.

Question 35.
Why is an oven made of double walls with the space in between filled with cork ?
Answer:
An oven is made of double walls and the space between them is filled with wool, cork etc. because the wool and cork are the insulator of heat. They prevent the heat of the oven to escape.

Question 36.
Why do we use cooking utensils made up of copper.
Answer:
Cooking utensils are made of metals such as copper, aluminium, brass, steel etc., so that heat is easily conducted through the base to their contents. But they are provided with handles of bad conductors (such as ebonite or wood) to hold them easily as handles will not conduct heat from the utensil to our hand.

Question 37.
Why is a tea kettle provided with an ebonite handle ?
Answer:
Tea kettles are provided with wooden or ebonite handles. The wood or the ebonite being the insulators of heat, does not pass heat from the utensils to our hand. Thus, we can hold the hot utensils or pans comfortably by their handles.

Question 38.
In summer, ice is kept wrapped in a gunny bag. Explain the reason.
Answer:
In summer, the ice is kept wrapped in a gunny bag or it is covered with saw dust. The air filled in the fine pores of the gunny bag or saw dust, is the insulator of heat. The air does not allow heat from outside to pass through it to the ice. Thus, the ice is prevented from melting rapidly.

Question 39.
Explain why
(a) we wear woolen clothes in winter.
(b) the water pipes are covered with cotton during very cold weather.
Answer:
(a) Woolen clothes have fine pores filled with air. Wool and air both are bad conductors of heat. Therefore in winter, we wear woolen clothes as they check the conduction of heat from the body to the surroundings and thus keeps the body warm.
(b) During very cold weather, the water pipes are covered with cotton. The cotton has air trapped in its fine pores. The cotton and air are the insulators of heat. They do not pass heat from water inside the pipes to the outside atmosphere. Thus, cotton prevents the water in the pipes from freezing.

Question 40.
Why are quilts filled with fluffy cotton ?
Answer:
Quilts are filled with fluffy cotton. Air is trapped in the fine pores of cotton. Cotton and air are the insulators of heat. They check heat from our body to escape and thus keep us warm.
The newly made quilts are warmer than the old ones because in the old quilts, there is no air trapped in the cotton.

Question 41.
State the direction of heat transfer by way of convection.
Answer:
By the process of convection, heat is always transferred vertically upwards. The reason is that the medium particles near the source of heat absorb heat from the source and they start moving faster. As a result, the medium at this place becomes less dense so it rises up and the medium from above being denser, moves down to take its place. Thus, current is set up in the medium which is called a convection current. The current continues till the entire medium acquires the same temperature.

Question 42.
Why is a ventilator provided in a room ?
Answer:
Ventilators and windows are provided in rooms for proper ventilation. The reason is that when we breathe out in a room, the air in the room becomes warm and impure. The warm air is less dense i.e. lighter, so it rises up and moves out through the ventilators. Then the cold fresh air comes in the room through the windows to take its place. Thus the continuous circulation of fresh air keeps the air in the room fresh.

Question 43.
Why are chimneys provided over furnace in factories ?
Answer:
Chimneys are provided over the furnace in factories. This is because the hot gases coming out of the furnace are less dense than the air. They rise up through the chimney. The smoke, fumes etc. around the furnace rush in so as to take their place and they are sucked out. Thus, the chimney helps to remove the undesired fumes, smoke etc. from the premises.

Question 44.
What are the land and sea breezes ? Explain their formation.
Answer:
LAND BREEZE : Blowing of breeze (air) from land towards sea is called land breeze.
During night land and sea water both lose heat. Specific heat capacity of land being very low as compared to that of sea water, land loses heat energy fast and cools more rapidly as compared to sea. Sea water being at higher temperature, the air above it becomes lighter and rise up. Air from land being at higher pressure. So air from land starts blowing towards sea and gives rise to Land Breeze.

SEA BREEZE : Blowing of breeze (cold air) from sea towards land during the day is called the SEA BREEZE. During day time land and sea both are heated equally by the sun, but land has very low specific heat capacity as compared to sea, is heated up more quickly. Thus air above land due to heat becomes lighter and rises up. Thus pressure decreases and cold and humid air above the sea starts blowing towards land, thereby giving rise to SEABREEZE.

Question 45.
Why is the freezing chest in a refrigerator fitted near its top?
Answer:
Freezing chest in a refrigerator is fitted near the top, because it cools the remaining space of the refrigerator by convection current. Air near the top comes in contact with the freezing chest gets cooled, becomes denser and therefore descends while the hot air from the lower part rises and hence convection currents produced cool the whole space inside.

Question 46.
Explain briefly the process of heat transfer by radiation.
Answer:
RADIATION. “The transfer of heat energy from a hot body to cold body directly, without heating the medium between two bodies is called RADIATION.”
The radiant heat or thermal radiation is of the form of ELECTROMAGNETIC WAVES. These waves can travel even in vacuum in all directions in straight line with the speed of light. They do not heat the medium through which they pass. Heat radiations are also called INFRA-RED RADIATIONS because the wavelength of heat radiations is longer than that of visible light. These radiations can cause heating effect only if they are absorbed by some material.

Question 47.
Give one example of heat transfer by radiation.
Answer:
When we sit in the sun, we feel warm. We cannot get heat from the sun by the process of conduction or convection because most of the space between the sun and the earth is a vacuum and both of these modes of heat transfer require medium. Hence, one must be getting heat from the sun by the mode of radiation.

Question 48.
Why do we prefer to wear white or light coloured clothes in summer and black or dark coloured clothes in winter ?
Answer:
We prefer to wear white clothes in summer. The reason is that the white clothes reflects most of the sun’s heat and absorb very little of the sun’s heat, thus they keep our body cool.
We prefer to wear black and dark coloured clothes in winter. The reason is that the black or dark colour clothes absorb most of the sun’s heat and keep our body warm.

Question 49.
The bottom of a cooking utensil is painted black. Give the reason.
Answer:
The bottom part of the cooking utensil or pan is painted black. The reason is that the black surface absorbs more heat and so the contents of utensil or pan get cooked rapidly if its bottom part is painted black.

Question 50.
Draw a labelled diagram of a thermo flask. Explain how the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.
Answer:
Heat transfer is minimised because of:
(1) The vacuum between the two walls, rubber, glass, cork and air do not allow the loss of heat by conduction.
(2) Cork in the neck of flask and the cup over it prevent loss of heat by convection.
(3) Heat cannot be lost by conduction or convection because of vacuum between the two walls.
(4) Heat loss is also minimised by radiation, by making outer surface of inner wall and inner surface of outer wall silvered. The inner wall is a BAD RADIATOR and the outer wall is a GOOD REFLECTOR of radiation.

C. Numericals

Question 1.
The temperature of a body rises by 1°C. What is the corresponding rise on the (a) Fahrenheit scale (b) Kelvin scale?
Answer:
(a) Since 100 divisions on Celsius scale =180 divisions on the Fahrenheit scale 1 division on Celsius scale
∴ 1 division on Celsius scale
= 1.80 / 1.00 × 1
= 1.8 divisions in the Fahrenheit scale.
For 1°C rise corresponding rise in Fahrenheit = 1.8°F
(b) Since 100 divisions in the Celsius scale = 100 divisions in the Kelvin scale
1 division on Celsius scale = 100 / 100 × 1
= 1 division on Kelvin scale
For 1°C rise corresponding rise in Kelvin is 1 K.

Question 2.
The temperature rises by 18°F. What is the rise on the Celsius scale ?
Answer:
Since 100 divisions on the Celsius scale =180 divisions on the Fahrenheit scale
∴ 18 divisions on Fahrenheit scale.

Question 3.
Convert 5°F to the Celsius scale.
Answer:

Question 4.
Convert 40°C to the (a) Fahrenheit scale (b) Kelvin Scale.
Answer:
(a) Fahrenheit scale
C = 40°C
Substitute value of C = 40° in below equation

Question 5.
Convert – 40°F to the Celsius scale.
Answer:

Selina Concise Physics Class 6 ICSE Solutions – Simple Machines

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Physics for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 4 Simple Machines

• Synposis
• Work is said to be done when a force applied on a body moves it. If the body does not move on applying a force on it, no work is done by the force.
• The capacity of doing work is called energy.
• A machine is a device which helps us to do work more easily.
• A machine enables us to apply a less effort for a load greater than the effort or to apply the effort at convenient point and in a desired direction.
• Some machines are simple and some are complex.
• The mechanical advantage of a machine is the ratio of the load to the effort, i.e., Mechanical advantage = Load / Effort
• Smaller the effort required for a certain load, greater is the mechanical advantage of the machine.
• The efficiency of a machine is the ratio of the useful work done on the load by the machine to the work put into the machine by the effort, i.e.
  Efficiency = Work output / Work input
• The efficiency of an ideal machine is 1 (or 100 per cent).
• The efficiency of an actual machine is less than 1 because some part of the work put into the machine is lost in overcoming the friction between the moving parts of the machine.
• A lever is a simple machine which we most commonly use in our daily life. It is a rod which can turn about a fixed point called the fulcrum.
• The mechanical advantage of a lever is equal to the ratio of the effort arm to the load arm, i.e. Mechanical advantage of a lever = Effort arm / Load arm
  The levers are of three kinds :
  Class I levers which have fulcrum in between the load and the effort.
  Class II levers which have load in between fulcrum and the effort.
  Class III levers which have effort in between the fulcrum and the load.
• The mechanical advantage of class I lever can be 1, more than 1 or less than 1.
• The mechanical advantage of class II levers is always more than 1.
• The mechanical advantage of class III levers is always less than 1.
• A pulley is a simple machine which is used for raising a load up by applying the effort downwards.
• The mechanical advantage of an ideal pulley is 1. In an actual pulley, due to friction, the mechanical advantage is less than 1 (i.e., the effort is more than the load).
• The pulley allow us to apply the effort downwards which is a convenient direction.
• The wheel and axle is a simple machine having a wheel and an axle. The linear motion of axle is obtained by rotating the wheel so as to reduce friction. Example: Steering wheel, screw drivers, water tap etc.
• An inclined plane is a simple machine which is used to move a load up with a less effort. It is a sloping (or slanting) surface.
• Less the slope of the inclined plane, less is the effort needed to push a load up.
• The mechanical advantage of an inclined plane is greater than 1 (i. e. a less effort is required to push a heavy load up an inclined plane).
• A wedge is a sharp edge formed by joining the two inclined planes together. Example: nail, knife, axe, plough etc.
• A screw is a modified form of an inclined plane.
• A screwjack is a simple machine having a combination of a screw and a lever. It is used to lift the heavy vehicles such as cars, trucks, buses etc.
• Machines are used for our convenience. Therefore, we should take proper care of a machine by painting the machine parts to avoid rusting, lubricating its parts to reduce friction etc. This increases the life span of the machine.

Test yourself

A. Objective Questions

1. State whether the following statements are True or False.

(a) A boy does work while pushing a wall.
Answer. False

(b) A machine performs work by itself.
Answer. False

(c) In an ideal machine, work done on load is equal to the work done by effort.
Answer. True

(d) All levers are force multipliers.
Answer. False

(e) A pulley changes the direction of force.
Answer. True

(f) An inclined plane always has the mechanical advantage more than 1.
Answer. True

2. Fill in the blanks

(a) The useful work done by an actual machine is always less than the work done on the machine.
(b) In class II levers, the load is in between fulcrum and effort.
(c) The mechanical advantage of class III lever is always less than 1.
(d) A pulley is used to change the direction of effort.
(e) Mechanical advantage of an inclined plane is always greater than 1.

3. Match the following

Answer.

4. Select the correct alternatives

(a) For an ideal machine, the efficiency is

1. greater than unity
2. less than unity
3. equal to unity
4. depends on the value of load

(b) Mechanical advantage of a machine is defined as:

1. Load X Effort
2. Load / Effort
3. Load + Effort
4. Effort / Load

(c) The mechanical advantage of a lever is equal to:

1. Load arm / Effort arm
2. Effort arm / Load arm
3. Load arm + Effort arm
4. Load arn — Effort arm

(d) A pulley is used because it

1. has the mechanical advantage greater than one
2. has 100% efficiency
3. helps to apply the force in a convenient direction
4. requires more effort to raise a less load.

(e) Wheel is used with axle because

1. sliding friction is less than the rolling friction
2. rolling friction is less than the sliding friction
3. they work as the inclined plane
4. They help us to change the direction of force.

B. Short/Long Answer Questions

Question 1.
When is work said to be done by a force ?
Answer:
Work is said to be done when a force moves an obj ect through a distance in its own direction.

Question 2.
What is energy ?
Answer:
Energy: The ability or capacity to do work is called energy.

Question 3.
What do you understand by a machine ?
Answer:
Machine: A machine is a device that allows us to do work with less effort. Machines make our work easier to do. Machines have made our li ves comfortable and faster.

Question 4.
What is the principle on which a machine works ?
Answer:
Principle of a Machine: The work output of a machine is equal to the work input.

Question 5.
State two functions of a machine.
Answer:
Various functions that a machine can perform are:

1. Changing the direction of applied force — Example: When a flag is hoisted with the help of a pulley.
2. Changing the magnitude of applied force — Example: Bottle opener multiplies the applied force and much less effort is required to open the cap.
3. Applying force at a convenient point — Example: In a pair of scissors, the input force is applied at the handle of the scissors which cuts the paper at the other end of the blade.
4. Changing the speed of an object — Example: While riding a bicycle, force is applied on pedals which multiplies the speed.

Question 6.
Name six simple machines. Give an example of each machine.
Answer:
The Simple Machines and there examples are as follows:

1. The lever: Examples are a crow bar, claw hammer, a pair of pilers etc.
2. The Inclined plane: Examples are ramp, staircase, hilly roads etc.
3. The wedge: Examples are knife, axe, plough, nail etc.
4. Screw: Examples are A screw.
5. The wheel and axle: Examples are steering wheel of a car, bicycle pedal etc.
6. The pulley: Examples are a pulley used in raising a load.

Question 7.
Define the term ‘work input’ and ‘work output’ in relation to a machine.
Answer:
Work input is work done on a machine equal to the effort force times the distance through which the force is applied.
Work output is work that is done by a machine equals resistance force times the distance through which the force applied.
For an ideal machine, the work output is equal to the work input i. e. the efficiency.

Question 8.
Explain the term mechanical advantage of a machine.
Answer:
The mechanical advantage of a machine is the ratio of the load to the effort. In other words

Question 9.
Define the term efficiency of a machine.
Answer:
The ratio of the work done by the machine to the work done on the machine is called efficiency of a machine

(Work done by a machine is called the output energy and the work done on a machine is called the input energy.)

Question 10.
What is an ideal machine ?
Answer:
A machine is which no part of the work done on the machine is wasted, is called an ideal or perfect machine. Thus, for an ideal machine, the work output is equal to the work input, i.e., the efficiency of an ideal machine is 1 (or 100 per cent).

Question 11.
Can a machine have an efficiency of 100% ? Give a reason to support your answer.
Answer:
Efficiency of a machine is always less than 100% as output energy is always less than the input energy, because some energy is lost to overcome friction.

Question 12.
A machine is 75% efficient’. What do you understand by this statement ?
Answer:
If a machine is 75% efficient, it means that 75% of the work input to the machine is obtained as the useful work output. The remaining 25% of the work input has been lost in overcoming the friction.

Question 13.
What is a lever ?
Answer:
Lever: A lever is a simple rigid bar which is free to move around a point called fulcrum.

Question 14.
Describe three orders of levers giving an example of each. Draw neat diagrams showing the positions of fulcrum, load and effort in each kind of lever.
Answer:
The levers are of three kinds :
Class I levers which have fulcrum in between the load and the effort.

Class II levers which have load in between the fulcrum and the effort.

Class III levers which has effort in between the fulcrum and the Load

Question 15.
What do you mean by the mechanical advantage of a lever ?
Answer:
The mechanical advantage of a lever is equal to the ratio of the effort arm to the load arm. This is also called the principle of a lever.

Question 16.
Which class of lever has the mechanical advantage always more than 1 ? Give an example.
Answer:
The mechanical advantage of class II levers is always more- than 1.
Example – Nut cracker, wheel barrow, bottle opener etc.

Question 17.
Which class of lever has the mechanical advantage always less than 1 ? Give an example.
Answer:
The mechanical advantage of class III levers is always less than 1.
Example: a pair of tongs, sugar tongs, knife, forceps etc.

Question 18.
Give one example of class I lever in each case where the mechanical advantage is

1. more than 1
2. equal to 1
3. less than 1.

Answer:

1. more than 1: Load arm of pliers
2. equal to 1: See – saw
3. less than 1: The load arm of a pair of scissors.

Question 19.
Name the class to which the following levers belong:

Answer:
(a) A pair of scissors — Class I lever
(b) a lemon squeezer — Class II lever
(c) a nut cracker — Class II lever
(d) a pair of sugar tongs — Class III lever
(e) a beam balance — Class I lever
(f) an oar rowing a boat — Class I lever
(g) a wheel barrow — Class II lever
(h) a see saw — Class I lever
(i) a pair of pilers — Class I lever
(j) a crow bar — Class I lever

Question 20.
The diagram given below shows the three kinds of levers. Name the class of each lever and give one example of each class.

Answer:

Examples : The examples of class I levers are : a see saw, a pair of scissors, a pair of pilers, crow bar, common balance, spoon opening the lid of a tin can, handle of a hand pump.

Examples : The examples of lever of class II are : nut cracker, wheel barrow, paper cutter, mango, lemon squeezer, bottle opener.

Examples: The examples of levers of class III are : a pair of tongs, sugar tongs, knife, forceps,-forearm of a person holding a load, spade for lifting soil or coal.

Question 21.
Draw diagrams to illustrate the positions of fulcrum, load and effort, in each of the following:
(a) a see saw
(b) a beam balance
(c) a nut cracker
(d) a pair of forceps
Answer:

Question 22.
How can you increase the mechanical advantage of a lever ?
Answer:
The mechanical advantage of a lever can be increased by increasing the effort arm or reducing the load arm.

Question 23.
How does the friction at the fulcrum affect the mechanical advantage of the lever ?
Answer:
Friction at the fulcrum reduces the mechanical advantage.

Question 24.
State three differences between the three classes of levers.
Answer:

Question 25.
What is a pulley ?
Answer:
Pulley: It is a flat circular disc with a groove in its edge and a rope passing through the groove. It is capable of rotating around a fixed point passing through its central axis called axle.

Question 26.
What is the mechanical advantage of an ideal pulley ?
Answer:
In an ideal pulley, the effort applied is equal to the load to be lifted.
i.e. Effort = Load
Mechanical advantage = Load / Effort = 1

Question 27.
The mechanical advantage of an actual pulley is less than 1. Give a reason. What is the justification for using the pulley then ?
Answer:
In an actual pulley due to friction, the mechanical advantage is less than 1 (i.e. the effort is more than the load).
The reason for using the pulley when its mechanical advantage is equal to 1 or less than 1 is that the pulley allows us to apply the effort downwards i.e. in a convenient direction. To raise a load directly upwards is difficult. But with the help of a pulley, the effort can be applied in the downward direction to move the load upwards. One can hang on it to make use of his own weight also in order to apply the effort.

Question 28.
Draw a neat labelled diagram showing a pulley being used to lift a load. How are load and effort related in an ideal situation?
Answer:
To raise a load, the load is attached to one end of the string and the effort is applied at the other end by pulling it is downward direction . as shown in fig.

Question 29.
What is an inclined plane? What is its use ? Give two examples where ¡t is used.
Answer:
An inclined plane is a rigid sloping surface over which heavy loads can be raised or lowered to a certain height or depth.
The mechanical advantage of an inclined plane is the ratio of the length of the plank to the vertical height of the load raised. Its value is greater than one. Therefore, an inclined plane acts as a force multiplier. Thus, it can be used to lift heavy loads.
Example : If a heavy box needs to be loaded on a lorry, it is far easier to push it over an inclined plane than to lift it up. Steeper the inclined plane, greater will be the effort required to push up the load.
Sloping ramps, flyovers, roads on hills and staircases are all examples of inclined planes.

Question 30.
What is a screw ? Give two examples.
Answer:
A screw is a simple machine which appears like an inclined plane wound around a rod with a pointed tip.
Examples : ajar lid, a drill.

Question 31.
What is wheel and axle ? Give two examples.
Answer:
The wheel and axle is a simple machine having a wheel and an axle. The linear motion of axle is obtained by rotating the wheel so as to reduce friction. Example: Steering wheel, screw drivers, water tap etc.

Question 32.
How does a wheel help in moving the axle ?
Answer:
Wheel-and-axle arrangement consists of two cylinders of different diameters joined together such that if one is made to rotate, the other also rotates. The axle is a cylindrical rod fixed to the centre of a circular disc-like object called the wheel.

This machine acts as a speed multiplier device.
In riding a bicycle, when we apply force on the wheel (by pedal), the fixed axle rotates with it easily. This force that turns the axle produces a much larger movement of the wheel.

Question 33.
What is a wedge ? Give two examples.
Answer:
A wedge is a double inclined plane such that the two sloping surfaces taper to form either a sharp edge or a pointed edge. Examples : A knife, an axe, a chisel.
In some special cases, the number of inclined planes used can be more than two as well. In such cases, the sloping surfaces generally taper to form either a very sharp or a pointed edge to split or pierce materials. Pins, nails and needles are examples of pointed wedges. The front end of a boat is shaped like a wedge so that it can easily cut across the flowing water.
The wedge works on a principle of an inclined plane.

Question 34.
Name the machine to which the following belong :

1. Beam balance
2. Lemon crusher
3. Sugar tongs
4. Ramp
5. Door knob
6. Needle

Answer:

1. Beam balance — A lever (lever of class I)
2. Lemon crusher — A lever (lever of class II)
3. Sugar tongs — A lever (lever of class III)
4. Ramp — An inclined plane
5. Door knob — Wheel and axle
6. Needle — Wedge

Question 35.
What care would you take to increase the life span of a machine which you use ?
Answer:
Taking care of machines: Some of the ways in which machines should be cared for are given below :

1. Machines should be kept in a clean environment, which is free from dust and moisture.
2. When not in use, machines should be kept covered to prevent collection of dust on them.
3. Machines made of iron should be protected from rust by coating them with paint.
4. The moving parts of a machine should be regularly oiled with a good-quality machine oil to reduce friction and wear and tear. The above care of machines increases their life.

Question 36.
Select the correct statement :
(a) A wheel barrow is a lever of class I.
(b) The efficiency of a machine is always 100%
(c) Friction in moving parts of a machine reduces its efficiency.
(d) No lever has the mechanical advantage greater than 1.
(e) It is easier to lift a load vertically up than to push it along an inclined plane.
(f) A screw is made by two inclined planes placed together.
Answer:
(c) Friction in moving parts of a machine reduces its efficiency.

C. Numericals

Question 1.
In a machine an effort of 10 kgf is applied to lift a load of 100 kgf. What is its mechanical advantage ?
Answer:
Given,
Load = loo kgf
Effort = 10 kgf

Question 2.
The mechanical advantage of a machine is 5. How much load it can exert for the effort of 2 kgf ?
Answer:
Given,
Mechanical advantage = 5
Effort 2 kgf

Question 3.
The mechanical advantage of a machine is 2. It is used to raise a load of 15 kgf. What effort is needed ?
Answer:
Given,
Mechanical advantage = 2
Load = 15 Kgf

Question 4.
A lever of length 100 cm has effort of 15 kgf at a distance of 40 cm from the fulcrum at one end. What load can be applied at its other end ?
Answer:

Question 5.
In a lever, fulcrum is at one end at a distance of 30 cm from the load and effort is at the other end at a distance of 90 cm from the load. Find :
(a) the length of load arm,
(b) the length of effort arm, and
(c) the mechanical advantage of the lever.
Answer:

Selina Concise Physics Class 6 ICSE Solutions – Matter

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Physics for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 6 Physics ICSE SolutionsChemistryBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Physics Chapter 1 Matter

Synposis

•  Matter is the substance which occupies space and has mass.
•  The three states of matter are

1.  solid,
2.  liquid
3.  gas.

•  Matter is composed of large number of molecules.
•  A molecule is the smallest particle which can exist freely in nature by itself and it retains the properties of the substance.
•  All molecules of a substance are identical, but the molecules of different substanes are different.
•  A molecule is very small in size (10^-10 m).
•  The molecules are separated from each other with spaces called inter-molecular spacing.
•  The molecules in a substance are held together by the forces acting between the molecules which are called the inter-molecular forces.
•  The force of attraction between the molecules of the same sub¬stance is called the force of cohesion, while the force of attraction between the molecules of two different substances is called the force of adhesion.
•  The forces of cohesion and adhesion are effective only when the separation between the molecules is 10^-9 m. When the separation becomes more, they vanish.
•  The molecules in a substance are not at rest, but they are con¬stantly in motion.
•  In a solid, the molecules are rigid, the inter-molecular spacing is least, the inter-molecular forces are strongest and the molecules remain in their fixed positions. They vibrate to and fro about their mean positions, but they do not leave their positions, so a solid has a definite shape and a definite volume.
•  In a liquid, the molecules are not rigid, the inter-molecular spacing is more than that in solids, the inter-molecular forces are weak and the molecules are free to move within the boundary of the liquid, so the liquid has a definite volume, but it does not have a definite shape.
•  In gases, the molecules are not rigid, the inter-molecular spacing is more than that in solids and liquids, the inter-molecular forces are weakest and the molecules are free to move anywhere in space. So the gas has neither a definite volume nor a definite shape.

Activity – 2

Classify 20 objects around you as solids, liquids and gases

Solids : Ice, Aluminium, Silver, Calcium, Gold, Iron, Sodium, chloride, Sugar, Wood.

Liquids : Water, benzene, chloroform, oil, honey, glycerine, hydrochloric acid, alcohol, dettol.

Gases : Steam, Air, Oxygen, Hydrogen, Chlorine, Nitrogen, Ammonia, Helium, Argon.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The molecules of each substance are identical.
Answer. False

(b) The inter-molecular forces are effective at all distances between the two molecules.
Answer. False

(c) The molecules in a substance arc in random motion.
Answer. Tme

(d) In a gas, the molecules can move anywhere in space. .
Answer. Tme

(e) The liquids are less viscous than the gases.
Answer. False

2. Fill in the blanks

(a) All the molecules of a substance are identical.
(b) The inter-molecular spacing is least in solids more in liquids and still more in gases.
(c) The molecular motion in liquid and gas is in zig-zag path.
(d) In a solid, the molecules vibrate to and fro but they remain at their fixed positions.
(e) The inter-molecular forces are the weakest in gases.
(f) A solid exerts pressure downwards on its base.
(g) The gases are least dense.
(h) A solid is most rigid.

3. Select the correct alternative

(a) The diameter of a molecule is approximately

1.  1 cm
2.  10 cm
3.  10^-10 m
4.  1 m

(b) The inter-molecular forces are strongest in

1.  solids
2.  liquids
3.  gases
4.  both (i) and (ii)

(c) The molecules

1.  in solid, liquid and gas, move freely anywhere.
2.  in a solid, move freely within its boundary.
3.  in a liquid, move within its boundary.
4.  in a gas, move only within its boundary.

(d) The solids are

1.  more dense
2.  less dense
3.  least dense
4.  highly compressible

(e) The inter-molecular forces in liquids are

1.  as strong as in solids
2.  stronger than in solids
3.  weaker than in solids
4.  weaker than in gases

4. Match the following columns

B. Short/Long answer questions

Question 1.
Define matter. What is its composition ?
Answer:
Matter is defined as anything which occupies space and has mass. It can be perceived by our sense of smell, touch, sight, hearing and taste.
Matter is composed of tiny particles known as atoms.

Question 2.
Name the three states of matter.
Answer:
The three states of matter are solids, liquids and gases.
Solids —A solid has a definite shape and definite volume.
Example – wood, stone, iron, ice etc.
Liquid —A liquid has a definite volume but not definite shape.
Example — water, juice, milk, oil, etc.
Gases —A gas neither has definite shape nor a definite volume.
Example – air, hydrogen, oxygen, water vapour etc.

Question 3.
What is a molecule ?
Answer:
The smallest unit of matter which can exist independently is called molecule.
Example: Oxygen molecule (O₂) made up of two (O) atoms.

Question 4.
What is the approximate size of a molecule ?
Answer:
Matter is made up of molecules which are very small in size (~10^-9 m).

Question 5.
One litre of water has 6.02 × 10²⁶ molecules. Estimate the size of a molecule.
Answer:
The size of a particle (or molecule of matter is very small. 1 litre of water has 6.02 × 10²⁶ molecules, so the volume of a particle of

Thus the diameter of a water molecule is nearly 1.27 × 10^-9 metre.

Question 6.
What do you mean by inter-molecular spacing ?
Answer:
Intermolecular space — The space between any two consecutive molecules of a substance is called intermolecular space

Question 7.
Describe a simple experiment to illustrate the existence of inter-molecular spacing.
Answer:
Take 100 ml of water in a measuring cylinder. Add 20 gram of salt in water gently and stir it well so as to dissolve the salt well in water. It is noticed that the level of water does not change. It shows that the particles of salt occupy spaces between the particles of water.

Question 8.
What do you mean by inter-molecular forces ?
Answer:
Intermolecular force of attraction — The force of attraction between the molecules (like molecules or unlike molecules) is called intermolecular force of attraction.

Question 9.
What are the forces of cohesion and adhesion ?
Answer:
The force of attraction between the molecules of similar kind is called force of cohesion.
Example: The forces between water molecules.
This force of cohesion keep the molecules of the substance bind together.
The force of attraction between different types of molecules is called force of adhesion.
Example: When a glass filled with water is emptied some water I particles remain stuck to the glass due to the adhesion between water molecules and glass.

Question 10.
State three characteristics of molecules of matter.
Answer:
The particles of matter called molecules, have the following characteristics:

1.  They are very small in size.
2.  They have spaces between them.
3.  They are in constant random motion.
4.  They always attract each other.

Question 11.
State the approximate spacing between two molecules of a matter.
Answer:
The spacing between particles of a matter is called inter-molecular space.

Question 12.
How do the solids, liquids and gases differ in their following properties
(a) Size
(b) Shape
(c) Density
Answer:

Question 13.
The molecules in a substance are in motion. What type of path do they follow ?
Answer:
The particles in a substance are not at rest (in motion),and they move randomly in all possible directions in a zig-zag Path

Question 14.
Describe a simple experiment to illustrate that molecules are not at rest, but they constantly move.
Answer:
Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place it on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below.

Question 15.
Write down five general properties of solids, liquids and gases.
Answer:
Solids:

1.  The molecules here are very tightly packed having negligible or very less intermolecular space.
2.  They have the strongest intermolecular force of attraction.
3.  The molecules have very small vibration about their mean position i.e. small amplitude.
4.  They have a definite shape and volume.
5.  They are generally hard and rigid.
6.  They are good conductors of heat.

Liquids:

1.  Molecules are less tightly packed.
2.  The intermolecular force of attraction is less than that of solids.
3.  The molecules here can move from one place to another
4.  Do not have any particular shape of their own and thus acquire the shape of the vessel.
5.  A particular quantity of a liquid has a definite volume at a given temperature.

Gases :

1.  The force of attraction between the molecules is the least.
2.  The intermolecular space is the largest.
3.  Neither have a definite shape nor a definite volume.
4.  The molecules move independently.
5.  Worst conductors of heat.

Question 16.
Give the molecular model for a solid and use it to explain why a solid has a definite volume and a definite shape.
Answer:

Here the molecules are very tighty packed that there is no or very less intermolecular space and there is high intermolecular force of attraction (force of cohesion).
The molecules do not move about their mean position and thus solids have a definite shape and volume.

Question 17.
Describe the molecular modcl for a liquid. I-low does it explain that a liquid has no definite shape, but has a definite Volume ?
Answer:

Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they do not have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.

Question 18.
A gas has neither a definite volume nor a definite shape. Describe the molecular model to explain it.
Answer:
Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not

bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.

Question 19.
Distinguish between the three states of matter—solid, liquid and gas on the basis of their molecular models.
Answer:
Solids:

Here the molecules are very tighty packed that there is no or very less inteimolecular space and there is high intermolecular force of attraction (force of cohesion).
The molecules do not move about their mean position and thus solids have a definite shape and volume.
Liquids:

Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they donot have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.
Gases :

Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.

Question 20.
Distinguish between solids, liquids and gases on the basis of their following properties :
(a) compressibility
(b) fluidity
(c) rigidity
(d) expansion on heating
Answer:

Question 21.
What do you mean by the change of state of matter ? Explain:
(a) the change of a solid into a liquid at a constant temperature, and
(b) the change of a liquid into a gas at a constant temperature.
Answer:
The change in state of matter of a substance from solid to liquid or from liquid to gas is brought by imparting heat energy to it at a constant temperature.
(a) The process of change of a substance from solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting or fusion i. e.

(b) The process of change of a substance from a liquid state to its gaseous state at a particular temperature, called the boiling point,
is called boiling or vaporisation, i.e.

Selina Concise Physics Class 6 ICSE Solutions – Physical Quantities and Measurement

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 2 Physical Quantities and Measurement

• Synposis
•  The observation of a phenomenon is made possible by using the five senses: sight, smell, touch, hearing and taste.
•  Our senses are not always reliable. They are subjective.
•  Sometimes it is necessary to make an exact measurement.
•  Physics is a science of measurement.
•  We use instruments to get an exact measurement.
•  Four basic measurements in our daily life are: measurement of length, measurement of mass, measurement of time, and measurement of temperature.
•  Measurement is basically a process of comparison of the given quantity with a standard unit.
•  For measuring a quantity we need a unit, and then we find the number of times that unit is contained in that quantity.
•  The unit selected for measurement should be of a convenient size and it must not change ‘with place or time.
•  The distance between two fixed points is called length.
•  The S.I. unit of length is metre (m). Its multiple is kilometre (km), where 1 km = 1000 m. Its sub multiples are centimetre (cm) and millimetre (mm), where 1 cm = 10^-2 m and 1 mm = 10^-3 m.
•  The FPS unit of length is foot (ft) and its sub multiple is inch where 1 ft = 12 inch and 1 ft = 30.48 cm.
•  The most common instruments used to measure length are the metre ruler and the measuring tape which are marked in cm and mm.
•  To measure a length accurately with a metre ruler, the scale should be placed with its markings close to the object and parallel to its r length. The eye is kept in front of and in line with the reading to be taken.
•  The quantity of matter contained in a body is called its mass.
•  The S.I. unit of mass is kilogram (kg). Its multiples are quintal and metric tonne. 1 quintal = 100 kg and 1 metric tonne =10 quintal = 1000 kg. Its sub multiples are gram (g) and milligram (mg) where 1 g = 10 kg and lmg = 10^-6 kg.
•  The FPS unit of mass is pound (lb) where 1 lb = 453.59 g.
•  Mass of a body is measured by using a beam balance or an electronic balance.
•  The interval between two instances or events is called time.
•  The S.l. unit of time is second (s), 1 s 1 / 86400 of a mean solar day. The C.G.S. and F.P.S. unit of time is also second (s).
• The multiple unit of time are minute (mm), hour (h), day and year where 1 min = 60 s, 1 h = 3600 s, 1 day = 86400 s and 1 year = 3.15 × 10⁷ s.
•  The time at any instant is recorded by a pendulum clock or watch and the time interval of an event is measured by using a stop watch or a stop clock.
•  The temperature is the measure of degree of hotness or coldness of a body.
•  The S.I. unit of temperature is kelvin(K), but the common unit of temperature is degree Celsius (°C) and degree fahrenheit (°F).
•  Doctors use a clinical thermometer to measure the patient’s body temperature.
•  The normal temperature of a human body is 37°C or 98.6°E
•  The total surface occupied by an object is called its area. Area is expressed as the product of measured length of two sides.
•  The S.I. unit of area is square metre (m²).
•  One square metre is the area of a square of each side one metre.
•  The bigger (or multiple) units of area are dam², hectare and square kilometre (km²), where 1 dam² =100 m²,1 hectare= 10⁴ m² and 1 km² = 10^6  m2
•  The smaller (or sub multiple) units of area are cm² and mm² where 1cm² = 10^-4 m² and 1 mm² = 10^-6  m²

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) S.I. unit of temperature is Fahrenheit.
Answer. False

(b) Every measurement involves two things – a number and a unit
Answer. True

(c) Mass is the measure of quantity of matter.
Answer. True

(d) The S.I. unit of time is hour.
Answer. False

(e) The area can be expressed as the product of length of two sides.
Answer. Tme

2. Fill in the blanks

(a) The S.I. unit of length is metre of time is second of mass is kilogram.
(b) °C is the unit of temperature.
(c) 1 metric tonne = 1000 kg
(d) The zero mark in Celsius thermometer is the melting point of ice
(e) The thermometer used to measure the human body temperature is called the clinical thermometer.
(f) The normal temperature of human body is 37 °C or 98.6 °F.
(g) The mass of an object is measured with the help of a beam balance.

3. Match the following columns

4. Select the correct alternative

(a) The symbol of degree celsius is

1. °C
2.  °F
3.  K
4.  °K

(b) lO mm is equal to

1.  1cm
2.  1m
3.  10dm
4.  10cm

(c) The amount of surface occupied by an object is called its:

1.  volume
2.  area
3.  mass
4.  length

(d) A metre ruler is graduated in:

1.  m
2.  cm
3.  mm
4.  km

(e) A thermometre is graduated in:

1.  kelvin
2.  °C
3.  g
4.  cm

B. Short/Long Answer Questions

Question 1.
What is measurement ? How is a measurement expressed ?
Answer:
Measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.
The value obtained on measuring a quantity is called its magnitude. The magnitude of a quantity is expressed as numbers in its unit.

Question 2.
State two characteristics of a unit.
Answer:
Two characteristics of a unit are

1.  It should be of convenient size.
2.  It must be universally accepted, i. e. its value must remain same at all places and at all times.

Question 3.
Name four basic measurements in our daily life.
Answer:
In our daily life we measure the following four basic physical quantities.

1.  Length
2.  Mass
3.  lime
4.  Temperature

Question 4.
What are the S.I. units of

1.  mass
2.  length
3.  time and
4.  temperature. Write their names and symbols.

Answer:
S.I. units are as follows

Question 5.
Define one metre, the S.I. unit of length. State its one multiple and one sub multiple.
Answer:
One metre is defined as the distance travelled by light in air in \(\frac { 1 }{ 299,792,458 }\) of a second
Multiple of metre = Kilometre
Submultiple of metre = Centimetre

Question 6.
Convert the following quantities as indicated
(a) 12 inch = ft
(b) 1 ft = cm
(c) 20 cm = m
(d) 4.2 m = cm
(e) 0.2 km = m
(f) 0.2 cm = mm
(g) 1 yard = m
Answer:
(a) 12 inch 1 ft
(b) 1 ft = 30.48cm
(c) 100 cm = 1m

Question 7.
(a) Describe in steps how would you measure the length of a pencil using a metre rule. Draw a diagram if necessary.
Answer:
To measure the length of a pencil using a metre rule, place metre rule with its marking close to the object. Let PQ be a pencil.
The end P of the pencil coincides with the zero mark on the ruler. The end Q of the pencil is read by keeping the eye at the position ‘B’ vertically above the end Q. So the length of pencil is 4.3 cm.

(b) Explain with an example how you will use the metre ruler in part (a) if the ends of ruler are broken.
Ans. The ends of the ruler get damaged with use and its zero mark may not be visible. To measure the length of an object with such a ruler, the object is placed close to a specific markings on the ruler and positions of both ends of the object are read on the ruler.

The difference of the two readings gives the length of the object. In fig. the reading on ruler at the end X is 1.0 cm and at the end Y is 4.3 cm. So the length of the rod XY is 4.3 — 1.0 = 3.3 cm.

Question 8.
Name the device which you will use to measure the perimeter of your play ground. Describe in steps how you will use it.
Answer:
We will use a measuring tape to measure the perimeter of our playground.
To measure the length of playground the tape is spread along the length of the curved area.

Question 9.
The diagram below shows a stick placed along a metre RULER. The length of the stick is measured keeping the eye at positions A, B and C.

Answer:
(a) Write the length if stick PQ as observed, for each position of the eye. Are they all same?
Ans. Length of stick PQ from
Position A =3.4 cm
Position B = 3.2 cm
Position C = 3.00 cm
No they are not same.
(b) Which is the correct position of the eye? Write the correct length of the stick.
Ans. ‘B’ is the correct position of the eye. Correct length of the stick PQ = 3.2 cm

Question 10.
Define mass. State its (1) S.I. (2) C.GS and (3) EP.S. units. How are they related ?
Answer:
The mass of a body is the quantity of matter contained in it. The S.I. unit of mass is kilogram. In short form, it is written as kg.
In C.GS. system, the unit of mass is gram, (symbol g).
In F.P.S. system, the unit of mass is pound (symbol lb)

Question 11.
Convert the following quantities as indicated:
(a) 2500 kg = ………. metric tonne.
(b) 150 kg = quintal
(e) 10 lb = ………. kg
(d) 250 g = … .kg
(e) 0.01 kg = ………. g
(f) 5 mg = ………. kg
Answer:

Question 12.
Name the instrument which is commonly used to measure the mass of a body. State how is it used ?
Answer:
Instrument commonly used to measure the mass of a body, is the beam balance.

When we hold up the balance, we observe that when there is nothing on either pan, the beam is horizontal. The body whose mass is to be measured is placed on the left pan. The standard weight are put on the right pan. They are so adjusted that the beam is again horizontal on holding the balance up. The total of the standard weights gives the mass of the given body.

Question 13.
Define one kilogram, the S.I. unit of mass. How is it related to (i) quintal (ii) metric tonne and (iii) gram.
Answer:
The mass of 1 litre of water at 4 °C is taken as 1 kilogram
1 quintal = 100 kg
1 metric ton = 10 quintal = 1000 kg

Question 14.
Name and define the S.I. unit of time. How is it related to (i) minute (ii) hour, (iii) day and (iv) year ?
Answer:
The S.I. unit of time is second. In short form we write it as ‘ S ’.
One second is the time interval between the two consecutive ticks that you hear from pendulum wall clock.
1 min = 60 s
1 h = 60 min. = 3600 s.
1 day = 24 h = 86400 s.
1 year = 365 days = 3.15 × 10⁷ s.

Question 15.
Name two devices used to measure the short time interval of an event.
Answer:
Two devices used to measure the time interval of an event are

1.  StopWatch
2.  Stop Clock

Question 16.
Express in second

1.  3 minute 15 second and
2.  5 hour 2 minute 5 second.

Answer:

1.  3 minute = 15 second
   1 minute = 60 second
   3 minutes 15 second = 60 × 3 + 15
   = 180 + 15
   = 195 seconds
2.  1 minute =60 second
   2 minutes =2 × 600 = 120 second …(1)
   1 hour 3600 second
   5 hour 3600 × 5 = 18000 second …(2)
   5 hour 2 minutes and 5 second
   = 18000 + 120 + 5 = 18125 seconds

Question 17.
What does the temperature measure ?
Answer:
Temperature measures the degree of coldness and hotness of a body.

Question 18.
Name the

1.  S.I. unit and
2.  one common unit of temperature. Write their symbols also.

Answer:
The S.I. unit of temperature is kelvin (symbol K).
Common unit of temperature is degree centigrade (symbol °C)

Question 19.
Name the instrument used for measuring of the temperature of a person. Draw its labelled neat diagram.
Answer:
The temperature is measured with a thermometer.

Question 20.
Write the temperature of (i) melting ice (ii) boiling water.
Answer:
The temperature of

1.  melting ice = 0 °C
2.  boiling water = 100 °C

Question 21.
What is a clinical thermometer ? State its special feature. Draw a labelled neat diagram of a clinical thermometer showing the range of temperature marked on it.
Answer:
Doctors use a special thermometer called the clinical thermometer for measuring the temperature of the patient’s body. This thermometer has the markings from 35°C to 42°C. It has a slight bend or kink in the stem just above the bulb. This kink is called the constriction. This constriction prevents the mercury from falling back all by itself. The temperature of a healthy person is 37°C. This temperature is marked by a red arrow.

Question 22.
What is the normal temperature of the human body ? How is it indicated in a clinical thermometer ?
Answer:
Normal temperature of a human body is 37°C or 98.6 °F.
To measure the temperature of a patient’s body, its bulb is kept either below the tongue or under the arm’s pit of the patient for about a minute. Then the thermometer is taken out and its reading is noted. When the temperature of patient’s body is above 37°C, he is said to suffer with fever.

Question 23.
Can a clinical thermometer be used to measure the temperature of the boiling water ? Give reason for your answer.
Answer:
No, a clinical thermometer cannot be used to measure the temperature of boiling water. ,
The reasons are

1.  It has a very small range.
2.  It can break on cooling and on excess heating.

Question 24.
Explain the term ‘area of a surface’.
Answer:
The total surface occupied by an object is called its area or surface area.

Question 25.
Name the S.I. unit of area and define it.
Answer:
The S.I. unit of area is square metre or meter² which in short form is written as m².

Question 26.
How are the units

1.  square yard
2.  hectare
3. km²
4. cm²
5. mm² related to the S.I. unit of area ?

Answer:

Question 27.
Explain how you will measure the area of (i) a square (b) a leaf?
Answer:
The area of a square can be calculated by using the following formula –
1. Area of square of side l
= side x side
= l × l = L² .
The area of a leaf is obtained by using a graph paper. A graph paper has small squares of each side 1 mm. The area of each big square is 1 cm².
Procedure: Place the leaf on graph paper. Draw its outline on the paper and remove it. Now count the number of complete squares. To this add the number of incomplete squares which are half or more than half. Ignore the squares which are less than half. Thus,
Approximate area = (No. of complete squares + no. of half or more than half of incomplete squares) × area of one square.

Selina Concise Physics Class 7 ICSE Solutions – Physical Quantities and Measurement

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Physics for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 1 Physical Quantities and Measurement

• Points to Remember
• The mass of a body is the quantity of matter contained in a body regardless of its volume or any force acting on it.
•  The weight of a body is the force with which every body is attracted towards its centre.
•  The unit of mass in S.I. system is kilogram (kg). Higher units of mass are quintal and metric tonne.
•  The weight of body changes with acceleration due to gravity.
•  Weight is zero at the centre of the earth.
•  Mass per unit volume of a substance is called density of the body.
•  The unit of density in S.I. system is kg m^-3 and gcm^-3 in C.G.S. system.
•  The density in S.I. system is 1000 × numerical value in C.G.S. system.
•  The density of liquids and gases decreases or increases with the rise or fall in temperature.
• The cycle of upward and downward movements of the fluid form currents in the medium which are known as convectional currents.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) The S.I. unit of volume is litre.
Answer. False.
The S.I. unit of volume is cubic metre.

(b) A measuring beaker of capacity 200 ml can measure only the volume. 200 ml of a liquid.
Answer. True.

(c) cm2 is a smaller unit of area than m².
Answer. True.

(d) Equal volumes of two different substances have equal masses.
Answer. False.
Equalvolumes of two different substances have different masses.

(e) The S.I. unit of density is g cm^-3.
Answer. False.
The S.I. unit of density is Kg m^-3.

(f) 1 g cm^-3 = 1000 kg m^-3.
Answer. True.

(g) The density of water is maximum at 4°C.
Answer. True.

(h) The speed 5 ms^-1 is less than 25 km h^-1.
Answer. True.

(i) The S.I. unit of speed is ms^-1.
Answer. True

2. Fill in the blanks

(a) l m³ = 10⁶ cm³
(b) The volume of an irregular solid is determined by the method of displacement of liquid.
(c) Volume of a cube = (one side)
(d) The area of an irregular lamina is measured by using a grapl paper.
(e) Mass = density × volume.
(f) The S.I. unit of density is kg m^-3.
(g) 1 g cm^-3 = 1000 kg m^-3.
(h) 36 km h^-1 = 10 ms^-1.
(i) Distance travelled d = speed v × time t.

3. Match the following

4. Select the correct alternative 

(a) One litre is equal to :

1.  1 cm^-3
2. 1 m³
3.  10^-3  cm³
4. 10^-3 m³

(b) A metallic piece displaces water of volume 15 ml. The volume of piece is :

1.  15 cm³
2.  15 m³
3.  15 × 10³ cm³
4.  15 × 10³ cm³

(c) A piece of paper of dimensions 1.5 m x 20 cm has area :

1.  30 m2
2.  300 cm2
3.  0.3 m2
4.  3000 m3

(d) The correct relation is :

1.  d = M × V
2.  M = d × Y  
3.  V = d × M
4.  d = M + V

(e) The density of alcohol is 0.8 g cm-3. In S.I. unit, it will be :

1.  0.8 kg m^-3
2.  0.0008 kg m^-3
3.  800 kg m^-3
4.  8 x 10³ kg m^-3

(f) The density of aluminium is 2.7 g cm-3 and of brass is 8.4 g cm^-3. For the same mass, the volume of:

1.  both will be same
2.  aluminium will be less than that of brass
3.  aluminium will be more than that of brass
4.  nothing can be said.

(g) A block of wood of density 0.8 g cm^-3 has a volume of 60 cm³. The mass of block will be :

1.  60.8 g
2.  75 g
3.  48 g
4.  0.013 g

(h) The correct relation for speed is

1.  Speed = distance x time
2.  speed = distance / time
3.  speed = time / distance
4.  speed = 1 / distance x time

(i) A boy travels a distance 150 m in 1 minute. His speed is

1.  150 m s^-1
2.  2.5 m s^-1
3.  25 m s^-1
4.  9 m s^-1

B. Short/Long Answer Questions

Question 1.
Define the term volume of an object.
Answer:
The space occupied by an object is called its volume.

Question 2.
State and define the S.I. unit of volume.
Answer:
S.I. unit of volume – The S.I. unit of volume is cubic metre. In short form, it is written as m³.
One cubic metre is the volume of a cube of each side 1 metre as shown in figure below i.e., 1 m³ = 1 m × 1 m × 1 m.

Question 3.
State two smaller units of volume. How are they related to the S.I. unit?
Answer:
A smaller unit of volume is cubic centimetre (symbol cm³) and cubic decimetre (symbol 1 dm³). One cubic centimetre is the volume of a cube of each side 1 centimetre, i.e.,
1 cm³ = 1 cm × 1 cm × 1 cm.
Relationship between m³ and cm³
l m³ = lm × lm × lm
= 100 cm × 100 cm × 100 cm
= 10,00,000 cm³ = 10⁶ cm³.
Relationship between m³ and dm³
l m³ = lm × 1 m × 1 m .
= 10 dm × 10 dm × 10 dm
= 1000 dm
= 10³ dm
Note 1 m = 10 dm

Question 4.
How will you determine the volume of a cuboid ? Write the formula you will use.
Answer:
Volume of a cuboid = length × breadth × height.

Question 5.
Name two devices which are used to measure the volume of an object. Draw their neat diagrams.
Answer:
Two devices that are used to measure the volume of an object are :
(i) Measuring cylinder and
(ii) Measuring beaker.

Question 6.
How can you determine the volume of an irregular solid (say a piece of brass) ? Describe in steps with neat diagrams.
Answer:
To measure the volume of a piece of stone.
Take a piece of brass, a measuring cylinder, fine thread of sufficient length and some water.
Place a measuring cylinder on a flat horizontal surface and fill it partially with water. Note the reading of the water level very carefully. Now tie the piece of brass with a thread and dip it completely into water. We see that the level of water rises. Note the reading of the new water level.

The difference in the two levels of water gives the volume of the piece of brass
Initial level of water = 60 ml
Level of water when brass is immersed = 80 ml
∴ Volume of water displaced = 80 ml – 60 ml = 20 ml
∴ Volume of the piece of brass = 20 cm³
Note : 1 ml = 1 cm³

Question 7.
You are required to take out 200 ml of milk from a bucket full of milk. How will you do it ?
Answer:
By using the measuring beaker A measuring beaker is used to measure a fixed volume of liquid from a large volume. Suppose it is required to measure 200 ml of milk from the milk contained in a bucket. For this, take the measuring beaker of capacity 200 ml. Wash it and dry it. Then, immerse the measuring beaker well inside the milk contained in the bucket so that the beaker gets completely filled with the milk.
Take out the measuring beaker from the bucket gently so that no milk splashes out and then pour the milk from the measuring beaker into the another empty vessel.

Question 8.
Describe the method in steps to find the area of an irregular lamina using a graph paper.
Answer:
Method to find the area of an irregular lamina using a graph paper : First, place the lamina over a graph paper and draw its boundary line on the graph paper with a pencil. Then remove the lamina and count and note the number of complete squares as well as the number of squares more than half within the boundary line (only the squares less than half, are left while counting). The area of lamina is equal to the sum of the area of complete squares and the area of squares more than half. Let n be the total number of complete and more than half or half squares within the boundary of lamina. Since area of one big square is 1 cm × 1cm = 1 cm², so the area of lamina will be n x

Question 9.
Define the term density of a substance.
Answer:
The density of a substance is defined as the mass of a unit volumx of that substance.

Question 10.
State the S.I. and C.G.S. units of density. How are they inte related ?
Answer:
The S.I. unit of mass is kilogram (symbol kg) and of volume is
cubic metre (symbol m³). Therefore S.I. unit of density is kg/m³
or kg m^-3.
The C.G.S. unit of mass is gram (symbol g) and of volume is cubic centimetre (symbol cm3). Therefore the C.G.S. unit of
density is g/cm³ or g cm^-3.

Question 11.
‘The density of brass is 8.4 g cm’3’. What do you mean by the statement ?
Answer:
Density of brass is 8.4 g cm^-3. This means that unit volume of brass contain 8.4 g mass.

Question 12.
Arrange the following substances in order of their increasing density:
(a) iron
(b) cork
(c) brass
(d) water
(e) mercury
Answer:
b<a<c<d<e

Question 13.
How does the density of water changes when :
(a) it is heated from 0°C to 4°C,
(b) it is heated from 4°C to 10°C ?
Answer:
(a) Water contracts on heating from 0°C to 4°C and expands on heating above 4°C.
(b) The density of water is maximum at 4°C. It decreases when it is cooled from 4°C to 0°C or it is heated above 4°C.

Question 14.
Write the density of water at 4°C.
Answer:
The density of water at 4°C is 1.0 g cm^-3, or 1,000 kg m^-3

Question 15.
Explain the meaning of the term speed.
Answer:
The distance covered or travelled by a body in one second is called the speed of the body, i.e.

Question 16.
Write the S.I. unit of speed.
Answer:
The S.I. unit of speed is metre/second or metre per second. Its symbol is m s^-1.

Question 17.
A car travels with a speed 12 m s”1, while a scooter travels with a speed 36 km h-1. Which of the two travels faster ?
Answer:
Speed of car = 12 m s^-1
Speed of scooter = 36 km h^-1
here, 1 km = 1000 m
1 hr = 3600 sec

∴ Speed of car is more. Car travels faster than scooter.

C. Numericals

Question 1.
The length, breadth and height of a water tank are 5 m, 2.5 m and 1.25 m respectively. Calculate the capacity of the water tank in (a) m³ (b) litre.
Answer:
Given,
Length (1) = 5m
Breadth (b) = 2.5 m
and Height (h) = 1.25 m

Question 2.
A solid silver piece is immersed in water contained in a measuring cylinder. The level of water rises from 50 ml to 62 ml. Find the volume of silver piece.
Answer:
Given, initial level of water .v₁ = 50 ml
Final level of water v₂ = 62 ml
Volume of silver piece V = v₂ – v₁
= 62 ml – 50 ml
= 12 ml or 12 cm³

Question 3.
Find the volume of a liquid present in a dish of dimensions 10 cm x 10 cm x 5 cm.
Answer:
Volume of water = Length × breadth × height
= 10 cm × 10 cm × 5 cm
= 500 cm³ or 500 ml.

Question 4.
A rectangular field is of length 60 m and breadth 35 m. Find the area of the field.
Answer:
Length of a rectangular field = 60 m
Breadth of rectangular field = 35 m
∴ Area = 60 m × 35 m
= 2100 m²

Question 5.
Find the approximate area of an irregular lamina of which boundary line is drawn on the graph paper shown in fig. 1.16. below.

Answer:
From figure, the number of complete squares = 11
The number of squares more than half = 9
∴ Total number of squares = 11 + 9 = 20
∴ Area of the 1 square = 1 cm × 1cm = 1 cm²
∴ Area of 20 squares = 20 × 1 cm² = 20 cm²
∴ Approximate area of irregular lamina = 20 cm²

Question 6.
A piece of brass of volume 30 cm³ has a mass of 252 g. Find the density of brass in (i) g cm^-3, (ii) kg m^-3.
Answer:

Question 7.
The mass of an iron ball is 312 g. The density of iron is 7.8 g cm^-3. Find the volume of the ball.
Answer:

Question 8.
A cork has a volume 25 cm³. The density of cork is 0.25 g cm^-3. Find the mass of cork.
Answer:

Question 9.
The mass of 5 litre of water is 5 kg. Find the density of water in g cm^-3.
Answer:

Question 10.
A cubical tank of side 1 m is filled with 800 kg of a liquid. Find: (i) the volume of tank, (ii) the density of liquid in kg m^-3.
Answer:

Question 11.
A block of iron has dimensions 2 m × 0.5 m × 0.25 m. The density of iron is 7.8 g cm^-3. Find the mass of block.
Answer:
Given, l = 2m
b = 0.5 m

Question 12.
The mass of a lead piece is 115 g. When it is immersed into a measuring cylinder, the water level rises from 20 ml mark to 30 ml mark.
Find:
(i) the volume of the lead piece,
(ii) the density of the lead in kg m^-3.
Answer:

Question 13.
The density of copper is 8.9 g cm^-3. What will be its density in kg m^-3 ?
Answer:

Question 14.
A car travels a distance of 15 km in 20 minute. Find the speed of the car in (i) km h^-1, (ii) m s^-1.
Answer:
Distance travelled by car =15 km
Time taken = 20 minutes
(i) Speed of car in km h^-1
Convert 20 minutes to hour

Question 15.
How long a train will take to travel a distance of 200 km with a speed of 60 km h^-1 ?
Answer:
Distance covered by train = 200 km
Speed of train = 60 km h^-1

Question 16.
A boy travels with a speed of 10 m s^-1 for 30 minute. How much distance does he travel ?
Answer:
Speed of boy = 10 m s^-1
Time taken = 30 minutes
speed = distance travelled / time taken
Distance travelled = Speed × Time taken
Convert 30 minutes to seconds
1 minute = 60 sec
30 minute 60 × 30 = 1800 seconds
Putting the value of speed and time we get
Distance travelled = 10 ms^-1 × (1800 sec) = 18000 m
= 18000 metre or 18 km Ans.

Question 17.
Express 36 km h^-1 in m s^-1
Answer:

Question 18.
Express 15 m s^-1 in km h^-1.
Answer:

Selina Concise Physics Class 6 ICSE Solutions – Magnetism

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Physics for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 6 Magnetism

• Synposis
• The first natural magnet was discovered in Magnesia, a town in Greece. It was called the lodestone.
• Artificial magnets are made of iron or steel. They are made of different shapes namely the bar magnet, cylinderical magnet, U-shaped magnet, horse-shoe magnet, magnetic needle and compass.
• The materials which are attracted by a magnet are called magnetic materials. Examples: iron, steel, cobalt.
• The materials which are not attracted by a magnet are called non-magnetic materials. Examples: paper, wood, brass, plastic, copper aluminium, etc.
• A magnet has two poles, a north and a south pole.
• A magnet has the following properties:

1. A magnet attracts the small pieces of iron.
2. A magnet always rests in the north-south direction, if it is free to swing.
3. Like poles repel each other and unlike poles attract each other.
4. Poles always exist in pairs, cannot be isolated.

• Magnets are used to separate iron and steel from their mixture with non-magnetic substances. –
• Magnets are used in many electrical appliances such as electric . bell, loud-speaker, etc.
• A magnetic compass is used by sailors and navigators to find the north-south direction.
• Magnetic induction is the process in which a piece of iron temporarily behaves like a magnet in the presence of another magnet.
• When a magnet is placed near an iron piece, the iron piece behaves like a magnet. The end of the iron piece near the north pole of the magnet becomes a south pole while the farther end becomes a north pole.
• It is because of magnetic induction that a magnet attracts a piece of iron.
• An iron piece can be made into a magnet by any of the following methods:

1. Magnetic induction
2. Single touch method
3. Double touch method
4. Electrical method

• In the single touch method, we need a single magnet, but in the double touch method we need two magnets, hi these methods, the end touched last by the magnet has the polarity opposite to that of the striking pole.
• Powerful magnets are made by the electrical method.
• Electromagnets or temporary magnets are made of soft iron.
• Permanent magnets are made of steel.
• Electromagnets are used in devices like electric bell, magnetic toys, telephone etc.
• Permanent magnets are used in devices like galvanometer, ammeter, voltmeter etc.
• A magnet can be destroyed by rough handling, by dropping it several tunes, by hammering it repeatedly and by heating it.
• The magnetic field around a magnet is the space in which a magnetic substance such as small iron piece experiences a force of attraction.
• The earth itself behaves like a magnet. It has its own magnetic field.
• The south polarity of the earth is near the geographic north pole and the north polarity of the earth is near the geographic south pole.
• Magnetic keepers are used to store the magnets.
• Magnetic keepers are small pieces of soft iron.

ACTIVITY 1

Magnetic objects
Iron, Steel, Cobalt, Nickel
Non-magnetic objects
Wood, Stone Plastic, Rubber Copper, Sand, Gold, Silver, Brass Paper, Aluminium

Test yourself

A. Objective Questions

1. Write true or false for each statement.

(a) Artificial magnets are weaker than the natural magnets.
Answer. False
Artificial magnets are stronger than the natural magnets.

(b) Poles of a magnet cannot be separated.
Answer. True

(c) A magnet can attract only a magnetic substance.
Answer. True

(d) A magnet has no effect when it is heated to a high temperature.
Answer. False.
A magnet get demagnetised when it is heated to a very high temperature.

(e) Permanent magnets get easily demagnetised.
Answer. False.
Permanent magnets cannot be demagnetised.

(f) Magnetic poles occur in pairs.
Answer. True

(g) Single touch method is better than the electrical method for making a magnet.
Answer. False.
Electrical method is better than single touch method.

(h) Magnetic keeper is a wooden piece.
Answer. False.
Magnetic keepers are the pieces of soft iron.

(i) Copper cannot be magnetised.
Answer. True

2. Fill in the blanks

(a) Temporary magnets are usually made up of soft iron.
(b) Rough handling destroys the magnetic properties of a magnet.
(c) Like poles repel each other.
(d) A freely suspended magnet points in the north-south direction.
(e) In a magnet, ends have the maximum attractive property.
(f) A magnet has two poles.

3. Match the following

4. Select the correct answer

(a) If we suspend a magnet freely, it will settle in .

1. east-west direction
2. north-south direction
3. north-east direction
4. east-south direction

(b) Making a magnetic substance a magnet by bringing it closer to another magnet without touching it, is

1. magnetic induction method
2. single touch method
3. double touch method
4. electrical method

(c) An example of natural magnet is

1. iron
2. steel
3. lodestone
4. none of above

(d) The artificial magnet used to detect direction in the laboratory is

1. U-shaped magnet
2. horse shoe magnet
3. electromagnet
4. magnetic compass

B. Short/Long Answer Questions

Question 1.
What is a magnet ?
Answer:
The substances which have the property of attracting iron, are called magnets.

Question 2.
What are magnetic and non-magnetic substances ? Give two examples of each.
Answer:
Magnetic substances: The substances that get attracted by a magnet are called magnetic substances. Iron, steel, cobalt and nickel are magnetic substance
Non-magnetic substances : The substances that do not get attracted by a magnet are called non-magnetic substances, e.g., wood, plastic, copper, paper, aluminium, rubber, stone.

Question 3.
What are natural and artificial -magnets ?
Answer:
Natural magnets: Natural magnets are those which are found in nature e.g. load stone.
Artificial magnets: Man made magnets are called artificial magnets, e.g. electromagnet.

Question 4.
How is an artificial magnet prepared from a natural magnet ?
Answer:
Pieces of iron or other materials are made magnets by rubbing them with natural magnets (or by passing direct current through a wire wound around them). This is how artificial magnets are made.

Question 5.
State two ways of magnetising an iron piece.
Answer:
The two ways of magnetising an iron piece are:

1. Magnetic induction method.
   Take a long iron nail and test it for magnetic properties by bringing near the magnetic substances. You will see nail does not attract the magnetic substances. Now bring near a pole of a bar magnet to the head of the nail. Now bring the iron paper clips near the pointed end of the nail, you will observe that the iron paper clips now get attracted towards the nail. This is because iron nail has become magnet. Now take the bar magnet away form the iron nail, paper clips fall off. This magnetism is temporary.
   
2. Single touch method: Take a demagnetised piece of iron. Place it on a table surface. Take a magnet and select its one pole. Now mb it with the selected pole on the iron in one direction for several times. After sometime, the iron piece turns into a magnet.
   

Question 6.
How can magnetic properties of a magnet be destroyed ?
Answer:

1. By hammering the magnet repeatedly.
2. By rough handling
3. By heating.

Question 7.
Why docs a freely suspended magnet always rest in north- south direction ?
Answer:
A freely suspended magnet always rest in north-south direction because the north-pole of the magnet lies in the geographic north direction and the south pole of the magnet lies in the geographic south direction. So it aligns itself in N-S direction. As unlike poles attract and like poles repel.

Question 8.
Draw diagrams of the artificial magnets of four different shapes.
Answer:

Question 9.
Why are the artificial magnets preferred over the natural magnets ?
Answer:
Artificial magnets are preferred over natural magnets because natural magnets are weak and often irregular in shape, they can readily be magnetised and demagnetised by turning the current on or off in the coil.

Question 10.
Describe an experiment to show that the maximum attractive property is at the poles of a magnet.
Answer:
Take a bar magnet and place a steel pin at some distance. We observe that nothing happens. Now, bring the steel pin near the pole of the bar magnet. We notice that pin sticks to the magnet. This experiment shows that maximum magnetic force acts at the poles of the magnet.

Question 11.
State four important properties of a bar magnet.
Answer:

1. Attractive property: A magnet can attract small pieces of iron filing or other ferromagnetic materials.
2. Directive property: If a magnet is suspended horizontally by a thin thread (say silk thread), it rests always pointing north- south direction of earth.
3. Like poles always repel each other and unlike poles attract each other.
4. Poles always exist in pairs : Single pole can never exist.

Question 12.
Explain the attractive property of a magnet with the help of an experiment.
Answer:
Take iron filling on a piece of paper. Bring a bar magnet near it. Iron filling will cling to it. It shows the attractive property of magnet.

Question 13.
Describe the method by which an iron bar can be made a magnet.
Answer:
Single touch method : Place the iron bar (or the needle) AB on a table. Take a bar magnet NS and place it almost vertical with its north pole (N) on the end A of the bar.
Move the magnet along tire iron bar till the other end B is reached.

Lift the magnet at the end B and again place it on the first end A Again stroke the bar. Repeat the process about 20 times. Then turn the iron bar AB upside down. Again stroke it with the magnet about 20 times. The bar now becomes a magnet.

Question 14.
How are the magnets kept safely ? What is the role of keepers in storing the magnets ?
Answer:
When magnets are not in use they should be kept and stored in magnetic keepers. The magnetic keeper are the pieces of soft iron. A magnetic keeper has a card board with one or two iron soft pieces. Two magnets are placed in such a way that their opposite poles are close to each other and then a soft iron keeper is attached with it.

Question 15.
Define the term magnetic field of a magnet. How will you recognise it experimentally ?
Answer:
The space around a magnet in which if a magnetic substance such as small pieces of iron, are placed, they get attracted to-wards the magnet, is called the magnetic field.
Recognition of the magnetic field around a magnet: If a magnet is placed below a sheet of stiff paper and some iron filings are spread on it, then on tapping the sheet gently, the iron filings are found to arrange themselves in a definite pattern as shown in fig.

Question 16.
How will you make an iron bar electromagnet ? Draw a diagram showing the polarities of the electromagnet.
Answer:
Take the given iron bar AB. Wound several turns of insulated copper wire over the bar. Connect the ends of the wire to a battery through a switch. Press the switch to pass current. After some time, the bar AB becomes a magnet.

The end A of the bar at which the current enters the coil in clock¬wise direction becomes the south pole (S) and the end B of the bar at which the current leaves the coil in anti-clockwise direction becomes the north pole (N).

Question 17.
State two ways of increasing the strength of an electro Magnet
Answer:
Strength of the electromagnet: The strength of the electromagnet can be increased:

1. by increasing the current in the coil, and
2. by increasing the total number of turns of the coil.

Question 18.
Suppose you are given a long bar magnet and you are asked to break it into four small magnets. Draw diagrams showing the polarities of each broken part.
Answer:

Question 19.
State three important uses of a magnet.
Answer:
Use of magnet:

1. Magnets are used in magnetic compass, door bells, refrigerators.
2. Magnets are used in dynamos, motors, loudspeakers, microphones etc.
3. Ceramic magnets are used in computers.
4. Magnets are used in toys to give magic effect.

Question 20.
What is magnetic induction ? Explain with the help of a diagram.
Answer:
Magnetic Induction: The property by which an ordinary piece of iron acquires magnetic properties temporarily due to the pres¬ence of another magnet close to it, is known as magnetic induction
Take a long nail. Put it on the arm of a stand. Spread some iron pins on the base of stand. You will find that the pins do not get attracted towards the nail. Now touch a magnet at the end of nail. As the end is brought close to the head of nail, some pins cling to the nail. It happens because the nail turns into magnet and acquires the properties of magnetism. The moment you remove the magnet from the head of the nail. The pins will fall down. It will no more be a magnet.

Question 21.
In which direction does a suspended bar magnet come to rest? Give reason.
Answer:
A magnet always rests in North and South direction, i.e. N-end always towards North of Earth and S-end towards South of Earth.

Question 22.
State three differences between the temporary and permanent magnets.
Answer:
Temporary magnet

1. It is made up of soft iron.
2. The magnet which loses its magnetism as soon as magnetising force is removed away from it.
3. Because of its weak power, it is not used to make iron piece into magnet.

Permanent magnets

1. It is made up of steel, cobalt and nickel.
2. The magnet, which does not lose its magnetic properties easily is called permanent magnet.
3. It can convert ordinary piece of iron into a temporary magnet.

Question 23.
State three ways of demagnetising a magnet.
Answer:
A magnet can be demagnetized in the following ways 

1. rough handling
2. hammering the magnet several times.
3. passing an alternating current around the magnet.
4. dropping the magnet on the floor several times.
5. heating the magnet to a very high temperature.

Question 24.
Suggest one way to recognise the magnetic field of the earth.
Answer:
If we suspend a magnet such that it is free to swing, we see that it always rests in the north-south direction. The north pole of the magnet lies in the geographic north direction and the south pole of the magnet lies in the geographic south direction. So it aligns itself in N-S direction.

Question 25.
Name the material of core of an electromagnet for

1. temporary magnet
2. permanent magnet.

Answer:

1. They are made of soft iron.
2. They are made of iron, steel, cobalt, nickel or an alloy called ANILCO.

Question 26.
You are given an iron nail, a torch cell and a long piece of insulated copper wire. With the help of a labelled neat diagram, describe in steps how you will make the nail, an electromagnet.
Answer:
Aim : To make an electromagnet.
Materials Required : An iron nail, a battery, a switch, some insulated copper wire and some iron paper pins.
Procedure : Coil the insulated copper wire around the iron nail. Connect the ends of the wire to the battery through the switch. Close the switch so that electric current flows through the copper wire. Now bring the iron paper pins close to the iron nail.
Observation And Conclusion : The paper pins get attracted to the nail, showing that the nail has turned into an electromagnet. Now, if you switch off the current, the iron paper pins will drop off the iron nail.

Question 27.
Describe an experiment to illustrate that like poles repel while the unlike poles attract.
Answer:

1. Take two bar magnets A and B. Suspend one magnet A with a silk thread from a support so that it is free to swing. The magnet will come to rest in the north-south direction. The north pole of the magnet is in the north direction and the south pole of the magnet is in the south direction.
2. Now holding the other magnet B in your hand, bring its north pole near the north pole of the suspended magnet A (such that the two magnets do not touch each other) as shown in figure. You will observe that the suspended magnet A moves away from the magnet B. This shows that the like poles repel each other.
   

Now bring the south pole of the magnet B near the north pole of the suspended magnet A as shown in figure, without touching it. You will observe that the magnet A moves towards the magnet B. This shows that the unlike poles attract each other.

Question 28.
What are magnetic keepers ? Name its material.
Answer:
Magnetic keepers are used to store the magnets. Magnetic keepers are small pieces .of soft iron.

Question 29.
How are the north and south poles of a magnet located ? Explain.
Answer:
Suspend a bar magnet with a silk thread from a wooden stand as shown in figure. The magnet swings for some time and then eventually comes to rest in a particular direction i.e., north-south direction. If we disturb the magnet a little, the magnet again comes to rest in the north-south direction.

The end of the magnet which points towards the north is called the north seeking pole or simply the north pole and the end which points towards the south is called the south seeking pole or simply the south pole. The north and south poles are marked by the letters N and S respectively.

 

Selina Concise Physics Class 7 ICSE Solutions – Light Energy

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Physics Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 4 Light Energy

• Points to Remember
•  Light is a form of energy which helps us to see objects.
•  Light always travels in a straight line in the form of rays.
•  Light sources are either natural or artificial.
•  The sun and stars are natural sources of light.
•  A bulb, a candle etc. are artificial sources of light.
•  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, star.
•  The bodies which do not emit light are called non-luminous bodies, e.g. wood, brick etc.
•  Objects are of three types, transparent, translucent or opaque.
•  The pinhole camera is a simple application of the rectilinear propagation of light.
•  When an object blocks light, it casts a shadow.
•  Eclipses are formed due to formation of shadows.
•  Solar and lunar eclipses are the examples of formation of shadow in nature.
•  An eclipse is the partial or complete hiding of one heavenly body by shadow of another.
•  When light strikes a polished surface it comes back in the same medium, is called reflection of light.
•  A straight highly polished, smooth and reflecting surface is known as a plane mirror.
•  Mirrors are of two types (a) plane mirrors (b) spherical mirrors.
•  Smooth and polished surface like a mirror causes reflection and is called a regular reflection.
•  Rough or diffused surface causes an irregular reflection.
•  According to first law of reflection. The incident ray, the normal and the reflected ray all lie in the same plane.
•  According to the second law of reflection, the angle of incidence is always equal to the angle of reflection.
•  The phenomenon due to which the left side of an object appears to be the right side of the object and right side appears left. This is known as lateral inversion.
•  Image is of two types (a) Real image (b) Virtual image.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) The image formed by a plane mirror is real.
Answer. False.
The image formed by a plane mirror is virtual.

(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.
Answer. False.
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.

(c) The image of the right hand in a plane mirror looks like that of a left hand.
Answer. True.

(d) The image formed by a plane mirror is upright.
Answer. True.

(c) The image formed by a plane mirror can be obtained on a screen.
Answer. False.
The image formed by a plane mirror cannot be obtained on a screen.

(f) The objects are seen around us due to irregular reflection of light.
Answer. True.

(g) The speed of light in vacuum is 3 × 10⁸ ms^-1.
Answer. True.

(h) A rose appears red in light of all the colours.
Answer. False. A rose appears red in white light.

(i) A black paper absorbs light of all the colours and reflects none.
Answer. True.

(j) The primary colours are red, blue and green.
Answer. True.

2. Fill in the blanks

(a) Angle of incidence = angle of reflection
(b) The incident ray, the reflected ray and the normal lie in one plane
(c) The image formed by a plane mirror is at a distance behind the mirror as the object is in front of it.
(d) The image formed by a plane mirror is erect and virtual.
(e) We are able to see the objects around us due to irregular reflection.
(f) A virtual image cannot be obtained on a screen.
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of paint of lead oxide. .
(h) A plane mirror does not reflect 100 percent light falling on it.
(i) The colour of an opaque object is the colour of light which it reflects.
(j) Magenta, cyan and yellow are the secondary colours.

3. Match the following

4. Select the correct alternative

(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 metre from himself. The distance of man from the mirror is

1. 6 m
2. 3 m
3. 2 m
4. 12 m

(b) The angle between the incident ray and the ray reflected from the plane mirror is 70°. The angle of incidence will be :

1. 70°
2. 30°
3. 35°
4. 90°

(c) The image formed by a plane mirror is

1. virtual and inverted
2. virtual and of same size
3. real and inverted
4. real and of same size

(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be:

1. 30°
2. 60°
3. 15°
4. 0°

(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray and the reflected ray is

1. 30°
2. 15°
3. 60°
4. 90°

(f) The property due to which a light ray striking a surface is returned back into the same medium is called

1. refraction
2. reflex action
3. reflection
4. regression

(g) A ray of light after reflection from a mirror is known as

1. reflected ray
2. normal
3. incident ray
4. refracted ray

(h) The speed of light is maximum in

1. glass
2. water
3. air
4. wood

(i) A red rose is seen in green light. It will appear.

1. red
2. blue
3. yellow
4. black

(j) The primary colours are

1. Red, Blue and Yellow
2. Magenta, Yellow and Cyan
3. Red, Blue and Cyan
4. Blue, Green and Red

B. Short/Long Answer Questions

Question 1.
What do you mean by the term reflection of light ?
Answer:
Reflection of light— When light strikes a polished surface it comes back in the same medium, is called reflection of light.

Question 2.
How is a plane mirror made ?
Answer:
To make a plane mirror, a thin piece of glass is taken. One surface of the mirror is made opaque by silvering it. On the top of that, another thin coating of red lead oxide is given which protects the silvering of the mirror.

Question 3.
Explain the following terms:
Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal.
Answer:
Incident ray— The ray of light falling on the surface AB is called the incident ray. In figure PN is the incident ray.
Reflected ray— The incident ray bouncing back in the same medium after striking the reflecting ourface is called reflected ray. In figure NQ is the reflected ray.

Angle of incidence— The angle formed between the incident ray and the normal is the angle of incidence. (PNM is the angle of incidence.)
Angle of reflection— The angle formed between the normal and the reflected ray is called angle of reflection (MNQ is the angle of reflection)
Normal— It is the line drawn perpendicular to the reflecting surface at the point of incidence. MN is the normal.

Question 4.
Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i and the angle of reflection r.
Answer:
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

Question 5.
State the two laws of reflection of light.
Answer:
Laws of reflection—
(i) The incident ray, normal and the reflected ray all lie in the same plane.
(ii) The angle of incidence is equal to the angle of reflection.

Question 6.
Describe an experiment to verify the laws of reflection of light. Ans. Laws of reflection.
Answer:
(i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.
(ii) The angle of incidence and angle of reflection are equal i.e. ∠i = ∠r.
Verification
Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK\ Mark a point B on it. Draw a perpendicular BN. Place a mirror XX’ on line KK’ such that polished side of mirror is along the line. Hold-the mirror in the mirror holder.

Fix two steel pins P and Q on the straight line AB atleast 10 cm apart. Look for the images of the pins P and Q and fix two pins P Q’ such that P’, Q’ and images of P and Q are all in the same straight line. Remove the pins and draw small circles around the pin pricks.
Remove the mirror also. Join P’Q’ and produce the straight line to meet at B.
Measure ∠ABN = i and ∠CBN = r.
It is found that ∠i = ∠r. This proves that Angle of Incidence is equal to Angle of Reflection.
As the incident ray, the reflected ray and the normal lie in the plane of paper, therefore, they lie in the same plane.

Question 7.
A ray of light falls normally on a plane mirror. What is the angle of incidence ?
Answer:
Angle of incidence is 0°. Since angle of incidence is’the angle between incident ray and normal. Direction of reflected ray is along BA opposite to the direction of incident ray.

Question 8.
Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.
Answer:
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

Question 9.
The diagram in Fig. shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30°. Draw normal on the plane mirror at the point O and find :
Answer:
(i) the angle of incidence
(ii) the angle of reflection

ON is normal on the plane mirror at point O
ON is perpendicular on a plane mirror
Angle of incidence ∠i = ∠AON
and angle of reflection ∠r = ∠BON
Since, ∠i – ∠r
∠AOB = 30°
⇒ ∠AON + ∠BON = 30°
⇒∠i + ∠i – 30°
⇒ 2 ∠i =30°
⇒ ∠i = 30 / 2 = 15°
∴Angle of incidence = ∠i = 15°
and Angle of reflection ∠i = 15°

Question 10.
In the following diagrams, measure and write the angle of incidence and draw the reflected ray in each case.
Answer:

Question 11.
The diagram in fig. shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.

Answer:

Law of reflection of light is used to draw the direction of the reflected ray.
This law states that angle of incidence is equal to the angle of reflection.
∠i = ∠r

Question 12.
The following diagram shows an incident ray AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.

Answer:
ON is perpendicular on a plane mirror
Angle of incidence ∠i (∠AON) i.e.
Angle between incident ray and normal ray = 90° – 30° = 60°
Angle of Reflection = 60°
∴ ∠i = ∠r
∴ Angle between incident and reflected ray i.e. ∠AOB
= 60 + 60 = 120°

Question 13.
State in words, how do you find the location of image of an object formed by a plane mirror.
Answer:
The location of image of a point object is as far behind the mirror as the object is in front of it.

Question 14.
Draw a ray diagram showing the formation of image of a point object by a plane mirror.
Answer:

Question 15.
The following diagram shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of O is formed and seen by the eye.

Answer:

Question 16.
State four characteristics of the image formed by a plane mirror.
Answer:
(i) The image formed is erect.
(ii) The image is of the same size as that of the object.
(iii) The image is laterally inverted. Right side appears to be left and left side appeared to be right.

Question 17.
How is the position of image formed by a plane mirror related to the position of the object ?
Answer:
The image formed by a plane mirror is laterally inverted, upright, of the same size and is formed far behind the mirror as the object is in front of it.

Question 18.
You are standing at a distance 2 metre from a plane mirror.
(a) What is the distance of your image from the mirror ?
(b) What is the distance between you and your image ?
Answer:
(a) Distance of image from the mirror is also 2 metre.
(b) Distance between me and my image is 4 metre.

Question 19.
What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.
Answer:
Lateral Inversion : Inter change of sides between the object and its image is called Lateral Inversion.
Example :

It means image formed behind the mirror is as far behind the mirror as object is in front of it.
i.e. distance of M = dist of M in distance of O in front of mirror = distance of O image, and so on.

Question 20.
Wirte down the letter C and I as seen in a plane mirror.
Answer:

Question 21.
What is irregular reflection ? Give an example.
Answer:
Irregular reflection – When a beam of light falls on such a surface which is not perfectly smooth and polished such as wall, wood, paper etc. the different portions of the surface reflect light in different directions. Such a reflection of light in different directions. Such a reflection of light from an uneven surface is called the irregular or diffused reflection.

Question 22.
How do we see objects around us ?
Answer:
Objects are seen when light after striking them, returns in the same medium and reach our eyes.

Question 23.
State two uses of a plane mirror.
Answer:
It is used as :
(i) Looking glass
(ii) In periscopes.

Question 24.
Can light travel in vacuum ?
Answer:
Yes, light can travels in vacuum or air, a distance of nearly 299, 792, 458 metre (or nearly 3 x 108 metre) in one second. Thus, the speed of light in vacuum (or air) is 3 x 108 m s’1 nearly.

Question 25.
State the speed of light in (a) air, (b) glass.
Answer:
(a) Air — 3 × 10⁸
(b) Glass — 2 × 10⁸

Question 26.
State whether light slows down or speeds up in the following cases :
(a) Light going from air to glass.
(b) Light going from glass to water.
(c) Light going from water to air.
Answer:
(a) Slows down
(b) Speeds up
(c) Speeds up

Question 27.
What are the primary colours ? Name the three primary colours.
Answer:
Primary colours are the colours of light by mixing which white light is obtained. They are : (i) red (ii) green and (iii) blue.
Red + Green + Blue = White

Question 28.
What are the secondary colours ? Name the three secondary colours.
Answer:
Secondary colours are the colours of light which are obtained by mixing the two primary colours. They are (i) yellow, (ii) cyan, and (iii) magenta.

Question 29.
Fill in the blanks with the appropriate colour
(a) Blue + ………… = Cyan
(b) Red + Blue + …………. = Vhite
(c) Red + Blue = …………
(d) Green + Red = …………
Answer:
(a) Blue + Green = Cyan
(b) Red + Blue + Green = White
(e) Red + Blue = Magenta
(d) Green + Red = Yellow

Question 30.
The leaves appear green when seen in white light. Give a reason.
Answer:
Leaves appear green in white light because they reflect only the green light and absorb the light of all the other colours.

Question 31.
A rose appears red in white light. How will it appear in
(i) green light, (ii) red light ? Give a reason for your answer for each.
Answer:
(i) If a red rose is seen in green light, it appears black. The reason is that the rose absorbs the green light falling on it and reflect none.
(ii) If a red rose is seen in red light, it appears bright red. This is because the rose reflects the red light falling on it and absorbs none of it.

Question 32.
Why does a piece of paper appear white in sunlight ? How would you expect it to appear when viewed in red light?
Answer:
A piece of paper appears white in sunlight because it reflects light of all the colours. It would appear red when viewed in red light.

Question 33.
A piece of paper appears black in sunlight. What will be it£ colour when seen in red light ?
Answer:
A piece of paper appear black in sunlight. It would appear black when seen in red light because it absorbs light of all the colours.

Selina Concise Physics Class 7 ICSE Solutions – Electricity and Magnetism

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 7 Electricity and Magnetism

• Points to Remember
•  Some materials behave in a particular manner showing magnetic properties.
•  A freely suspended bar magnet always point in North-South direction.
•  Like poles repel each other while unlike poles attract each other.
•  Magnet is neutral at its centre and has maximum magnetic effects at its ends called the poles.
•  Poles always exist in pairs with opposite polarities and can never be isolated.
•  Examples of magnetic substances are:- iron, cobalt, nickel.
•  Examples of non-magnetic substances are:- wood, plastic, aluminium, copper.
•  The poles are not exactly the ends of the magnet but thev are slightly inside.
•  For a given magnet, both the poles are of equal strength.
•  Earth’s magnet has its south pole situated near the geographic north.
•  Electromagnets are the temporary magnets which are made up of soft iron core with a winding of insulated copper wire.
•  Freely suspended current carrying solenoid always rests in north- south direction.
•  A direct current source has the fixed positive and negative terminals, e.g. a battery.
•  Hans Christian Oersted discovered that if an electric current is passed through a conductor, a magnetic field is developed around it.
•  The study of magnetic effects produced due to electric current is known as electromagnetism.
•  The direction of magnetic field due to a straight current carrying conductor is obtained by any of the following rules:
  (a) Right Hand Thumb Rule.
  (b) Right Hand Cork Screw Rule.
•  The property due to which a changing magnetic field within a closed conducting coil induces electric current in the coil is called electromagnetic induction.
•  The current produced in a closed coil when magnetic lines of force rapidly change within it is called the induced current.
•  The symbol for alternating current is 0.
•  The strength of induced current can be increased by increasing the
  (a) the number of turns in the coil
  (b) strength of the magnet used
  (c) relative speed between the magnet and closed coil.
•  We cannot think of modem life without electricity. We light our homes and other places of work with electricity.
•  It is used to run electric fans, televisions, geyser, electric irons, room heaters, refrigerators, music system etc.
•  Cell is a primary source of electricity. A combination of two or more cells is a battery.
•  Some other sources of electricity are generator and solar cells.
•  The path along which an electric current flows is called a circuit.
•  Electricity has the following effects
  (a) Heating effect
  (b) Magnetic effect
  (c) Chemical effect
  (d) Mechanical effect
•  When an electric path is complete is called closed circuit and the path with a break is called open circuit.
•  The substances which allow the electricity to flow through them are called conductors, e.g. metals, human body etc.
•  The substances which do not allow electricity to flow through them are called insulators, e.g. wood, paper, glass etc.
•  The consumption of electricity is calculated from the meter in kWh.
•  Electric fuse is a device which limits the current in an electric circuit.
•  All electrical appliances are connected in parallel in household circuits.
•  We should be cautious in using electricity. Certain precautions should be taken before working on an electrical gadget or circuit.

Activity 6

List five such electrical gadgets in your house in which electromagnet is used.

1. ……………………
2. ……………………
3. ……………………
4. ……………………
5. ……………………

Answer.

1. Computer
2. Electric motor
3. Fan, Toaster
4. Refrigerator
5. Television
6. Electric Bell

Activity 9

Answer.

 

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) A current carrying coil when suspended freely can rest in any direction.
Answer. False.

(b) A coil carrying current behaves like a magnet.
Answer. True.

(c) In an electromagnet, the core is made up of copper.
Answer. False.

(d) An electric bell uses an electromagnet.
Answer. True.

(e) An electromagnet with soft iron core is a temporary magnet.
Answer. True.

(f) We use cell as the source of electricity to run an electric immersion rod.
Answer. False.

(g) A torch bulb glows if the terminals of the bulb are connected to the terminals of a cell by the metallic wire.
Answer. True.

(h) Wool is a conductor of electricity.
Answer. False.
Wool is a insulator of electricity.

(i) Silver is an insulator of electricity.
Answer. False.
Silver is good conductor of electricity.

(j) Our body is a conductor of electricity.
Answer. True.

(k) For a circuit to be complete, every part of it must be made up of conductors.
Answer. True.

(l) All metals are conductors of electricity.
Answer. True.

(m) The switch should not be touched with wet hands.
Answer. True.

(n) A switch is an on-off device in an electric circuit.
Answer. True.

2. Fill in the blanks

(a) A magnet has two poles.
(b) Like poles repel each other and unlike poles attract.
(c) An electromagnet is used to separate large mass of iron scrap.
(d) The strength of magnetic field of an electromagnet is increased by inserting a core of soft iron.
(e) In a torch we use dry cell as the source of electricity.
(f) To light a table lamp and to run a refrigerator, we use mains as the source of electricity.
(g) A group of two or more cells is called a battery.
(h) Conductors pass electricity through them.
(i) Insulators do not pass electricity through them.

3. Match the following

4. Select the correct alternative

(a) A freely suspended magnet rests in

1.  east-west direction
2.  north-south direction
3.  north-east direction
4.  north-west direction.

(b) Electromagnets are made up of

1.  steel
2.  copper
3.  brass
4.  soft iron.

(c) An electromagnet is used in

1.  electric oven
2.  ammeter
3.  electric bell
4.  radio set.

(d) The purpose of armature in an electric bell is

1.  to make and break the circuit
2.  to produce sound
3.  to produce magnetic field
4.  to provide spring action.

(e) In a torch, the source of electricity is

1.  the bulb
2.  the switch
3.  the cell
4.  the mains.

(f) Electricity can flow through

1. wood
2.  rubber
3.  plastic
4.  copper wire.

(g) Electricity does not flow through

1.  human body
2.  animals body
3.  rubber
4.  silver.

(h) We should not touch the switch with wet hands otherwise

1.  electricity may pass through our body
2.  electricity may not pass through the appliance
3.  circuit may break
4.  the switch may get off.

B. Short/Long Answer Questions

Question 1.
State two properties of a bar magnet.
Answer:
Properties of a bar magnet

1.  Attractive property: A magnet attracts small pieces of iron, cobalt or nickel.
2.  Directive property: A magnet when suspended freely, always point towards North and South direction.
3.  Like poles, i.e. North and North or South and South poles repel each other.
4.  Unlike poles i.e. North and South attract each other.
5.  Poles always exist in pairs, i.e. poles of a magnet cannot be separated.

Question 2.
How will you test whether a given rod is a magnet or not?
Answer:
Aim— To test whether a given rod is a magnet or not.
Apparatus —

1.  rod to be tested
2.  a bar magnet
3.  a stand
4.  a thread

Procedure —

1.  Suspend the rod to be tested with a thread on the stand.
2.  Bring the bar magnet near the rod with its north pole towards the rod’s end.
3.  Observe.

Inference — In every case, the magnet comes to rest in a north- south direction which shows its directive property.
Observation and Inference —
— If the rod is attracted towards the bar magnet, the rod may be magnet or not.
— But if the rod remains in the same direction of rest as in the start of the experiment, it show it is not a magnet.

Question 3.
How will you test whether a given rod is made of iron or not?
Answer:
Bring a magnet near the rod if the rod is attracted by the magnet then it will be made of iron else not.

Question 4.
You are given two similar bars. One is a magnet and the other is of soft iron. How will you distinguish and identify them ?
Answer:
Take first bar and suspend it in a stand with the help of a thread. So that it is free to rotate in horizontal plane. Note the direction in which it sets itself. If the direction is North and South it may be magnet. Again rotate it, if this time again it sets itself in north and south direction, it is a magnet, otherwise, it is iron.
Now repeat above experiment with second bar and in the same way find it if it sets always in north and south direction then it will be a magnet.

Question 5.
You are given a magnet. How will you use it to find north-south direction at a place?
Answer:
The earth is a huge magnet in itself with its North and South poles. The North and South of the place can easily be detected with the help of a bar magnet. A freely suspended bar magnet always rest in the geographic N-S direction. Since the South pole of the earth’s magnet is closer to the geographic North, the North pole of the suspended magnet will always rest in the geographic North, and the South pole of the freely suspended magnet will point towards geographic South.

Question 6.
Describe a simple experiment to illustrate that like poles of two magnets repel each other while the unlike poles attract.
Answer:
Like poles repel and unlike poles attract.
Two like poles (both North poles or both South poles) repel each other. Two unlike poles (one North pole and the other South pole) attract each other. This can be demonstrated by the following simple experiment.
Take two bar magnets A and B. Suspend one magnet A with a silk thread from a support so that it is free to swing. The magnet A will come to rest in the North-South direction. The North pole of the magnet A is in the North direction and its South pole is in the South direction. Now holding the other magnet B in your hand if you bring its North pole near the North pole of the suspended magnet A as shown in figure you will observe that the two poles repel each other. Care is taken that the two magnets do not touch each other.

Now if you bring the South pole of the magnet B near the north pole of the suspended magnet A as shown in figure without touching it, you will observe that the two poles attract each other.

The above experiment shows that the like poles repels each other while the unlike poles attract each other.

Question 7.
“Poles exist in pair”. Comment on this statement.
Answer:
The magnetic poles always exist in pairs. It is not possible to separate the two poles of a magnet.
If a bar magnet is broken at the middle in two parts each part is found to be a magnet. Each part has the property to attract the small iron pieces. Each part rests in the North-South direction when suspended such as to swing freely. This shows that the new poles are formed at the broken ends.
If these pieces are broken again and again, each part will still found to be a complete magnet. Each part contains both the poles (N-pole and S-pole). Thus, the two poles of a magnet exist simultaneously.

Question 8.
What is a magnetic compass ? State its use.
Answer:
Magnetic compass is a device which is used to locate the direction of a place. It always rests in a North-South direction. It is used in the navigators in ships, submarines, aeroplanes etc.

Question 9.
Explain the meaning of the term magnetic field.
Answer:
The space around the magnet where its influence can be experienced is known as magnetic field. This field is formed by the magnetic lines of force which run from the North pole to the South pole. These lines can be found to be maximum crowded at the two ends of the magnet which are the poles i.e. the North pole and the South pole.

Question 10.
What is an electromagnet ?
Answer:
An electromagnet — An electromagnet is a temporary magnet which behaves as a magnet when electric current is passed through the insulated copper wire and loses its magnetism when current is stopped. It has a soft iron piece called the core with an insulated copper wire wound on it.

Question 11.
Name the material of an electromagnet.
Answer:
Iron bar, insulated copper wire, battery.

Question 12.
Draw a labelled diagram to make a soft iron bar as an electromagnet. Describe in steps the procedure.
Answer:
Usually, the electromagnets are made in two shapes :
(1) bar or I shaped magnet and (2) horse shoe or U shaped magnet.

1.  To make a fiar or I shaped electromagnet: Take a soft iron bar PQ and wind a thin insulated copper wire around the bar. Connect a cell or a battery B, and a key K in series between the ends of the coil. The circuit diagram is shown in figure.
   
   When key K is closed, current passes through the winding of the coil and the bar becomes a magnet. As the key K is opened, the current stops flowing in the coil and the bar loses its magnetism. Thus, the bar behaves like an electromagnet.
2.  To make a horse shoe or U shaped electromagnet: Take a U shaped soft iron piece. Wind a thin insulated copper wire on its arms such that the winding in the two arms is in opposite direction. In figure winding in the arm A starts from the front and is in clockwise direction (when seen from the bottom).
   On reaching the upper end of the arm A, winding starts from the back at the top of the arm B and is in anticlockwise direction. Connect a battery B and a key K between the two ends of the wire.
   

Question 13.
You are given a U shaped soft iron piece, insulated copper wire and a battery. Draw a circuit diagram to make a horse shoe electromagnet.
Answer:
End A becomes S-pole and B becomes N-pole.

Question 14.
Name two factors on which the strength of magnetic field of an electromagnet depends.
Answer:
The magnetic field of an electromagnet (I or U-shaped) can be increased by the following two ways :

1.  By increasing the number of turns of winding in the solenoid.
2.  By increasing the current through the solenoid.

Question 15.
State two ways by which the strength of magnetic field of an electromagnet can be increased.
Answer:
The magnetic field of the electromagnet can be increase in the following two ways :

1.  By inserting a rod of soft iron or steel inside the cylindrical tube. This rod is called the core.
2.  By increasing the total number of turns of the coil.

Question 16.
State two common uses of electromagnets.
Answer:
Uses of electromagnet —

1.  In electrical appliances such as electric bell, fan etc.
2.  In lifting heavy loads of iron scrap.
3.  To remove tiny particles of iron from the wound.
4.  In loading furnaces with iron.
5.  In separation of magnetic substances from the non-magnetic substances.

Question 17.
Name a domestic device in which an electromagnet is used.
Answer:
Electromagnet is used in ELECTRICAL APPLIANCES like ELECTRIC BELL, RADIO, T.V., FAN and MOTORS etc.

Question 18.
Draw a neat and labelled diagram of an electric bell and describe its working.
Answer:
Working of the bell— Principle of working of electromagnetism When the switch is pushed on, the circuit gets completed and current stalls flowing through the U-shaped electromagnet which creates magnetic field in the core. This attracts the iron armature. Now when the armature moves towards the electro-magnet, the hammer strikes the gong and the bell rings. But as the armature
moves towards the electromagnet, the contact with the adjustment screw breaks which breaks the closed circuit and stops the current. Now when there is no current there is no electromagnetism and the armature returns to the original position. This making and breaking of the circuit of the electromagnet continues as long as the button remains pressed.

Question 19.
The incomplete diagram of an electric bell is given in fig. Complete the diagram and label its different parts.

Answer:

Question 20.
What is declination ? Draw a diagram to show the angle between the declination and true direction of geographic north.
Answer:

Magnetic declination is the angle of the horizontal plane between the magnetic North and the geographic North (or true North). This angle is shown in figure by symbol θ.
The angle of declination is different at different places on the earth surface and it also changes at a place with time. The declination is taken positive if the magnetic North is towards the east of the true North as in figure and is negative if the magnetic north if towards the west of the true North.

Question 21.
Define the term current.
Answer:
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire.
The S.I. unit of electric current is the ampere.

Question 22.
Name four appliances which work using electricity.
Answer:

1.  an electric iron
2.  an electric heater
3.  an electric kettle
4.  an immersion rod

Question 23.
Name two sources of electricity.
Answer:

1.  dry cell and battery
2.  generator and solar cell

Question 24.
What is a battery?
Answer:
If we use a group of two or more cells, it is called a battery. A battery is used where we require more electricity.

Question 25.
What is an electric circuit?
Answer:
For a smooth flow of electric current, a complete circuit is needed. This is known as electric circuit.

Question 26.
Describe an experiment to show that electricity flows only if the circuit is complete and it does not flow if the circuit is incomplete.
Answer:
Take two torch bulbs A and B. Connect them to a cell through a switch as shown in fig. The bulbs are said to be in series. Close the switch (i.e., the circuit it completed), you will see that both the bulbs glow.

Now take out the connection of the bulb B as shown in fig. Now close the switch, you will observe that the bulb A does not glow because the circuit is now incomplete.

Now replace the bulb B by a fused bulb fig. and close the switch. Again you will see that the bulb A does not glow. This is because the circuit being in series, is still incomplete.

Question 27.
You are provided with a torch bulb, a cell and two plastic coated f metal wires. Draw a diagram to show a complete circuit to light the bulb.
Answer:
Take two bulbs A and B. Connect them through switches S₁ and S₂ in parallel as shown in fig. Close both the switches. You will see that both the bulbs glow.

Question 28.
In which of the following case the bulb will glow :

1.  Only one terminal of a cell is joined with a metal wire to one terminal of the bulb.
2.  Both terminals of the bulb are joined with two metal wires to one terminal of the cell.
3.  One terminal of the cell is joined to one terminal of the bulb and other terminal of the cell to the other terminal of the bulb.
   Answer:
   The bulb will glow in (3) case i.e.

Question 29.
Distinguish between conductors and insulators of electricity. Give two examples of each.
Answer:
Conductors

1.  Conductors are those substances which allow electricity to flow through them.
2.  e.g. all metals, human body.
   Insulators
   Insulators are those which do not allow electricity to pass through them.
   e.g. wood, paper, glass.

Question 30.
Select conductors and insulators from the following :
Glass, silver, copper, wood, paper, pure water, impure water, aluminium, iron, leather, plastic, steel, human body and ebonite.
Answer:
Conductors — Silver, copper, impure water, aluminium, iron, steel, human body.
Insulators — Glass, wood, paper, pure water, leather, plastic and ebonite.

Question 31.
The following diagram shows four circuits A, B, C and D. Each circuit has a cell and a torch bulb. Name the circuits in which the bulb will glow ? Give a reason to your answer

Answer:
The bulb will glow in circuit (D).
This is because copper is the best conductor of electricity as compared to aluminium. Silk is a non-conductor of electricity.

Question 32.
The diagram given below shows a bulb connected with a cell having terminals A and B. Mark the direction of current in the bulb.

Answer:

Current always flows from +ve terminal to the -ve terminal of a cell.

Question 33.
State the function of each of the following in an electric circuit and draw its symbolic representation: (1) Switch and (2) Cell.
Answer:

1.  Switch – A switch or key is used to put the circuit on and off. fig. shows the symbol of a switch or key when it is open (to put the circuit off and when it is closed (to put the circuit on)
   
2.  Cell – A cell or a group of cells is generally used as a source of electricity. A positive (+) and a negative (-). It is represented by the two vertical lines of unequal lengths. The long vertical line represents the positive terminal and the short line represents the negative terminal as shown in fig.
   

Question 34.
Draw a circuit diagram for a bulb connected to a cell with a switch. Mark arrow in the diagram to indicate the direction of flow of current.
Answer:

Question 35.
In which arrangement are the appliances connected in the electric circuit of our homes, Series or Parallel ? Give one reason for your answer.
Answer:
Parallel circuit: When the circuit is in parallel, the appliances work independently. This is the reason that in our household wiring system, all the circuits are in parallel. Every appliance when put on, works on its own without the interruption of the other appliance.

Question 36.
State two precautions that you must take when switching on an electric circuit.
Answer:
Precautions to be taken before the circuit is switched on
Before the circuit is switched on, following precautions must be taken :

1.  See that all the components of the circuit are properly connected.
2.  See that the connection wire is tightly connected to each appliance or component.
3.  Do not touch the switch or any component with wet hands.

 

Selina Concise Physics Class 6 ICSE Solutions – Force

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 3 Force

• Synposis
• A body not changing position with time with respect to a nearby fixed object is said to be at rest.
• A body changing position with time with respect to a nearby fixed object is said to be in motion.
•  Force is a push or pull which can change the state of rest or motion of the body or can change the size and shape of the body (i.e. it can deform a body).
• A force applied on a body can
  (a) move it, if it is not in motion
  (b) stop it, if it is moving
  (c) increase or decrease its speed
  (d) change its direction of motion
  (e) change its shape or size if it is not free to move.
• Force is defined as that cause which changes the state of rest or the state of motion of a body and can also deform it.
• A force has both the magnitude and the direction.
• When two forces act in opposite directions, the net force is equal to the difference of these forces, in the direction of the bigger force.
• When two forces act on a body which are equal in magnitude but opposite in direction, the net force on the body is zero.
• Forces are of two types :
  1. contact forces and
  2. non-contact forces (or forces at a distance).
• Contact forces are
  1. the muscular force applied as push or pull,
  2. force of friction
  3. the force of reaction normal to the surface and
  4. the force of tension in a string pulled by a load.
• Non-contact forces are
  1. gravitational force
  2. electrostatic force and
  3. magnetic force
• The weight of a body is the force with which the earth pulls the body.
• The unit of weight (or force) is kgf not kg which is unit of mass.
• Friction is a force that opposes the motion.
• Friction always acts in a direction opposite to the direction of motion.
• The cause of friction is the interlocking of the irregular projections on the two surfaces in contact. ‘
• The force of friction depends on:
  (a) the smoothness (or roughness) of the surfaces in contact, and
  (b) the weight of the sliding (or rolling) body.
• The force of friction does not depend on the area of the surfaces in contact.
• The disadvantages of friction are :
  (a) Friction opposes the motion
  (b) Friction produces heat
  (c) Friction causes wear and tear
  (d) Friction reduces efficiency
• Friction can be reduced by
  (a) making the surfaces smooth
  (b) the use of lubricants
  (c) the use of ball bearings
  (d) streamlining the shape of the moving body.
• The maximum force exerted by a surface on a body so long as it remains stationary is called the force of static friction.
• The minimum force required to keep the body moving over a surface such that it moves equal distances in equal intervals of time is called the force of sliding friction.
• The minimum force required to roll a body on a surface is called the force of rolling friction.
• Rolling friction is less than the sliding friction and sliding friction is less than the static friction.
• Friction is advantageous to us in almost all activities of our life.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The frictional force acts in the direction of motion of body
Answer. False

(b) The unit of weight is kilogram
Answer. False

(c) A force can change the direction of motion of a moving body.
Answer. True

(d) A force increases the mass of the body when applied on it.
Answer. False

(e) The force of friction is always disadvantageous.
Answer. False

(f) The sliding friction is more than the rolling friction.
Answer. True

(g) Liquids offer more friction than the gases.
Answer. True

(h) A wet oily road offers more friction than a dry rough road.
Answer. False

2. Fill in the blanks

(a) Force is applied as push or pull.
(b) On squeezing a gum tube, its shape changes.
(c) On pulling a string, its length increases.
(d) A moving football when kicked, its direction of motion changes.
(e) On applying brakes on a moving car, its speed slows down.
(f) We use ball bearings to reduce the friction.
(g) Friction opposes the motion.
(h) Lubricants are used to reduce friction.
(i) Friction causes wear and tear of moving parts of machine.

3. Match the following columns

Answer.

4. Select the correct alternative 
(a) A body falls downwards because of

1. electrical force
2. gravitational force
3. mechanical force
4. magnetic force.

(b) A force does not change

1. mass
2. length
3. shape
4. state of motion.

(c) A force to be expressed correctly requires

1. only the magnitude
2. only the direction
3. both the magnitude and direction
4. none of the above.

(d) Friction

1. promotes motion
2. opposes motion
3. acts in the direction of motion
4. is always a nuisance.

(e) Friction is reduced by

1. making the surfaces wet
2. making the surfaces dry
3. making the surfaces rough
4. sprinkling sand on the surface.

(f) Friction

1. causes wear and tear
2. produces heat
3. stops a moving body
4. has all the above disadvantages

(g) Friction is increased if

1. an oil is sprayed
2. the surfaces are made wet
3. the surfaces are made dry
4. the surfaces are polished

B. Short/Long Answer Questions

Question 1.
Name the term used for the push or pull ?
Answer:
Force

Question 2.
Give one example each of a force as

1. a push
2. as pull
3. a stretch and
4. a squeeze.

Answer:

1. a push — To1 open a door, we push it.
2. as pull — To move a grass roller on a lawn, it is pulled by a gardener.
3. a stretch — Stretching a rubber string.
4. a squeeze —Change in shape of a sponge on squeezing.

Question 3.
Explain the meaning of the term force.
Answer:
Force: Force is a physical cause that changes or may tend to change the state of rest or the state of motion of an object. The S.I. unit of force is Newton.

Question 4.
What effect can a force have on a stationary body ?
Answer:
When a force is applied on a stationary body, it begins to move.

Question 5.
What effects can a force have on a moving body ?
Answer:
When a force is applied on a moving body, it can be made to stop or it can change the direction of motion.

Question 6.
What effect can a force produce on a body which is not allowed to move ?
Answer:
When a force is applied on a body which is not free to move, it gets deformed i. e., the shape or size of the body changes.

Question 7.
Give one example each to indicate that the application of a force

1. produces motion
2. stops motion
3. slows down motion
4. changes the direction of motion
5. deforms a body

Answer:

1. A car originally at rest when pushed, begins to move.
2. A moving bicycle is stopped by applying the brakes.
3. The speed of a moving vehicle is slowed down by applying the brakes.
4. A player kicks a moving football to change its direction of motion.
5. On stretching a rubber string, its length increases.

Question 8.
State the effect produced by a force in the following cases :
(a) The sling of a rubber catapult is stretched
(b) A man pushes a heavy cart
(c) A player uses his stick to deflect the ball .
(d) A cyclist applies brakes
(e) A spring is compressed.
Answer:
(a) The shape and size of catapult changes i.e., its length increases.
(b) The heavy cart begins to move.
(c) The direction of the ball changes.
(d) The speed of the moving cycle is slowed down.
(e) There is change in size and shape of spring.

Question 9.
Name the two kinds of forces in nature.
Answer:
Two kinds of forces in nature are :

1. Contact forces
2. Non contact forces

Question 10.
Name the type of force which acts in the following cases:
Answer:
(a) A coolie lifts a luggage
Answer. Muscular force
(b) A bicycle comes to rest slowly when the cyclist stops pedalling
Answer. Frictional force
(c) A stone falls from a roof
Answer. Gravitational force.
(d) A comb rubbed with silk attracts the bits of paper
Answer. Electrostatic force
(e) A string hangs with a load
Answer. Force of tension.
(f) A horse moves a cart
Answer. Muscular force
(g) A magnet attracts an iron pin
Answer. Magnetic force
(h) A boy opens the door
Answer. Muscular force
(i) An apple falls from a tree
Answer.Gravitational force
(j) A man rows a boat.
Answer. Muscular force.

Question 11.
What do you mean by the gravitational force ? Give an example to illustrate it.
Answer:
The force of attraction on a body by earth is called gravitational force.
Example : The leaves and fruits fall from a tree downwards towards the ground, water in a river flows down streams, a ball thrown up goes to a height and then returns back on ground are some examples of motion due to gravitational force.

Question 12.
Define the term “weight of a body”
Answer:
Weight: The weight of the body is the force with which the earth attracts it towards the centre. It depends on acceleration due to gravity.

Question 13.
What do you understand by the term friction ?
Answer:
Friction: Friction is that force which opposes the relative motion between the two surfaces that are in contact with each other.

Question 14.
Give an example to illustrate the existence of force of friction.
Answer:
If we stop paddling our bicycle, it gradually slows down and ultimately it stops after travelling a certain distance. This is due to frictional force between bicycle and ground.

Question 15.
What is the cause of friction ?
Answer:
The cause of friction is the interlocking of the irregular projections on the two surfaces in contact.

Question 16.
State two factors which directly affect the force of friction.
Answer:
Two factors which directly affect the force of friction are :

1. The smoothness of the surface.
2. The presence of solid, liquid or gas around the moving body.

Question 17.
In which case will there be more friction between the truck and the road : when the truck is empty or when it is loaded ?
Answer:
When the truck is loaded there will be more friction between the truck and the road.

Question 18.
Which offers more friction on a body : a glass surface or a wooden surface ?
Answer:
Wooden surface offers move friction on a body.

Question 19.
Name the three kinds of friction.
Answer:
Friction is of three kinds :

1. Static friction
2. Sliding friction,
3. Rolling friction

Question 20.
List three disadvantages of friction.
Answer:
Disadvantages of friction:

1. Friction produces heat which damages the moving parts of a machine.
2. Friction produces wear and tear on the contacting surfaces. This reduces the life of machine parts, tyres and shoe soles.
3. A lot of energy is wasted due to friction to overcome it before moving.

Question 21.
When you apply the brakes, the bicycle stops and the rim of the wheel becomes hot. Explain the reason.
Answer:
It is due to friction between the brakes and the rim of the wheel that it becomes hot.

Question 22.
The eraser gets smaller and smaller as you use it more and more. Explain the reason.
Answer:
The eraser gets smaller and smaller as we use it more and more due to frictional force causing wear and tear of the eraser.

Question 23.
List three ways of reducing friction.
Answer:
Ways to reduce friction:

1. Providing ball bearings or wheels between the moving parts of machine or vehicles reduce friction and allow smooth movement as rolling friction is less than sliding friction.
2. Oiling or lubricating (with graphite or grease) the moving parts of a machine reduces friction. Fine powder like talcum powder also works as a lubricant to reduce friction.
3. Polishing the rough surface reduces friction offered by it.
4. Streamlining (giving special shape to experience minimum drag) the bodies of aeroplanes, cars, boats and ships help reduce drag (fluid friction) while travelling through air or water.

Question 24.
It is difficult to open an inkpot with greasy or oily hands. Explain.
Answer:
When the hands are oily, then the oil acts as lubricant and reduces the friction.
As the friction force is less, it is difficult to get grip of the inkpot and it becomes difficult to open it.

Question 25.
It is difficult to walk on a wet road. Explain.
Answer:
When the road becomes wet after rain, friction is reduced and hence, the road becomes slippery.

Question 26.
Give three examples to illustrate that friction is a necessary evil.
Answer:
The examples to illustrate that friction is a necessary evil are:

1. If friction were absent, we would not be able to walk.
2. Friction is necessary to burn a matchstick.
3. It is due to friction that we can write on a board by a chalk.

Question 27.
Define

1. static friction
2. sliding friction and
3. rolling friction

Answer:

1. The maximum force exerted by a surface on a body so long as it remains stationary is called the force of static friction.
2. The minimum force required to keep the body moving over a surface such that it moves equal distances in equal intervals of time is called the force of sliding friction..
3. The minimum force required to roll a body on a surface is called the force of rolling friction.

Question 28.
Arrange the following in descending order :

1. static friction
2. sliding friction and
3. rolling friction?

Answer:
Static friction > Sliding friction > Rolling friction.

Question 29.
A body needs a force F₁ just to start motion on a surface, a force F₂ to continue its motion and a force F₃ to roll on the surface. What is

1. the static friction
2. sliding friction and
3. rolling friction ? State whether F₂ is equal, less than or greater than (1) F₁ and (2) F₃.

Answer:

1. F₁ = Static friction
2. F₂ = Sliding friction
3. F₃ = Rolling friction
   F₁ > F₂ > F₃
   F₂ is less than F₁ but greater than F₃.