Selina Class 6 ICSE Solutions

Selina Concise Physics Class 6 ICSE Solutions – Simple Machines

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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 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.