Electricity and Magnetism

Objective Questions

Question 1

Write true or false for each statement:

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

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

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

(d) An electric bell uses an electromagnet.

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

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

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

(h) Wool is a conductor of electricity.

(i) Silver is an insulator of electricity.

(j) Our body is a conductor of electricity.

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

(l) All metals are conductors of electricity.

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

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

Answer

(a) False
Correct Statement — A current carrying coil when suspended freely rest in north-south direction.

(b) True

(c) False
Correct Statement — In an electromagnet, the core is made up of soft iron.

(d) True

(e) True

(f) False
Correct Statement — We do not use cell as the source of electricity to run an electric immersion rod.

(g) True

(h) False
Correct Statement — Wool is an insulator of electricity.

(i) False
Correct Statement — Silver is a conductor of electricity.

(j) True

(k) True

(l) True

(m) True

(n) True

Question 2

Fill in the blanks:

(a) A magnet has ............... poles.

(b) Like poles ............... each other and unlike poles ............... .

(c) An electromagnet is used to separate large mass of ............... scrap.

(d) The strength of magnetic field of an electromagnet is increased by inserting a core of ............... .

(e) In a torch, we use ............... as the source of electricity.

(f) To light a table lamp and to run a refrigerator, we use ............... as the source of electricity.

(g) A group of two or more cells is called a ............... .

(h) ............... pass electricity through them.

(i) ............... do not pass electricity through them.

Answer

(a) Two

(b) repel, attract

(c) iron

(d) soft iron

(e) dry cell

(f) mains

(g) battery

(h) Conductors

(i) Insulators

Question 3

Match the following:

Answer

Question 4a

A freely suspended magnet rests in:

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

Answer

north-south direction

Reason — The earth has a magnetic field of its own and its magnetic South pole is in geographic north and its North pole in geographic south. Since unlike poles attract so when a magnet is suspended freely, it rests in north-south direction.

Question 4b

Electromagnets are made up of:

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

Answer

soft iron

Reason — Electromagnets are temporary magnets made up of soft iron because they can easily be magnetized on passing electric current and gets de-magnetized when flow of electric current is stopped.

Question 4c

An electromagnet is used in:

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

Answer

electric bell

Reason — An electromagnet is used in electric bell as it is a temporary magnet.

Question 4d

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

Answer

to make and break the circuit

Reason — The movement of armature towards and away from the electromagnet in an electric bell makes and breaks the circuit.

Question 4e

In a torch, the source of electricity is:

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

Answer

the cell

Reason — Dry cell is used as a source of electricity in a torch.

Question 4f

Electricity can flow through:

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

Answer

copper wire

Reason — Copper is a good conductor of electricity.

Question 4g

Electricity does not flow through:

1. human body
2. animal's body
3. rubber
4. silver

Answer

rubber

Reason — Rubber is an insulator, it does not allow the flow of electricity through it.

Question 4h

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

Answer

electricity may pass through our body

Reason — We should not touch the switch with wet hands as tap water conducts electricity. Electricity will pass through our body and we can get electric shock.

Short/Long Answer Questions

Question 1

State two properties of a bar magnet.

Answer

The two properties of a bar magnet are:

1. It attracts small pieces of iron i.e. it has attractive property.
2. It always rests in the north-south direction when suspended freely i.e. it has directive property.

Question 2

How will you test whether a given rod is a magnet or not?

Answer

Suspend the given rod such that it swings freely and note the direction in which the rod rests. Repeat the process two to three times. If the rod rests in a particular direction pointing north and south in each case then the rod is a magnet. But if it rests in any random direction, then it is not a magnet.

Question 3

How will you test whether a given rod is made of iron or not?

Answer

1. Take the rod and place it on a table.
2. Take a magnet and touch the rod at one end, then at another end and then in middle. If the rod sticks to the magnet at both the ends and also at the middle, then the rod is made up of iron.

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

Let the two identical bars be bar A and bar B.

1. Take the bar A and place it on a table.
2. Take the bar B and touch it to one end of bar A, then at another end and then in middle. If the bar B sticks to the bar A at both the ends and also at the middle, then the bar A is made up of soft iron.
3. Repeat the above process by placing the bar B on the table and touch the bar A on one end, then on another end and finally on middle of bar B. We will find that the bar A will be attracted towards both the ends of bar B, but not to the middle which shows that bar B is a magnet.

Question 5

You are given a magnet. How will you use it to find north-south direction at a place?

Answer

A magnet when suspended freely, always rests in geographic north-south direction, so we can easily find the north south direction of the place by seeing the position of magnet.

Question 6

Describe a simple experiment to illustrate that like poles of two magnets repel each other while unlike poles attract.

Answer

1. Take two bar magnets A and B.
2. Suspend one magnet A with a silk thread from a support so that it is free to swing.
3. The magnet A will come to rest in the north-south direction. The north pole of the magnet A is in north direction and its south pole is in the south direction.
4. Now hold the other magnet B in your hand and bring its north pole near the north pole of the suspended magnet A without touching the magnet.

We will observe that two poles repel each other.

5. Now bring the south pole of the magnet B near the north pole of the suspended magnet A without touching it.

We will observe that the two poles attract each other.

The above experiment shows that like poles repel and unlike poles attract each other.

Question 7

"Poles exist in pairs". Comment on this statement.

Answer

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 small iron pieces. Each part when suspended freely, rests in north-south direction. This shows that new poles are formed at each broken parts. If these parts are broken again and again, still each part will be a complete magnet with north and south poles. This shows that poles exists in pairs.

Question 8

What is a magnetic compass? State its use.

Answer

A magnetic compass is a small magnetic needle pivoted at the centre of a small brass box which has a glass top. It is used to locate directions at a place.

Question 9

Explain the meaning of the term magnetic field.

Answer

The space around a magnet in which the magnetic needle of the magnetic compass gets influenced is called its magnetic field.

Question 10

What is an electromagnet?

Answer

An electromagnet is a temporary magnet made by winding wire around a soft iron core and on passing electric current it creates a magnetic field around itself and behaves like a magnet.

Question 11

Name the material of an electromagnet.

Answer

Soft iron

Question 12

Draw a labelled diagram to make a soft iron bar as an electromagnet. Describe in steps the procedure.

Answer

1. Take a soft iron bar PQ and wind a thin insulated copper wire around the bar.
2. Connect a cell or a battery and a key K in series between the ends of a coil as shown in the below figure.
3. When key is closed current passes through the winding of the coil and the bar becomes a magnet.
4. But when key is opened the current stops flowing in the coil and bar gets demagnetized.

So, soft iron bar behaves as an electromagnet when current passes through the coil.

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

Circuit diagram to make a horse shoe electromagnet is given below:

Question 14

Name two factors on which the strength of magnetic field of an electromagnet depends.

Answer

The two factors on which the strength of magnetic field of an electromagnet depends are:

1. Number of turns of the coil.
2. Strength of current passing through the coil.

Question 15

State two ways by which the strength of magnetic field of an electromagnet can be increased.

Answer

The two ways by which the strength of magnetic field of an electromagnet can be increased are:

1. By increasing the total number of turns of the coil.
2. By increasing the strength of current passing through the coil.

Question 16

State two common uses of electromagnets.

Answer

The two common uses of electromagnets are:

1. For loading furnaces with iron.
2. It is used in electrical gadgets like electric bell, electric tram, microphone, loudspeaker, electric motor etc.

Question 17

Name a domestic device in which an electromagnet is used.

Answer

Electric bell

Question 18

Draw a neat and labelled diagram of an electric bell and describe its working.

Answer

Labelled diagram of the electric bell is shown below:

Working of electric bell:

1. When the switch (bell push) is pressed, current flows through the coil of electromagnet.
2. The electromagnet gets magnetized and attracts the armature. As the armature moves the hammer attached to it moves and strikes the gong so the bell rings.
3. During this process the contact between the metallic springy strip and adjusting screw breaks so the circuit becomes incomplete.
4. The current stops flowing through the coil of electromagnet and it gets demagnetized.
5. As the electromagnet gets demagnetized, the armature comes back to its original position due to spring action of the metallic springy strip and comes in contact with the adjusting screw.
6. The circuit becomes complete, if the bell push is pressed again, current will flows through the coils of electromagnet and it gets magnetized.
7. It attracts the armature and hammer attached with it strikes the gong again and the bell rings.
8. This process of making and breaking the circuit continues and the bell rings as long as the bell push remains pressed.

Question 19

The incomplete diagram of an electric bell is given in figure below. Complete the diagram and label its different parts.

Answer

The complete and labelled diagram of the electric bell is shown below:

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).

Question 21

Define the term current.

Answer

Current is defined as the rate of flow of charge i.e., the amount of charge flowing in one second.

Question 22

Name four appliances which work using electricity.

Answer

The four appliances which work using electricity are:

1. Electric fan
2. Electric heater
3. Television
4. Electric iron

Question 23

Name two sources of electricity.

Answer

The two sources of electricity are:

1. Electric cell and battery
2. Generator

Question 24

What is a battery?

Answer

A battery is a group of two or more cells and it is used where we require more electricity.

Question 25

What is an electric circuit?

Answer

The path along which the electricity flows is called the 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

1. Take about 1 metre long insulated metal wire and cut it into two halves by a blade.
2. Remove the plastic covering at the ends of each wire.
3. Take a torch bulb and with help of cello tape fix one end of each wire to each terminal of the bulb.
4. Take a cell.
5. Fix the other ends of the wires to the terminals X and Y of the cell.

We will see the bulb will glow.

Electricity flows from terminal X of the cell to the bulb through wire 1, then through filament of the bulb to wire 2 then to terminal Y of the cell. As the circuit is complete or closed the electricity flows along the circuit and the bulb glows.

Now remove the wire 1 from the bulb connected to terminal X of the cell as shown below.

We will see bulb will not glow.

As wire 1 is removed so circuit is incomplete or open, so electricity does not flow, hence bulb will not glow.

The above experiment show that electricity flows only if the circuit is complete and it does not flow if the circuit is incomplete.

Question 27

You are provided with a torch bulb, a cell and two plastic coated metal wires. Draw a diagram to show a complete circuit to light the bulb.

Answer

Diagram showing a complete circuit to light the bulb is given below:

Question 28

In which of the following cases the bulb will glow:

(i) Only one terminal of a cell is joined with a metal wire to one terminal of the bulb.

(ii) Both terminals of the bulb are joined with two metal wires to one terminal of the cell.

(iii) 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 third case i.e. 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.
In this case the circuit is complete allowing the flow of electricity so bulb will glow.

Question 29

Distinguish between conductors and insulators of electricity. Give two examples of each.

Answer

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

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 reason to your answer.

Answer

The bulb will glow in circuit A as both the wires i.e. copper and aluminium are good conductors of electricity. But in other circuits B, C and D silk, cotton etc. are used which are insulators i.e. they do not allow the flow of electricity through them. So bulbs do not glow in circuits B, C and D.

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

The direction of current is shown in the below diagram:

Question 33

State the function of each of the following in an electric circuit and draw its symbolic representation :

(i) Switch and

(ii) Cell.

Answer

(i) Switch

Symbol:

Function:
A Switch is used to put the current on and off in a circuit.

(ii) Cell

Symbol:

Function:
A cell acts as a source of direct current for the circuit.

Question 34

Draw a circuit diagram for a bulb connected to a cell with a switch. Mark arrows in the diagram to indicate the direction of flow of current.

Answer

Circuit diagram for a bulb connected to a cell with a switch with direction of flow of current indicated by arrows is shown below:

Question 35

In which arrangements are the appliances connected in the electric circuit of our homes, Series or Parallel? Give one reason for your answer.

Answer

Appliances are connected in parallel arrangements in the electric circuit of our homes because of the following reasons:

1. Each appliance works independently.
2. If one appliance is out of order, other appliances are not affected.
3. Each appliance works on its own without interruption of other appliances.

Question 36

State two precautions that you must take when switching on an electric circuit.

Answer

The two precautions that must be taken when switching on an electric circuit are:

1. Do not touch the switch or any component with wet hands.
2. Connecting wires should not be naked, they should be properly insulated everywhere.