Electricity and Magnetism: Selina Concise Physics Class 7 Exercise Questions

ELECTRICITY AND MAGNETISM: Selina Concise Physics Class 7

**No error in answers- unlike the other sites which are full of them. This article has been cross-checked and proofread by three Physics teachers simultaneously.

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A. Objective Questions

Question 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, 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.

(i) Our body is a conductor of electricity. 

Answer: True.

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

Question 2. Fill in the blanks(Answers in bold)

(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 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.

Question 3. Match the following

Answer:

(a) Human body - (iii) conductor

(b) Silk thread- (v) insulator

(c) Charge in motion-(ii) current

(d) Soft iron-(iv) electromagnet

(e) Electromagnet- (i) electric bell

Question 4.Select the correct alternative: (Answers in bold)

(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 Books.guru

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

i. Attractive property: A magnet attracts small pieces of iron, cobalt or nickel.

 ii. Directive property: A magnet when suspended freely, always point towards North and South direction.

 Like poles, i.e. North and North or South and South poles repel each other.

 Unlike poles i.e. North and South attract each other.

 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:

 Suspend the rod to be tested with a thread on the stand.

 Bring the bar magnet near the rod with its north pole towards the rod’s end.

 Observation— In every case, the magnet comes to rest in a north- south direction which shows its directive property.

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:

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:

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, you will observe that the two poles repel each other. Now if you bring the South pole of the magnet B near the north pole of the suspended magnet A 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 its 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 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 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 :

 By increasing the number of turns of winding in the solenoid.

 By increasing the current through the solenoid.

Question 12. 

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 13.

Describe the working of an electric bell.

Answer:

 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 14.

What is declination ? 

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 15.

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 16.

Name four appliances which work using electricity.

Answer:

 i. refrigerator

ii. an electric heater

iii. an electric toaster

iv. washing machine

Question 17.

Name two sources of electricity.

Answer:

 dry cell and battery

 generator and solar cell

Question 18.

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 19.

What is an electric circuit?

Answer:

An electric circuit is a closed path in which electrons move to produce electric currents. ... It is a very simple concept that incorporates three different components—a source of electrical energy, a device, and a closed loop of conducting material.

Question 20.

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. 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. 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 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 21.

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

Answer:

 Conductors are those substances which allow electricity to flow through them.

 e.g. all metals, human body.

Insulators are those which do not allow electricity to pass through them.

e.g. wood, paper, glass.

Question 22.

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 23.

State the function of each of the following in an electric circuit : (1) Switch and (2) Cell.

Answer:

 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)

 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.

Question 24.

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

Answer:

Before the circuit is switched on, following precautions must be taken :

i. See that all the components of the circuit are properly connected.

 ii. See that the connection wire is tightly connected to each appliance or component.

iii. Do not touch the switch or any component with wet hands.

Question 25.

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 26. 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: 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. Draw the symbolic representation: (1) Switch and (2) Cell.

1) switch

2) cell

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.