Magnetism and Electromagnetism

Question 1

Differences between magnetic and non-magnetic materials

Answer 1

Magnetic materials are attracted to a magnet while non-magnetic materials are not attracted by a magnet

Question 2

Define a magnet

Answer 2

A material which attracts magnetic materials

Question 3

What are non-magnetic materials?

Answer 3

They are substances which are not attracted by a magnet

Question 4

What are the types of magnets?

Answer 4

Natural/Artificial
Permanent/Temperary

Question 5

Why are artificial magnets used more than natural magnets?

Answer 5

Artificial magnets are used because their strength may be increased or decreased whereas strength of natural magnet decreases with time

Question 6

Where are the poles of a magnet situated?

Answer 6

The poles are at the end of a magnet where magnetism is concentrated

Question 7

What are the laws of magnetism?

Answer 7

1) Like poles repel each other
2) Unlike poles attract each other

Question 8

What is the magnetic force?

Answer 8

It is the force between 2 magnets and it acts along the line joining their 2 poles

Question 9

Describe magnetic induction and its effect

Answer 9

It is an effect that takes place when a bar magnet is brought near a magnetic material, magnetic poles will be induced at the ends of the magnetic material. There is an attraction between the magnet and the magnetic material.

Question 10

What are the methods to magnetise iron/steel? (Produce artificial magnets) (3)

Answer 10

1) Single touch method
2) Divided touch method
3) Electrical method

Question 11

Explain the principle of single touch method and its rule

Answer 11

The iron rod is stroked with a pole of a bar magnet end to end in a single direction about 100 times it will be weakly magnetised. The end of the iron rod where the stroke finishes acquires a polarity opposite to the stroking pole of the bar magnet

Question 12

Explain the divided touch method

Answer 12

2 opposite poles of a magnet are places in the middle of the iron rod and they are moved outwards in opposite directions while stroking the iron rods. The iron rods will be weakly magnetised after about 100 times

Question 13

Why is the electrical method preferred when magnetising iron/steel?

Answer 13

It is quicker and the metal is magnetised more strongly

Question 14

Describe the electrical method to magnetise a metal

Answer 14

The solenoid is connected to a battery via a switch. The iron rod is placed inside the solenoid. A suitable current is allowed to flow and after a few minutes the iron rod will be magnetised.

Question 15

How can we increase the strength of the magnetised iron rod when using the electrical method?

Answer 15

By increasing the current in the circuit and increasing the number of turns of the solenoid

Question 16

How to determine the induced polarity of the magnetised rod (electrical method)?

Answer 16

The direction of the current in the solenoid is viewed at the ends along its axis. Clockwise - south. Anticlockwise - north

Question 17

What happens if the iron rod is replaced by a copper rod? Why?

Answer 17

Copper will not get magnetised because it is a non-magnetic material

Question 18

What are the methods to demagnetise a magnet?

Answer 18

1) By heating it strongly, it will lose its magnetism partially
2) Electrical method

Question 19

Describe the electrical method to demagnetise a magnet

Answer 19

The solenoid with the bar magnet inside it is connected to an alternating power supply via a switch. After the alternating current flows for 10-15 mins, the bar magnet is removed from the solenoid along the east-west direction.

Question 20

What is a magnetic field?

Answer 20

It is the space surrounding a magnet in which a magnetic force is experienced. There are invisible lines of force in that region which emerge from a north pole and point towards a south pole.

Question 21

Describe the plotting of magnetic field lines using a compass (6)

Answer 21

1) A bar magnet is placed in the middle of a piece of paper.
2) The plotting compass is placed near one end of the bar magnet. The position of the magnetic needle is marked by 2 pencil dots A and B.
3) The plotting compass is displaced such that the south end of the needle lies of dot B.
4)The new position of the north end is marked by dot C.
5) By repeating these steps a series of dots are produced which when joined using a pencil will represent a magnetic line of force.
6) The experiment is carried out at other places near the bar magnet and other magnetic lines of force are drawn

Question 22

What is the direction of the magnetic field?

Answer 22

The direction in which the north end of the needle points when placed at any point

Question 23

Properties of iron (electromagnet) (3)

Answer 23

1) It gets magnetised very easily and rapidly
2) It gets demagnetised easily (as soon as current in solenoid is switched off)
3) It is a temporary magnet

Question 24

Properties of steel (permanent magnets) (3)

Answer 24

1) It takes a very long time to be magnetised
2) It does not lose its magnetism easily
3) Permanent magnet

Question 25

Use of steel

Answer 25

In a compass the needle is made of steel

Question 26

Uses of electromagnet (

Answer 26

1) electric bell 2) magnetic relay 3) circuit breaker 4) magnetic speaker

Question 27

What is magnetic shielding and how is it done?

Answer 27

It is used to protect magnetic materials from unwanted external magnetic fields. The magnetic material is placed inside an iron ring because that space is free from magnetic lines of force

Question 28

What happens when a current flows in an electrical conductor?

Answer 28

A magnetic field will be produced in its surroundings

Question 29

How can the strength of the magnetic field be altered?

Answer 29

By varying the magnitude of the current

Question 30

How can we determine the direction of the magnetic field?

Answer 30

Using right hand grip rule. Thumb is direction of current and the direction of the finger curl is direction of magnetic field

Question 31

What is the effect on the magnetic field after changing the magnitude and direction of current?

Answer 31

If a higher current is used there will be a greater magnetic field produced and when current is reversed the direction of the field reverses

Question 32

What does a magnetic relay consist of?

Answer 32

2 separate circuits which are linked magnetically

Question 33

What is the principle of a magnetic relay? (4)

Answer 33

1) When the switch is closed, a current flows in circuit A and the soft iron rod becomes magnetised. 2) The shorter end of the L-shaped soft-iron rod is attracted to the soft iron rod and rotates clockwise about the pivot. 3) The longer end of the L-shape soft iron rod compresses the springy contacts together. Circuit B will be activated. 4) When the switch in circuit A is opened the electromagnet is demagnetised and the L-shaped rod returns to its position. A gab will be left between the two springy contact and therefore circuit B will be deactivated

Question 34

Uses of a magnetic relay

Answer 34

In any appliance which needs the flow of a large current. Reduces the risk of coming in contact with high voltage and hence electrocution

Question 35

What is a circuit breaker?

Answer 35

A device used in an electric circuit to protect the electric appliance from the flow of a large current

Question 36

What is the principle of a circuit breaker? (3)

Answer 36

1) When the normal current flows in a circuit the electro magnet is not strong enough to attract the iron lever towards it. 2) In case a large current is developed (short circuit) the magnetic field in the soft iron increases and the electromagnet is strong enough to pull the iron lever towards it. The spring pulls the spring metal away from the contacts and contact is broken. 3) Current is cut off and the soft iron is demagnetised, the iron lever drops down due to its weight and tension in the spring decreases. The contact will remain broken since the springy metal is still in an inclined position

Question 37

What are the advantages of a circuit breaker over a fuse? (2)

Answer 37

1) It operates more quickly 2) The fuse needs to be replaced whereas the circuit breaker can be re-operated by pressing the reset button

Question 38

What is the principle of a loudspeaker? (5)

Answer 38

1) The permanent magnet of a loudspeaker is a ring magnet consisting of a ring north pole and a central south pole. This provides a strong radial field inside its air gap. 2) The coil is wound round a thin hollow former attached to a paper cone. The turns of the coil are situated inside the air gap of the ring magnet and perpendicular to the radial field. 3) When an audio frequency alternating current passes through the coil of the loudspeaker, the coil experiences a force. When direction of current changes the direction of he force is reversed therefore the movement of the coil is reversed. 4) The a.c. current causes the coil and the cone to move in and out of the magnet at the same frequency of current. 5) The vibrations of the paper cone moves a large volume of air and therefore a loud sound having the same frequency as its original sound is produced.