Keeping things moving

Question 1

When is a magnetic field produced around a wire?

Answer 1

When a current flows through the wire.

Question 2

Where is the electromagnetic effect round a wire used?

Answer 2

1. On cranes for lifting iron and steel.
2. In circuit breakers.
3. In loudspeakers.
4. In electric bells.

Question 3

What is the motor effect?

Answer 3

The movement of a conducting wire in a magnetic field. It occurs when a wire carrying a current is placed in an external magnetic field. The magnetic field formed around the wire interacts with this permanent magnetic field causing the wire to experience a force that makes it move.

Question 4

When will a wire not experience a force against a magnetic field?

Answer 4

When it’s parallel to the magnetic field.

Question 5

How can the size of the force on a wire be increased?

Answer 5

1. By increasing the size of the current.

2. By increasing the strength of the magnetic field.

Question 6

How can the direction of the force on a wire be reversed?

Answer 6

1. By reversing the direction of flow of the current.

2. By reversing the direction of the magnetic field.

Question 7

How can the direction of the force between a wire and magnetic field be identified?

Answer 7

Using Fleming’s left hand rule:

1. The first finger points in the direction of the magnetic field.
2. The second finger points in the direction of the current.
3. The thumb points in the direction of the movement.

Question 8

What is electromagnetic induction?

Answer 8

The induced current in a wire as it moves through a magnetic field. It uses movement to produce a current.

Question 9

Why do generators use electromagnetic induction?

Answer 9

To produce electricity.

Question 10

What happens if a conducting wire, or coil of wires, is moved through or cuts through a magnetic field?

Answer 10

A potential difference is induced across the ends of the wire. If the coil of wire forms part of a complete circuit, an electrical current will be induced.

Question 11

What must there be in order for current to flow?

Answer 11

Movement.

Question 12

What is an induced current?

Answer 12

Electromagnetic induction when the coil of wire is stationary and the magnetic field is moved. They are used in a bicycle dynamo.

Question 13

How can potential difference be increased in an electromagnetic circuit?

Answer 13

1. By increasing the speed of the movement of the magnet or coils.
2. By increasing the strength of the magnetic field.
3. By increasing the number of turns on the coil.

Question 14

What is produced when a current flows through a conducting wire in a magnetic field?

Answer 14

A magnetic field is produced around the wire.

Question 15

Give five examples of where the motor effect has been applied.

Answer 15

1. Cranes for lifting iron.
2. Circuit breakers.
3. Electric bell.
4. Loudspeaker.
5. Electric relay.

Question 16

A loop of conducting wire carrying a current crosses an external magnetic field at 90^o. What effect is observed?

Answer 16

The wire experiences a force and moves.

Question 17

Briefly describe Fleming’s left hand rule.

Answer 17

Thumb points in the direction of movement, first finger points in the direction of the magnetic field, second finger points in the direction of the current.

Question 18

What does a transformer do?

Answer 18

It changes electrical energy from one potential difference to another potential difference.

Question 19

What are transformers made of?

Answer 19

Two coils called the primary and secondary coils, wrapped around a soft iron core.

Question 20

How do transformers work?

Answer 20

1. Insulated wires supply an alternating current in the primary coil.
2. This produces an alternating magnetic field in the iron core.
3. This magnetic field links with the secondary coil.
4. This induces an alternating potential difference across the secondary coil.
5. And so an alternating current in the secondary coil.

Question 21

How is the potential difference across the primary and secondary coils of a transformer related?

Answer 21

(Vp) / (Vs) = (np) / (ns)
Vp is the potential difference across the primary coil in volts, V.
Vs is the potential difference across the secondary coil in volts, V
np is the number of turns on the primary coil
ns is the number of turns on the secondary coil.

Question 22

What is a step-up transformer?

Answer 22

A transformer with more turns in the secondary coil than the primary coil. The potential difference across the secondary coil is greater than that across the primary coil.

Question 23

What is a step-down transformer?

Answer 23

A transformer with fewer turns in the secondary coil than the primary coil. The potential difference across the secondary coil is less than across the primary coil.

Question 24

Where are step-up and step-down transformers used and why?

Answer 24

In the National Grid to ensure the efficient transmission of electricity.

Question 25

What would be the case if transformers were assumed to be 100% efficient?

Answer 25

The electrical power output would equal the electrical power input.

Question 26

What equation can be used to show that if transformers were 100%efficient the electrical power output would equal the electrical power input?

Answer 26

Vp x Ip = Vs x Is
Vp is the potential difference across the primary coil in volts, V
Ip is the current in the primary coil in amperes, A
Vs is the potential difference across the secondary coil in volts, V
Is is the current in the secondary coil in amperes, A

Question 27

What are switch mode transformers?

Answer 27

Transformers that are much lighter and smaller than traditional transformers and operate at a much higher frequency.

Question 28

At what frequency do switch mode transformers operate?

Answer 28

Between 50 kHz and 200 kHz.

Question 29

What can switch mode transformers be used as?

Answer 29

Chargers for mobile phones, digital camera and computer laptops.

Question 30

Why do switch mode transformers use very little power?

Answer 30

Because no load is applied.

Question 31

What are the main components of a transformer?

Answer 31

1. Soft iron core,

2. Two coils.

Question 32

Do transformers use a direct current or an alternating current?

Answer 32

Alternating current.

Question 33

Give two advantages of switch mode transformers over traditional transformers.

Answer 33

1. They are lighter.

2. They are smaller.

Question 34

Over what frequency range do switch mode transformers work?

Answer 34

50 kHz to 200 kHz.

Question 35

Give three examples of the application of switch mode transformers.

Answer 35

1. Mobile phone chargers.
2. Digital camera chargers.
3. Laptop chargers.