Electromagnetic Induction

Question 1

What is electromagnetic induction

Answer 1

Electromagnetic induction is the process through which an induced e.m.f is produced in a conductor due to a changing magnetic field.

Question 2

Magnitude of induced e.m.f could be increased by increasing:

Answer 2

No of turns in the solenoid
Strength of magnet
Speed at which magnet moves with respect to solenoid.

Question 3

What is Faraday Law of electromagnetic induction

Answer 3

Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux in the circuit.

Question 4

What is Len’s Law

Answer 4

Len’s Law states that the direction of the induced e.m.f and hence the induced current in a closed circuit, is always such that the magnetic effect opposes the motion or change producing it.

Question 5

Explain the shape of the graph sketched in a)
Refer to pg 429
Part 1

Answer 5

• As the North Pole of magnet enters solenoid, magnetic influx in solenoid changes.
• By Faraday’s Law, the change in magnetic influx in the solenoid results in an induced e.m.f in the circuit. This e.m.f drives an induced current through the closed circuit.
• By Len’z law, the induced current creates an N pole at the right end of the solenoid to repel incoming N pole. Thus, the needle is deflected momentarily to 1 side.

Question 6

Part 2

Answer 6

• At the instant when the magnet travels past the mid-length point of the solenoid, there is no change in magnetic flux in the solenoid.
• There is no induced e.m.f and hence no induced current to cause the needle to be deflected

Question 7

Part 3

Answer 7

• As the S pole of the magnet exits the solenoid, there is again a change in magnetic flux in the solenoid
• By Faradays Law, this produces an induced e.m.f and hence an induced current
• By Len’s Law, the induced current creates a N pole at the left side of the solenoid to attract the S pole of magnet.
• This causes needle to be deflected momentarily to 1 side

Question 8

Simple a.c. Generator part 1

Answer 8

• A rectangular coil of wire ABCD is mounted on an axle.
• By rotating the handle, the coil rotates between the poles of the 2 permanent magnets.
• As the coil rotates in the magnetic field, the change in magnetic flux creates an induced e.m.f and hence an induced current.

Question 9

A.c generator part 2

Answer 9

• Slip rings are always in contact with the carbon brushes as the axle rotates. The slip rings Ensure that the induced current in the coil is transferred to the external circuit.
• The induced current powers the electrical load connected to the external circuit, lighting up the lamp.

Question 10

Output voltage against time graph for simple a.c. generator .
Part 1

Answer 10

1) When the plane of the coil is parallel to the magnetic field, the arms AD and BC cut across the magnetic field lines at the greatest rate. Since the rate of change in magnetic flux is maximum, the magnitude of induced e.m.f is maximum

Question 11

Output voltage against time graph for a simple a.c. Generator part 2

Answer 11

2) when the plane of the coil is perpendicular to the magnetic field, the arms AD and BC do not cut across the magnetic field lines. Since the rate of change of magnetic flux is 0, the magnitude of induced e.m.f is 0.

Question 12

Output voltage against time graph for simple a.c. generator

Part 3

Answer 12

3) As the coil rotates half a cycle, it is parallel to the magnetic field again. The magnitude of induced e.m.f. is maximum. Note that since arms AD and CB are moving in directions opposite to those in step 1, the direction of induced e.m.f is opposite to that in step 1

Question 13

Output voltage against time graph for simple a.c. Generator

Part 4 and 5

Answer 13

4) The arms AD and BC do not cut across the magnetic field lines. The magnitude of induced e.m.f is 0.
5) The coil has rotated 1 complete cycle. It is parallel to the magnetic field again. Hence, maximum induced e.m.f is produced

Question 14

Fleming left hand rule and right hand rule

Answer 14

Left hand rule used for d.c. motors

Right hand rule used for a.c. Generators

Question 15

We can increase the magnitude of the induced e.m.f of an a.c. Generator by:

Answer 15

• Increasing the number of turns in the coil
• Using stronger permanent magnets
• Increasing frequency of rotation of coil
• Winding the coil around a soft iron core to strengthen the magnetic flux linking the coil.

Question 16

Practical design of a.c. Generator

Part 1

Answer 16

1) A magnet is attached to an axle and a wheel. As the wheel rotates, the magnet rotates too.

Question 17

Practical design of a.c. Generator

Part 2

Answer 17

2) The magnetic field rotates with respect to a fixed coil. This changes the magnetic flux in the coil. An induced e.m.f is generated and hence an induced current.

Question 18

Practical design of a.c generators

Part 3

Answer 18

3 ) The induced current is channelled directly to the external circuit through the output terminals. Slip rings and carbon brushes are not needed.

Question 19

A fixed coil a.c. Generator is favoured for the following reasons:

Answer 19

• It does not require carbon brushes, which wear out easily and need to be replaced frequently.
• It is Less likely to break down from overheating. This is as it does not use slip rings and carbon brushes. An eroded connection between slip rings and carbon brushes has increased resistance, which can generate large quantities of heat.
• It is more compact

Question 20

What is a transformer?

Answer 20

A transformer is a device that can change high alternating voltage (at Low current) to a Low alternating voltage(at high current), or vice versa

Question 21

Transformers are used in:

Answer 21

• Electrical power transmission from power stations to households and industries
• Regulating voltages for the proper operation of electrical appliances

Question 22

Structure if transformer (Don’t really need to memorise except for important points)

Answer 22

• The primary coil and the secondary coil are wound around a laminated soft iron core.
• Each coil has a certain number of turns.
• The laminated soft iron core comprises thin sheets of soft iron. These sheets are insulated from one another by lacquer.
• Soft iron is used as it is easily magnetised and demagnetised. This ensures better magnetic flux between the 2 coils.
• The lamination reduces heat loss.

Question 23

How does a transformer work

Answer 23

1) The primary coil is connected to an alternating voltage- the input voltage Vp
2) A varying magnetic field is set up in the laminated soft iron core.
3) An e.m.f. is induced in the secondary coil. This voltage is called the output voltage. Since the circuit is closed, a current is also induced in the coil.

Question 24

Formula for transformer

Answer 24

Vs/Vp= Ns/Np
Where;
Vs= secondary(output voltage)
Vp= primary(input voltage)
Ns=no. of turns in secondary coil
Np= No. of turns in primary coil.

Question 25

What is a transformer?

Answer 25

A transformer is a device that can change high alternating voltage (at Low current) to a Low alternating voltage(at high current), or vice versa

Question 26

Transformers are used in:

Answer 26

- Electrical power transmission from power stations to households and industries - Regulating voltages for the proper operation of electrical appliances

Question 27

Structure if transformer (Don’t really need to memorise except for important points)

Answer 27

- The primary coil and the secondary coil are wound around a laminated soft iron core. - Each coil has a certain number of turns. - The laminated soft iron core comprises thin sheets of soft iron. These sheets are insulated from one another by lacquer. - Soft iron is used as it is easily magnetised and demagnetised. This ensures better magnetic flux between the 2 coils. - The lamination reduces heat loss.

Question 28

How does a transformer work

Answer 28

1) The primary coil is connected to an alternating voltage- the input voltage Vp 2) A varying magnetic field is set up in the laminated soft iron core. 3) An e.m.f. is induced in the secondary coil. This voltage is called the output voltage. Since the circuit is closed, a current is also induced in the coil.

Question 29

Formula for transformer

Answer 29

``` Vs/Vp= Ns/Np Where; Vs= secondary(output voltage) Vp= primary(input voltage) Ns=no. of turns in secondary coil Np= No. of turns in primary coil. ```

Question 30

Step-up and step-down transformer

Answer 30

In a step up transformer, the number of coils in the secondary coil is greater than that in the primary coil. This results in an output voltage that is higher than input voltage. In a step down transformer, the number of coils in the secondary coil is less than that in the primary coil. This results in an output voltage that is lower than the input voltage.

Question 31

Power transmission in a transformer.

Answer 31

Power in primary coil= power in secondary coil ``` VpIp=VsIs where; Vs= secondary(output) voltage Vp=primary(input) voltage Is=current in secondary coil Ip=current in primary coil ```

Question 32

Comparing step-up and step-down transformers

Answer 32

Step up transformer Ns>Np Vs>VP IsIp

Question 33

Efficiency formula

Answer 33

Efficiency= output power in secondary coil/input power in primary coil x100%

Question 34

Transformers and the transmission of electricity | Possible solutions to minimise loss of power due to joule heating:

Answer 34

- Use very thick cables so that resistance is Low. In this way, power lost as heat is reduced. However, thicker cables increase cable and construction costs. - Reduce magnitude of current flowing in the cables. This can be done with a step- up transformer. When the transmission voltage is stepped up, current in cables is stepped down.

Question 35

Power output and power loss

Answer 35

P out= VI | P loss=I square R

Question 36

The function of step down and step up transformers

Answer 36

As electricity is transmitted more efficiently at high voltages, electricity produced in power stations has its voltage stepped up by the step up transformers. The high voltage electricity is then transmitted to households and industries through transmission cables. Step down transformers then reduce the voltage to suitable values, so that households and industries can use electricity safely.