Electromagnetic Induction

Question 1

Electromagnetic induction

Answer 1

The phenomenon of inducing an e.m.f. in a circuit by changing the magnetic field.

Question 2

Faraday’s Law of Electromagnetic Induction:

Answer 2

The magnitude of the e.m.f. induced in a conductor is proportional to the rate of change of magnetic flux linkage with the conductor

Question 3

Lenz’s Law:

Answer 3

The direction of the induced e.m.f (and hence the direction of induced current in a closed circuit) is always such that
its magnetic effect opposes the motion or change causing it.

Question 4

Factors affecting the magnitude of the induced e.m.f.

Answer 4

1. the number of turns per unit length in the solenoid,
2. the strength of the magnetic field,
3. the speed at which the magnet is inserted into or withdrawn from the solenoid.

Question 5

No e.m.f. is induced if the conductor is:

Answer 5

at rest,
• moving parallel to the field (i.e. the conductor does not cut the magnetic field lines), or
• moving at constant speed in the field.

Question 6

Simple ac generator

Answer 6

• It transforms mechanical energy into electrical energy.
• The coil is turned and it rotates between the poles of a permanent magnet.
• As the coil rotates, the rate at which it cuts the magnetic field lines changes with time.
• This induces an alternating e.m.f. between the ends of the coil, which in turn drives an alternating
induced current in the external circuit where an electrical load (e.g. resistor or lamp) is connected.

Question 7

1. Slip rings

- in continuous contact with the carbon brushes

Answer 7

PURPOSE:

i. Ensures that the coil is not entangled (main reason)
ii. Ensures that the induced current is transferred to the external circuit
iii. Ensures that the induced current is alternating

Question 8

Fixed coil generator

Answer 8

e.m.f. is induced by rotating magnets around a stationary coil.

Question 9

Advantages of fixed coil generator

Answer 9

1. Carbon brushes wear and tear easily and need to be replaced frequently, increasing cost
2. Connection with slip rings becomes loose when the carbon brush is eroded. A loose connection in a circuit increases resistance at the connecting point, which causes unnecessary heating (the
   thermal energy generated can cause the a.c. generator to breakdown). Thus slip rings and carbon brushes are seldom used to carry large currents from a.c. generators.
3. The fixed coil generator is more compact and space-saving.

Question 10

Factors affecting the magnitude of the induced e.m.f. in an a.c. generator:

Answer 10

1. Doubling the number of turns of coil:
   → the output voltage is doubled
   (Note: the frequency and period remains the same, as the coil is still being rotated at the same rate.)
2. Doubling the frequency of rotation of coil:
   → the output voltage is doubled
   → the output frequency is doubled (as period T is halved)

Question 11

To increase the induced e.m.f. in an a.c. generator:

Answer 11

1. Increase the no. of turns per unit length on the coil
2. Increase the frequency of rotation of the coil
3. Use stronger permanent magnets
4. Wind the coil on a soft-iron core to concentrate the magnetic field lines through the coil, thus strengthening the magnetic field)

Question 12

Transformer

Answer 12

An electrical device that changes a high alternating voltage (at low current) to a low alternating voltage (at high current), and vice versa.

Question 13

Usage of transformers

Answer 13

electrical power transmission from power stations to households and factories
• regulating voltages for proper operation of electrical appliances

Question 14

How a transformer works

Answer 14

• It transfers electrical energy supplied from the primary coil to the secondary coil by electromagnetic induction.
• At the primary coil, the applied alternating voltage (and current) sets up a constantly changing magnetic field.
• This magnetic field is linked to the secondary coil through the soft iron core and an e.m.f. is induced in the secondary coil (according to Faraday’s Law of Electromagnetic Induction).
• For a transformer to work, an alternating voltage is needed. A constant voltage in the primary coil will not work as the magnetic field set up is constant.

Question 15

Laminated soft iron core

Answer 15

• thin sheets of soft iron insulated from each other
  • core: concentrates the magnetic field lines so that the magnetic field is stronger (to minimise ‘leakage’ of magnetic field lines to maximise efficiency of transformer)
  • soft iron: allows the magnetic field to change direction easily within the core
  • lamination: reduces heat loss due to eddy currents
  (undesirable currents that are induced in the soft iron core itself and can result in thermal energy loss)

Question 16

Step up/down transformer

Answer 16

Step-up transformer:
The e.m.f. (voltage) in the secondary coil is larger (more turns). Step-down transformer:
The e.m.f. (voltage) in the secondary coil is smaller (less turns).

• When the secondary coil has less turns than the primary coil, the constantly changing magnetic field due to the primary coil cuts through less turns in the secondary coil
• The rate of change of magnetic field lines linked to the secondary coil is less and e.m.f. induced is less (than e.m.f. in primary coil)

Vs/Vp=Ns/Np(=Ip/Is for ideal transformer)

Question 17

Causes of power loss in a non-ideal transformer:

Answer 17

Heat loss due to resistance of the coils

2. Leakage of magnetic field lines between the primary and secondary coils
3. Heat loss due to eddy currents induced in the iron core