### Question 4

- State the following laws of electromagnetic induction:

(i) Faraday’s law;

(ii) Lenz’s law.

- Trace the magnetic lines of force for the current carrying conductor in Figure 2 below:

- Calculate the energy stored in the magnetic field of a coil of inductance 10mH carrying a current of 2mA.

### Observation

The expected answers were:

(a) (i) Faraday’s law: States that when there is relative motion between a

conductor and a magnetic field, e.m.f. is induced.

The magnitude of induced e.m.f. in the conductor is proportional to the

rate of change of flux linkage.

(ii) Lenz’s law: States that the direction of the induced e.m.f. is such that it opposes the change inducing it.

(b)

(c) E = ½ LI2

= 10 x 10-3 x 4/2 x 10-6

= 20 x 10-9 J

= 20nJ

Faraday’s law consists of two statements: when there is relative motion between the magnetic flux linking a conductor, an e.m.f is induced across the terminals of the conductor; and the magnitude of the induced e.m.f is directly proportional (not equal) to the rate of change of flux linkage. (Some candidates surprisingly stated the Faraday’s Law of electrolysis).