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).