Basic Electronics Paper 2, May/June 2015

Question 4

  1. State the following laws of electromagnetic induction:

(i)   Faraday’s law;
(ii)  Lenz’s law.

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


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




                       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.     
      (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).

No candidate gave the correct response to question 4(b).