Basic Electricity Paper 2 , May/June 2014  
Questions:   1 2 3 4 5 6 7   Main
General Comments

Question 2
  1. (a)        State Lenz’s law.
    (b)        Fig. 1 shows a current-carrying conductor in a magnetic field.

    The conductor in Fig. 1 has a length of 40 cm and carries a current of 20 A in a magnetic field of flux density of 0.8 T. If the conductor moves at a velocity of 30 ms-1, calculate the:
    (i)         force acting on the conductor;
    (ii)        induced e.m.f.


The expected answers were:
(a)        Lenz’s  law states that the direction of an induced e.m.f. is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that e.m.f.                                                                                                                       
  (b)(i)  B = 0.8 T,  I = 20 A,  l = 40 cm = 0.40 m                                                      
                        Force F = BIlsin                                                                   
                       F = 0.8 x 20 x 0.4 x sin900                                                     
                        F = 6.4 N                                                                                            
                  (ii)  v = 30 m/s                                                                                                      
                        e.m.f., E = Blvsin                                                                                       
                        E  = 0.8 x 30 x 0.40 x sin900                                                              
                        E = 9.6 V

The question was on Magnetic Field and Electromagnetism. The question was unpopular with the candidates. Many candidates as reported could not state the law, some mixed up the expressions for computing force and induce emf while others did not convert the conductor length to metres before using it. The candidates’ performance was not encouraging.


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