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Physics Paper 2, Nov/Dec. 2013  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 14

Question 14

  1. (i)      What is a ‘lightning conductor’?

(ii)     Why are lightning conductors used to protect tall buildings from the
          undesirable effect of thunderstorms?

  1.  Draw the lines of force associated with the following situations:


  1.   Two unlike point charges of equal magnitude;
  2.    A positive charge in isolation;
  3.   Two parallel plates carrying unlike charges of equal magnitude.

      (c)    Two equal point charges exert a force F on each other when they are 5.0 x 10-2m
              apart in a vacuum.  Determine the distance between the charges if the force is to
              be doubled. 


   This question was not popular among the candidates as it was attempted by few             candidates.  Performance was as follows
Part (a)(i):  Candidates were able to describe lightning conductor correctly.
           (ii)   Candidates were not able to answer this question correctly due to poor
                  knowledge of principle of action of lightning conductors.
Part(b):  Performance in this part was fair.  However, few candidates indicated the arrows             on the lines force wrongly.

  1.  Correctly drawn and performance was fair
  2.  Correctly drawn and performance was fair
  3.  Poorly drawn.  They did not mind the positive and negative charges performance was poor.

Part(c)   Most candidates could not calculate this because they did not find their forces              differently before calculating the distance  required performance was poor.
 The expected responses are:
(a)(i)   A lightning conductor is a long metal rod/ bar with a pointed end placed above a (tall) building and connected by a thick cable attached to a metal plate buried  in the ground.                                                                                                               
        (ii)    Positive charges are repelled to the charged cloud to neutralize the charges in it   However, if enough charges are not repelled to neutralize the charged cloud and a flash of lightning occurs, the current is conducted to the earth through the copper plate thereby protecting the building  







(c)                      F1  =                                                                                                          
                          F2  =                                                                                              
                              =       =  2                                                                                  
                            =    =                                                                   

                            =  3.54 x 10 -2 m
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