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Physics Paper 2, Nov/Dec. 2009  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 14
Part II:      Candidates were required to answer any five questions from this part.
  1. State:
    1. three ways by which the e.m.f. of an a.c. generator can be increased;
    2. how an a.c. generator may be modified to produce direct current.
  1. (i)         Define the root mean square value of an alternating current.
    1. Explain the term resonant frequency as it relates to an

L-C-R series circuit.

  1. A particle of charge -6 μC released from rest in a uniform electric field E moves a distance of 5cm.  If its kinetic energy is 4 x 10-3 J, calculate the:
    1. work done by the electric field;
    2. potential at its initial position;
    3. magnitude of the electric field intensity.



Part (a) was fairly well attempted except few candidates that mistook slip rings for split rings in ways to modify a.c generator to produce d.c.
Part (b) was fairly well attempted.   However, some candidates merely define resonance rather than resonance frequency which is a different concept.

Part (c) was unpopular question.  Most of the responding candidates failed to evolve and sustain logical reasoning i.e. work done by the electric field is equal to the kinetic energy of the particle.  However, most candidates were using wrong equations such as

V  =       Q    ;      E  =    Q       ;    W = Q1 Q2 
          4πεor              4πεor2              4πεor

The expected answers are:
(a)  (i)  E.m.f. of an a.c. generator can be increased by:

  1. increasing the number of turns in the coil of the armature;
  2. using larger magnets/increasing strengths of the magnetic field;
  3. increasing the speed of rotation of the armature;
  4. the core should be made of soft iron.

Accept any other valid example.

  1. By replacing the slip rings with split rings (commutator)                                        


  (b)(i)   The root mean square value is the direct current that would dissipate the same
  amount of energy in a resistor as is dissipated by the actual alternating current.
The root mean square value is that steady current that dissipates the same energy or  quantity of heat in the same time in the same resistance.                                      




        (ii)    Resonance frequency fo is the frequency at which minimum impedance occurs.
                 Resonance frequency fo is the frequency at which XL  =  Xc .

Resonance frequency fo is the frequency when maximum current is obtained from such a circuit.

(c)(i)    Work done = kinetic energy  =  4.0 x 10-3 J                                                               

     (ii)   W  = QV                                                                                                        

             V  =  W/Q  = 4 x 10-3
                               6  x 10-6                                                                                               

             V  =  0.667 X x 10-3   = 667V                                                                                               

   (iii)    E  = V/d                                                                                                                       

                =   667   

                =  13333.4 NC-1                       
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