Physics Paper 2, May/June. 2014  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 Main

General Comments

Question 11

(a)     Define electromotive force.

(b)     State:

  1. the principle of operation of a potentiometer;
  2. two advantages that a potentiometer has over a voltmeter in measuring potential difference.

(c)     (i)       Sketch and label a diagram of a gold-leaf electroscope
        (ii)        Give one use of a gold-leaf electroscope

(d)    (i)       Explain the action of a magnetic relay:
        (ii)       List two factors which determine the magnitude of an induced emf in a coil.
       (iii)       A current of 5 A passes through a straight wire in a uniform magnetic field of flux density
                    2.0 x 10-3 T.  Calculate the force per unit length exerted on the wire when it is inclined at
                    30o to the field.


Part (a).    This question attracted many candidates and their performance was above average.
Part(b)      This was another popular question.  In the first part; performance was low while in the second
                 part performance was above average.
Part(c)      This was also popular among the candidates and the performance was above average.  However,
                 some diagrams are not workable.  Their diagrams either has the gold leaf or earthting omitted.
Part(d)    (i)   This was not a popular question among the candidates.  Candidates were evidently unfamiliar
                      with the device.  Teachers should teach electromagnetic induction fully.  Performance was
(ii)    The performance of candidates was above average in this part as majority of the responding    candidates were able to list correctly the two factors.

  1. Many candidates were unable to state the correct formula.  Hence substitution and arithmetic calculation was affected performance was low.


The expected answers are:
11.     (a)    Electromotive force emf
                   The emf of a cell is the energy needed to take a unit positive charge round a circuit in
                   which the cell is connected.                                                                       
                  The p.d across the terminals of the cell when the cell/battery/generator is on open                                      circuit                                                                                                           
                  The energy per unit charge transformed in the cell.                                   
         (b)(i)  Principle of operation of a Potentiometer

                   The p.d across the wire is (directly) proportional to the length of the wire                                                                                           OR
                    When a steady current flows through a uniform wire, equal
                    lengths of the wire will have equal potential difference               
                   (Accept V  L  where  V and L are correctly specified).
          (ii)     Advantages of Potentiometer over Voltmeter

                    e.g -  It is more accurate.
                             It is more sensitive
                             It eliminates error due to internal resistance of the cell                                                                                                                  
     (c)(i)             Labelled sketch of Gold Leaf Electroscope

       (ii)  Uses of Gold-leaf Electroscope                                                                     
              e.g  To:
  -  detect electrostatic  charge
                            -  detect the nature of electrostatic charge
                            -   determine the magnitude of the induced charges.


(d)(i)    Magnetic Relay

             The mode of action of magnetic relay relies on the principle of electromagnetic induction .
            A small current passing through one circuit controls another circuit containing devices which
            requires larger current 

  (ii)     Factors affecting induced emf in a coil

            e.g the  -  speed of rotation of the coil
                         -  magnetic field strength
                         -  cross-sectional area of the coil
                         -  number of turns in the coil

(iii)        F  =  IBLsin
              Force per unit length  =    =  IB sin                                                               
                                                             =  5 x 2.0 x 10-3x sin30°                                     

                                                             =  5.0 x 10-3 Nm-1             




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