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Applied Electricity Paper 2 , May/June 2009  
Questions:   1 2 3 4 5 6   Main
General Comments
Weakness/Remedies
Strength





















Question 3

(a)      State Faraday’s Law of electromagnetic induction.

(b)     A capacitor has seven parallel plates separated by sheets of mica, 0.5mm thick and a relative permittivity of 6.  The capacitor whose area of each plate is 500cm2 is connected to a 400v supply

Calculate the:
(i) capacitance of the capacitor;
(ii) charge in the capacitor.
                                              { ∑o = 8.85  x  10-12 F/m}

_____________________________________________________________________________________________________
Observation

The expected answers were:

(a)    Faraday’s Law of electromagnetic induction states that whenever there is a change in the magnetic flux linked with a conductor in a closed circuit, e.m.f. is induced in the conductor which causes a current to flow in it.
                                                          OR
(b)  The strength of the induced e.m.f. is directly proportional to the rate of change of magnetic flux.

(i)      Capacitance of the capacitor
                                   C  =  ∑o ∑r A/d (n-1)
                                   C  =  8.85  x  10-12 x (6 * 6 * 0.05) / 0.0005
                                   C  =  31.86nF
                    (ii)     Charge in the capacitor
                                   Q = CV
                                       = 31.86  x  10-9  x  400 ; Q  =  12.74μC   

This question was on electromagnetic field.  Many candidates attempted the part (a) of the question but avoided part (b).  However, the expression for calculating C for multiple plate capacitor was wrongly quoted by many candidates and consequently missed it.               

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