Physics Paper 3 May/June 2015

Question 3B




This was another popular question among the candidates and satisfactory readings were obtained.  The challenges encountered by the candidates were in the plotting of the graph and interpreting it because of the small values got from the evaluation of the reciprocal of I.  Precautions are well stated.

Part (b).  Most candidates were able to explain the electromotive force of a cell but could not explain how a galvanometer may be adapted to read as an ammeter satisfactorily.

Candidates were expected to:

Read and record value of Io  to at least 1 d.p in Ampere                     

-           Evaluate Value of Io-1  to at least 3 s.f.          
-           Read and record five values of I   to at least 1 d.p in amperes and in trend.


-  as R increases, I decreases             
-         Evaluate five  values of   to at least 3 s.f               
-           List R, I and  in composite table
-           Distinguish both axes
-           Plot graph using appropriate scales
-           Draw line of best fit
-           evaluate slope, intercept and k  =
-           state any two of the following precautions:                                                                                     

Key opened when readings are not being taken

Ensured clean / tight connections

Noted/adjusted/corrected zero error of ammeter

Avoided parallax error on ammeter

Repeated readings shown on the table

The expected answers for part (b) are:

b(i)      The electromotive force of a cell is the work done in driving a unit of electric charge round a complete circuit                                   


      The electromotive force of a cell is the terminal potential difference across a cell when it      does not supply electric current to an external resistor.                          


      The emf of a cell is the total energy per coulomb of charge obtained from a cell


                                                                               Correct diagram                          

                                                          G  =  Galvanometer , S = Shunt or low resistor wire