You are provided with two standard resistors Rand S, a voltmeter, an accumulator, a rheostat and
other necessary apparatus.
(i)Set up a circuit as illustrated in the diagram above.
(ii) Connect the voltmeter across R.
(iii)Close the key K and vary the rheostat to set the voltmeter reading VI across R to 0.3 V.
(iv)Open the key and disconnect the voltmeter.
(v) Now connect the voltmeter across S and close the key.
(vi)Read and record the voltmeter reading V2•
(vii) Repeat the procedure for other values of VI = 0.6,0.9, 1.2 and 1.5 V by varying the
rheostat. In each case, read and record the corresponding value ofV2.
(viii) Tabulate your readings.
(ix)Plot a graph with VI on the vertical axis and V2 on the horizontal axis.
(x)Determine the slope s, of the graph.
(xi) What is the physical significance of the slope?
(xii)State tHe precautions taken to obtain accurate results.
(b) (i)State two reasons why the key in the circuit should be opened when readings
are not being taken.
(ii)Explain why a battery of eight dry Leclanche cells each of emf 1.5 V is not used in
place olu car battery of 12 V to start a car engine.
This was the most popular question and well attempted by majority of the candidates.
Majority of the respondent candidates were unable to explain the physical significance of the slope.
The part (b) was poorly attempted. Many candidates only state that the battery will not run down but
they are not conversant with the fact that the ohmic resistance of the circuit will increase which will
in turn increase the temperature of the circuit.
(i)Five values of VI read and recorded in volts to at least 1 decimal place.
(ii) Five values of V, read and recorded in volts to at least 1
decimal place and in trend.
[Trend: As V I increases, V 2 increases]
(iii)Composite table showing VI, and V2
(i) Axes correctly distinguished, 12 mark each
(ii)Reasonable scales, 12 mark each
(iii)Five points correctly plotted
(iv)Line of best fit
(i)Large right angled triangle
(ii)∆VI correctly determined
(iii)∆V 2 correctly determined
(iv) ∆VI/∆V2correctly evaluated
Based on teachers values of Rand S
Slope = R/S ± 10%
The physcial significance of the slope is the ratio S to R or R/S
Award 1 mark each for any two correct precautions
Tight connections/clean terminals was ensured
Key removed when readings were not being taken
Parallax error avoided in reading voltmeter
zero error noted and corrected for in the voltmeter
(Accept any other valid precautions).
(b )(i) The key in the circuit should be opened
so as not to drain the battery/cell.
so as not to increase the ohmic resistance/temperature of the circuit
(ii) The current produced by the dry Lec1anche cells is too low compared to
the current produced by an accumulator due to the high internal resistance of
the Leclanche-cell (or due to the low internal resistance of the accumulator).