Physics Paper 3, May/June 2014
 Questions: 1 2 3 Main
Weakness/Remedies
Strength

Question3

(a)

You have been provided with a battery E, a standard resistor Rx, two resistance boxes RB1 and RB2, two keys, K1 and K2, a voltmeter and other necessary materials.

1. Measure and record the emf of the battery.
2. Connect a circuit as shown in the diagram above.
3. Set the resistance R, in the resistance boxes such that R in RB1 = R in RB2 = 2Ω
4. With K1 open and K2 closed, measure and record the potential difference Vo across the standard resistor Rx.
5. Close K1 and K2.  Read and record the potential difference V1 and Rx.
6. Evaluate V1-1.
7. Repeat the procedure (v) for four other values of R = 4 Ω, 6 Ω , 8 Ω and 10 Ω respectively.  In each case, ensure that the value of R in RB1 is equal to the value of R in RB2.
8. Evaluate V1-1 in each case.
10. Plot a graph with V1-1 on the vertical axis and R on the horizontal axis starting both axes from the origin (0, 0).
11. Determine the slopes, s of the graph and the intercept, c on the vertical axis.
12. Evaluate y = .
13. State two precautions taken to obtain accurate results.

(b)        (i)        Explain what is meant by the potential difference between two points in an electric circuit.

(ii)        A cell has an emf of 3V.  When it is connected across a resistor of resistance 4 Ω, a current of 0.5.  A passes through the circuit. Calculate the internal resistance of the cell.

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Observation

Part (a):  This was not a popular question among the candidates probably due to the fact that most schools could not afford to two sets of resistance boxes for each set-up.  However, the few responding candidates performed averagely.  The determination of  y =   was not correctly evaluated.

Part (b):  Many candidates gave good responses.

In part (a), candidates are expected to:

• Measure and record Emf of battery to at least 1 d.p in volts.
• Measure and record Vo to at least 1 d.p in volts.
• Record five values of R in ohms.
• Read and record five values of V1 to at least 1 d.p in volts and in trend.

(Trend: As R increases, V1 decreases)

• Show composite table containing R, V1 and V1-1
• Plot a graph using reasonable scales
• Draw line of best fit
• Determine the slope of the graph
• Evaluate   y =

State any two of the following precautions:

e.g  -    Key removed when not taking readings
-    Ensured tight/clean terminal
-    Avoided parallax error in reading voltmeter
-    Noted and corrected/avoided zero error on the voltmeter
-    Repeated readings shown on table.

In part (b), the expected answers are:

(i)     The potential difference (in volts) between two points in a circuit is the
work done (in joules) when one unit of positive charge (coulomb) moves from
one point to the other

E  =  IR  + Ir

3   =   0.5 x 4 + 0.5r
3 = 2 + 0.5r
r  =  3 - 2
0.5
=   2Ω