Alternative A
Question 3A
You are provided with a resistance box R, voltmeter, key,cell of e.m.f. E, constantan wire, standard resistor, Rx, an ammeter and other necessary apparatus.
- Measure and record the e.m.f. E of the cell provided.
- Set up the circuit as shown in the diagram above.
- Set R to 1Ω. Close the key, read and record the current I and the corresponding voltage V.
- Repeat the procedure for four other values of R = 2 Ω,4 Ω, 6Ω and 8 Ω.
- In each case, read and record I and V.
- Tabulate the readings.
- Plot a graph of V on the vertical axis and I on the horizontal axis.
- Determine the slope, s, of the graph.
- State two precautions taken to ensure accurate results.
Observation
Part (a) Performance was fair. This was another popular question and it was attempted by majority of the candidate. However, few of the candidates could not chose convenient scales because of the decimal numbers involved.
The expected response:
(i) Value of E correctly measured and recorded to at least 1 d.p. in volts
(ii) Five values of R correctly recorded in ohms.
(Deduct ½ mark for each wrong or missing value)
(iii) Five values of I correctly recorded to at least 1 d.p. in amperes and
in trend.
Trend: As R increases, I decreases
(Deduct 1 mark for each wrong or missing value)
(iv) Five values of V correctly recorded to at least 1 d.p. in volts and in trend
Trend: As R increases, V decreases
(Deduct 1 mark for each wrong or missing value)
(v) Composite table showing at least R, I and V.
GRAPH
(i) Both axes correctly distinguished (½ mark each)
(ii) Reasonable scales (½ mark each)
(iii) Five points correctly plotted
(Deduct 1 mark for each wrong or missing point)
(iv) Line of best fit
SLOPE [02]
(i) Large right-angled triangle
(ii) DV correctly determined
(iii) DI correctly determined
(iv) 
correctly evaluated
ACCURACY [01]
Based on the slope = teacher’s value for Rx ± 10%
PRECAUTIONS [02]
Award 1 mark for any 2 correct precautions stated in acceptable tense.
e.g
- Key opened in between readings/key opened when readings were not taken
- Ensured tight connections
- Avoided parallax error when taking reading on voltmeter/ammeter
- Repeated readings (shown on table)
- Noted/corrected zero error on voltmeter/ammeter
- Clean terminals ensured