Alternative A
Question 3A
You are provided with an ammeter, resistor, key, metre bridge and other necessary apparatus.
(i) Connect a circuit as shown in the diagram above.
(ii) Close the key and use the jockey to make contact with AB at N such that AN = d = 25 cm.
(iii) Read and record the ammeter reading I.
(iv) Evaluate I-1
(v) Repeat the procedure for values of d = 35 cm, 50 cm, 65 cm, and 80 cm. In each case, record I and determine I-1
(vi) Tabulate your results.
(vii) Plot a graph with I-1 on the vertical axis and d on the horizontal axis.
(viii) Determine the slope, s, of the graph.
(ix) State two precautions taken to obtain accurate results.
(b) (i) Use the graph to determine the value of d =I =1.5 A.
(ii) State two factors which affects the resistance of a wire.
Observation
Part (a) Performance was poor. Many candidates who attempted this question failed short in so many ways. Candidates were unable to record d to at least 1 d.p in cm, I to at least 1 d.p in A and evaluation of I-1 and log I-1 to 3 s.f each.
Part (b) Candidates responded unsatisfactorily. Many candidates were unable to determine the value of I = 1.5 A.
The expected response:
- Five values of d correctly read and recorded to at least 1 d.p in cm
- Five values of I correctly read and recorded to at least 1 d.p in A
- Five values of I-1 correctly evaluated to at least 3 s.f.
- (Five values of log I-1 correctly evaluated to 3 s.f
- Composite table showing at least d, I, I-1 and log I-1
- Both axes correctly distinguished
- Plot a graph using reasonable scales
- Draw line of best fit
- Determine the slope of the graph
- State any two of the following precautions.
e.g.
- Key opened in between readings/key opened when readings were not taken
- Tight connections ensured
- Avoided parallax error when taking reading on ammeter/metre rule
- Repeated readings shown
- Avoided/Noted/corrected zero error on ammeter/metre rule
- Clean terminals ensured
- Avoided dragging jockey on the potentiometer
