Question 1
AIM: To determine the relationship between the lamp rating and current drawn from the battery.
- Connect the circuit as shown in Fig. 1.
- Ask the supervisor to check the circuit connection.
- Close switch S.
- Read and record in Table 1 the ammeter reading.
- Open switch S.
- Repeat steps (c) to (e) for the other lamp ratings in Table 1.
Table 1
S/N |
Lamp rating (W) |
Current (A) |
1. |
5 |
|
2. |
10 |
|
3. |
15 |
|
4. |
20 |
|
5. |
25 |
|
- Plot a graph of current (A) on the vertical axis against lamp rating (W) on the horizontal axis.
- Calculate the gradient of your graph.
- Comment on your graph.
- From your graph, determine the current when:
(i) lamp rating is 7W;
(ii) lamp rating is 22W.
- Given that P = I2R, calculate the resistance of the lamp when:
(i) lamp rating is 7W;
(ii) lamp rating is 22W.
- The circuit contains a fuse rated 5A. State:
(i) the purpose of the fuse;
(ii) why the fuse is suitable for the experiment.
Observation
The expected answers were
Table 1
S/N |
Lamp rating (W) |
Current (A) |
1. |
5 |
0.42 |
2. |
10 |
0.83 |
3. |
15 |
1.25 |
4. |
20 |
1.67 |
5. |
25 |
2.08 |
NOTE: Readings of current (A) to ± 0.1 accuracy.
(g) A linear graph from origin (0,0) with appropriate scale and line of best fit.
(h) 
= 0.08333 A/W OR 0.08333 V-1
(i) The higher the lamp rating, the higher the amount of current drawn.
OR The lamp rating is directly proportional to the current.
OR The relationship is a linear/straight line graph.
(j) The current is:
(i) 0.6A;
(ii) 1.85A.
(k) Given that 
, therefore 
(i) 
R = 19.44Ω.
(ii) 
R = 6.42Ω.
(l) Fuse rated 5A.
(i) To protect the circuit against surge/high current.
(ii) -The highest value of current obtained from the experiment is less than 5A
-It allows the flow of the maximum current in the experiment.
(m) Precautions in carrying out the experiment
- Ensure tight circuit connection.
- Ensure the correct polarity of battery.
- Avoid error due to parallax in meter reading
Question 1 required candidates to investigate the relationship between the rating of lamps and amount of current drawn from battery. Majority of the candidates that attempted Question 1 had difficulties in plotting a suitable graph.