Auto Electrical Works Paper 3, WASSCE (SC), 2018

Question 1

 

AIM: To determine the relationship between the lamp rating and current drawn  from the battery.
           

 

  1. Connect the circuit as shown in Fig. 1.
  2. Ask the supervisor to check the circuit connection.
  3. Close switch S.
  4. Read and record in Table 1 the ammeter reading.
  5. Open switch S.
  6. 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

 

  1. Plot a graph of current (A) on the vertical axis against lamp rating (W) on the horizontal axis.
  2. Calculate the gradient of your graph.
  3. Comment on your graph.
  4. From your graph, determine the current when:

(i)      lamp rating is 7W;
(ii)     lamp rating is 22W.

  1. Given that P = I2R, calculate the resistance of the lamp when: 

(i)      lamp rating is 7W;
(ii)     lamp rating is 22W.

  1. The circuit contains a fuse rated 5A. State:

(i)      the purpose of the fuse;
(ii)     why the fuse is suitable for the experiment.

State two precautions taken in carrying out 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.