Question 2
AIM: To investigate the characteristics of a parallel a.c. circuit.
Figure 2
(a) Connect the circuit as shown in Figure 2.
(b) Ask the supervisor to check the circuit connection.
(c) You are provided with Table 2.
Table 2
VS (V) |
AT = A1 + A2 |
A1 (mA) |
A2 (mA) |
V(V) |
0 |
|
|
|
|
5 |
|
|
|
|
10 |
|
|
|
|
15 |
|
|
|
|
20 |
|
|
|
|
25 |
|
|
|
|
30 |
|
|
|
|
(d) Set the variac (VS) to zero volt.
(e) Close the switch S.
(f) Read and record in Table 2 the reading on the voltmeters V and ammeters A1, and A2.
(g) Open the switch S, adjust the variac to 5V.
(h) Close the switch and repeat step (f)
(i) Repeat steps (g) and (h) for other values of Vs as listed in Table 2.
(j) Open the switch S.
(k) Complete Table 2.
(l) Plot the graph of Vs (V) on the vertical axis and AT (mA) on the horizontal axis.
(m) Determine the slope of the graph.
(n) What does the slope of the graph represent?
(o) Compare the slope of the graph with R1 + R2
Observation
The expected responses were:
- Circuit connection
TABLE 2
Vs (V) |
AT = A1 + A2 |
A1 (mA) |
A2(mA) |
V (V) |
0 |
0 |
0 |
0 |
0 |
5 |
38 |
23 |
15 |
5 |
10 |
75 |
45 |
30 |
10 |
15 |
114 |
68 |
46 |
15 |
20 |
152 |
91 |
61 |
20 |
25 |
189 |
114 |
75 |
25 |
30 |
227 |
136 |
91 |
30 |
- Correct values of AT (mA)
- Correct values of A1 (mA)
- Correct values of A2 (mA)
- Correct values of V(V)
- Correct Axes
- Reasonable Scale
- Plotting of points
- Line of best fit
- Slope
VS1 = 5, VS2 = 30
AT1 = 40, AT2 = 227
Slope = 
= 
= 
= 133.69 Ω
Comments
- Slope represents the equivalent resistance in the circuit
- The slope in less than R1 + R2
Candidates were required to investigate the characteristics of a parallel a.c. circuit. The question was well handled by the candidates. Many presented the required answer and statement corresponding to what was expected of them.