The candidates were expected to connect the circuit as shown in Fig.1, measure and record in Table 1 the ammeter readings, voltmeter readings and make some observations and calculate some values.
The expected answers were:
Variac voltage(V) |
Condition of Bulbs |
A(A) |
V1(V) |
V2 (V) |
40 W |
60 W |
100 |
ON |
OFF |
0.042 |
60.48 |
40.32 |
120 |
ON |
OFF |
0.050 |
72.00 |
48.00 |
140 |
ON |
OFF |
0.058 |
83.52 |
55.68 |
160 |
ON |
DIM |
0.067 |
96.48 |
64.32 |
180 |
ON |
DIM |
0.075 |
108.00 |
72.00 |
200 |
ON |
DIM |
0.083 |
119.52 |
79.68 |
- The results of V1 (V) and V2 (V) are not equal.
- The difference in values of V1 (V) and V2 (V) is due to the difference in the voltage of the bulbs
OR
The 40 W bulb possesses lower resistance while the 60 W possess higher resistance to current leading to different voltage drop.
- P = IV = I2R
R60W = = = 960
The candidates, as reported, understood the question and carried out the actions required. But they failed to obtain accurate readings from the instruments which marred their performances. The observations and calculations they were required to make were all based on the readings. The moment the readings are not accurate, all other actions on it are also inaccurate.