Question 1

AIM: To show the relationship between current and voltage in series-connected loads

1.  Connect the circuit as shown in Figure 1.
2.  Ask your supervisor to check the circuit connection.
3.  Copy Table 1 into your answer booklet.

Variac voltage(V)	Condition of bulbs		A(A)	V1(V)	V2 (V)
	40 W	60 W
100
120
140
160
180
200

1.  Close switch S.
2.  Set the variac to 100 V.
3.  Read and record in Table 1 the meter readings A, V1 and V2.
4.  Repeat steps (e) and (f) for voltages 120 V to 200 V as shown in Table 1.
5.  Observe and state the conditions of the bulb as the voltage is varied from      100 V to 200 V.
6.  Compare the results of V1 and V2.

1.  State your observations on whether there are differences or similarities in  the values of V1 and V2.
2. Calculate R of the 60 W bulb when the variac voltage is 200 V.

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

1. The results of V1 (V) and V2 (V) are not equal.
2. 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.

1.  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.