Question

The circuit shownin Fig. `7.33` contains a battery , a rheostat , and two identical lamps. What will happen to the brightness of the lamps if the resistance of the rheostat is increased?

A. Lamp `P` - Less bright , Lamp `Q` - Brighter
B. Lamp `P` - Less brighter , Lamp `Q` - Less brighter
C. Lamp `P` - Brighter , Lamp `Q` - Less brighter
D. Lamp `P` - No change , Lamp `Q` - Brighter

Answer 1

Correct Answer - A
Consider two extreme cases : (i) When the resistance of the rheostat is zero , the current through `Q` is zero since `Q` is short - circuited. The circuit is then essentially a battery in series with lamp `P`.(ii) When the resistance of the rheostat is very large, almost no current flows through it . So the currents through `P and Q` are almost equal . The circuit is essentially a battery in series with lamps `P and Q`