Current Electricity and Electrical InstrumentHard

Question

The circuit shown in figure, contains a battery, a rheostat and two identical lamp. What will happen to the brightness of the lamps if the remittance of the rheostat is increased?

Options

A.Lamp P - Less brighter
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

Solution

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

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