The circuit shown in the diagram contains a battery "B" a rheostat "Rh" and identical lamps P and Q. What will happen to the broghtness of the lamps, if the resistance through the rheostat is increased? Give reasons.

Explanation

Rh is connected in parallel with lamp Q. As the resistance through Rh is increased, the resistance of the parallel combination increases and so, the total resistance of the circuit increases. This results in decrease in the current in the whole circuit and also, potential drop across it will decrease. This will decrease the brightness of lamp P. On the other hand, brightness of lamp Q increases because its resistance remains same but due to decrease in potential difference across P, potential acorss it increases.