A battery E is connected to three identical lamps P, Q and R as shown in figure. Initially, the switch S is kept open and the lamp P and Q are observed to glow with same brightness. Then, switch S is closed.

How will the brightness of glow of bulbs P and O will change? Justify your answer.

The brightness of glow of bulb P will increase and brightness of glow of bulb Q will decrease.

Because on closing S, bulbs Q and R will be in parallel and the combination will be in series with bulb P. Hence, the total resistance of the circuit will decrease and the current flowing in the circuit will increase. Therefore, the glow of bulb P will increase.

Also, bulbs Q and R will be in parallel in this case. So, the current gets divided and lesser current flows through Q and hence the glow of bulb Q decreases.