Three bulbs A, B and C are connected as shown in the figure. What changes are seen in brightness of the bulbs if switch S is closed?

1. The brightness of bulb A decreases and that of C increases.
2. The brightness of bulb A increases but that of C decreases.
3. The brightness of both bulbs A and C decreases.
4. The brightness of both bulbs A and C remains the same.

The brightness of bulb A increases but that of C decreases.

Reason — When switch S is open, bulb B is out of the circuit, so bulbs A and C are in series. When the switch is closed, bulbs B and C form a parallel combination. This reduces the equivalent resistance of that part of the circuit, so the total resistance of the circuit decreases.

As a result, the current in the main circuit increases. Since bulb A is in series with the whole circuit, it gets more current and glows brighter.

But after closing the switch, the current in the right-hand part divides between bulbs B and C. Therefore, the current through bulb C decreases, so its brightness decreases.

Hence, the brightness of bulb A increases while that of bulb C decreases.