Question

In the figure A, B and C are three identical bulbs. Now the bulbs A and B are glowing. Which of the following statements is correct if switched on ?

• A. The brightness of bulb A decreases
• B. The brightness of bulb B increases
• C. The brightness of bulb A decreases and that of B increases
• D. The brightness of bulb A increases and that of B decreases

Answer 1

Answer:

D. The brightness of bulb A increases and that of B decreases

• The correct answer is indeed The brightness of bulb A increases and that of B decreases.

• When bulb C is switched on, the total resistance of the circuit decreases, and the total current increases.

• Since bulbs A and B are in series with each other, the increased current flows through both bulbs, causing:

• The brightness of bulb A to increase, as more current flows through it.

• The brightness of bulb B to decrease, as the increased current is shared between the two bulbs, reducing the voltage across bulb B.

Note that the brightness of a bulb is directly proportional to the current flowing through it, so an increase in current results in an increase in brightness.