Question

In the circuit shown, the total power output is $P$ when $S$ is closed. As the switch $S$ is opened, the power output becomes (all bulbs are identical)

• A. $\frac{2}{3} F$
• B. $\frac{3}{2} F$
• C. $\frac{4}{9} P$
• D. $\frac{9}{4} P$

Answer 1

Answer: (A)

When switch $S$ is closed $R_{e f f}=2 R(R$ is resistance of one bulb)
$P=\frac{V^{2}}{2 R}$
When switch $S$ is open $R_{e f f}=3 R$
$P^{\prime}=\frac{V^{2}}{3 R}$
$ \Rightarrow \frac{P^{\prime}}{P}=\frac{2}{3}$