Question

An electric bulb P rated $220V,60W$ is working at full efficiency Another identical bulb Q is connected across the mains as shown in the figure. What will be the total power if the bulbs are connected in parallel?

• A. $120W$
• B. $60W$
• C. $200W$
• D. $70W$

Answer 1

Answer: (A)

$120W$

The answer is A.

The resistance across the bulb is calculated as follows.
That is, $P=\frac{V^2}{R}.So,R=\frac{V^2}{P}$.
$R=\frac{{220}^2}{60}=806.67ohms$.
The bulb Q is also identical as bulb P. So it will also have the same resistance. The total resistance in the circuit is $\frac{1}{806.34}+\frac{1}{806.34}=\frac{1}{0.00248}=403.17ohms$ (parallel circuit).
Total power in the circuit is $P=\frac{V^2}{R}=\frac{{220}^2}{403.17}=120W$.

Hence, the total power consumed is 120 W.