First slide

Electrical power

Question

When three identical bulbs of 60 W, 200 V rating are connected in series to a 200 V supply, the power drawn by them will be

Easy

A

60 W
B

180 W
C

10 W
D

20 W

Solution

Let R₁, R₂ and R₃ be the resistances of three bulbs, respectively.
In series order,
R = R₁ + R₂ + R₃
But R = V²/P and supply voltage is the same as the rated voltage
$\begin{array}{l} ∴ \frac{V^{2}}{P} = \frac{V^{2}}{P_{1}} + \frac{V^{2}}{P_{2}} + \frac{V^{2}}{P_{3}} \\ or \frac{1}{P} = \frac{1}{60} + \frac{1}{60} + \frac{1}{60} \\ or P = \frac{60}{3} = 20 W \end{array}$

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