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

# 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

1. A

60 W

2. B

180 W

3. C

10 W

4. D

20 W

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### 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

$\begin{array}{l}\therefore \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\frac{{\mathrm{V}}^{2}}{\mathrm{P}}\text{\hspace{0.17em}}=\text{\hspace{0.17em}}\frac{{\mathrm{V}}^{2}}{{\mathrm{P}}_{1}}\text{\hspace{0.17em}}+\text{\hspace{0.17em}\hspace{0.17em}}\frac{{\mathrm{V}}^{2}}{{\mathrm{P}}_{2}}\text{\hspace{0.17em}}+\text{\hspace{0.17em}}\frac{{\mathrm{V}}^{2}}{{\mathrm{P}}_{3}}\\ \mathrm{or}\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\frac{1}{\mathrm{P}}\text{\hspace{0.17em}}=\frac{1}{60}\text{\hspace{0.17em}}+\frac{1}{60}\text{\hspace{0.17em}}+\frac{1}{60}\text{\hspace{0.17em}}\\ \mathrm{or}\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{P}=\frac{60}{3}\text{\hspace{0.17em}}=20\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{W}\end{array}$

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