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

• A. 60 W
• B. 180 W
• C. 10 W
• D. 20 W

Answer 1

Answer: (D)

20 W

Answer 2

Key Idea : In series order, the resistances of three bulbs must be added to give resultant resistance of the circuit. Let $R_{1}, R_{2}$ and $R_{3}$ are the resistances of three bulbs respectively. In series order $R=R_{1}+R_{2}+R_{3}$ but $R=\frac{V^{2}}{P}$ and supply voltage in series order is the same as the rated voltage. $\therefore \frac{V^{2}}{P}=\frac{V^{2}}{P_{1}}+\frac{V^{2}}{P_{2}}+\frac{V^{2}}{P_{3}}$ or $\frac{1}{P}=\frac{1}{60}+\frac{1}{60}+\frac{1}{60}$ or $P=\frac{60}{3}=20 \,W$