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A torch bulb rated at 4.5 W, 1.5 V is connected as shown in the figure. The e.m.f. of the cell needed to make the bulb glow at full intensity is

1. 4.5 V
2. 1.5 V
3. 2.67 V
4. 13.5 V

Correct answer: 13.5 V

Solution

A bulb rated 4.5 W, 1.5 V draws current \(I = P/V = 4.5/1.5 = 3\,\text{A}\). If the figure has 9 such bulbs in series, the total emf needed is \(9 \times 1.5 = 13.5\,\text{V}\). Hence the correct answer is 13.5 V.

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