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

4.5 V

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1.5 V

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2.67 V

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13.5 V

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Solution

The correct option is D 13.5 V
When bulb glows with full intensity, potential difference across it is 1.5 V. So current through the bulb and resistance of $1 Ω$ are 3 A and 1.5 A respectively. So main current from the cell i = 3 + 1.5 = 4.5 A. By using $E = V + i r \Rightarrow E = 1.5 + 4.5 \times 2.67 = 13.5 V$ .

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

The correct option is D 13.5 VWhen bulb glows with full intensity, potential difference across it is 1.5 V. So current through the bulb and resistance of 1Ω are 3 A and 1.5 A respectively. So main current from the cell i = 3 + 1.5 = 4.5 A. By using E=V+ir⇒E=1.5+4.5×2.67=13.5 V.