A torch bulb rated as 4.5W−1.5V is connected as shown in the figure. The emf of the cell needed to make the bulb glow at full intensity is
A
4.5V
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B
1.5V
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C
2.67V
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D
13.5V
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Solution
The correct option is D13.5V
Given, P=4.5W,V=1.5V
current passing through the bulb= 4.51.5=3A
In the given situation, potential difference across bulb is due to potential difference due to 1Ω resistor because of parallel connection.
Hence, current in 1Ω resistor=1.51=1.5A
itotal=1.5+3=3.5A
EMF of the battery =V=e−itotal×rinternale=1.5+4.5×2.67=13.5V