First slide

Electrical power

Question

A torch bulb rated as 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

Moderate

A

4.5 V
B

1.5 V
C

2.67 V
D

13.5 V

Solution

Current in the bulb $= \frac{P}{V} = \frac{4 .5}{1 .5} = 3 A$
Current in $1 Ω$ resistance $= \frac{1.5}{1} = 1.5 A$
Hence, total current from the cell, i = 3 + 1.5 = 4.5 A
By using $E = V + ir \Rightarrow E = 1 .5 + 4 .5 \times (2 .67) = 13 .5 V$

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