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A torch bulb rated 4.5 W, 1.5 V is conne...

A torch bulb rated `4.5 W, 1.5 V` is connected as shown in Fig. 7.35. The emf of the cell needed to make the bulb glow at full intensity is

A

`4.5 V`

B

`1.5 V`

C

`2.67 V`

D

`13.5 V`

Text Solution

Verified by Experts

The correct Answer is:
D
Resistance of bulb is
`(1.5 xx 1.5 ) /( 4.5 ) Omega = 0.5 Omega`
Resistance of parallel combination,
`R = (1 xx (1)/(2))/(1 + (1)/(2)) Omega = (1)/( 3) Omega`
Now, `r = ( E - V) /( V) R or (8) /(3) = ( E - 1.5 )/( 1.5) xx (1)/(3) or E = 13.5 V`
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