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

Options

A. `4.5 V`
B. `1.5 V`
C. `2.67 V`
D. `13.5 V`

Answer

D

Explanation

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`