Question

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`

Answer 1

Correct Answer - D
(d) Current in the bulb `= (P).(V) = (4.5)/(1.5) = 3 A`
Current in `1 Omega` resistance `= (1.5)/(1) = 1.5 A`
Hence total current from the cell `i= 3 + 1.5 = 4.5 A`
By using `E = V + ir implies E 1.5 + 4.5 xx (2.67) = 13.5 V)`