If a `30 V, 90 W` bulb is to be worked on a `120 V` line, resistance of how many ohms should be connected in series with the bulb

Options

A. 40
B. 10
C. 20
D. 30

Answer

D

Explanation

Resistance of the lamp,
`R_(0) =V^(2)/P = (30)^(2)/90 = 10 Omega`
Current in the lamp,
`i = 30/10 = 3 A`
When lamp is operated on a 120 V, then resistance,
`R. V^(.)/i = 120/3 = 40 Omega`
Thus, for proper glow, the resistance required to the put in series will be:
`R = R^(.) - R_(0) = 40 - 10 = 30 Omega`