VITEEE 2015 Paper

Resistance of lamp
R =

C2

P

=

(30)2

90

= 10 Ω
Current in the lamp
I =

V

R0

=

30

10

= 3 A
As the lamp is operated on 120V DC, then resistance becomes
R' =

V′

i

=

120

3

= 40 Ω
For proper glow, a resistance R is joined in series with the bulb
R' = R + R0
⇒ Rα = R' - R0 = 40 - 10 = 30 Ω