Question

Two bulbs 60 W and 100 W designed for voltage 220 V are connected in series across 220 V source. The net power dissipated is

• A. 80 W
• B. 160 W
• C. 37.5 W
• D. 60 W

Answer 1

Answer: (C)

37.5 W

Answer 2

Given :
For Bulb B1 :
P1 = 60 W , V = 220 V
$R_1=\frac{V^2}{P_1}=\frac{(220)^2}{60}$
For Bulb B2 :
P2 = 100 W, V = 220 V
$R_2=\frac{V^2}{P_2}=\frac{(220)^2}{100}$
Required is R1 + R2 :
$=\frac{(220^2}{60}+\frac{(220)^2}{100}$
Now, $Req=\frac{V^2}{Req}=\frac{(220)^2}{(220)^2}[\frac{1}{60}+\frac{1}{100}]$
$=\frac{1}{\frac{1}{60}+\frac{1}{100}}$
$Req=\frac{1}{\frac{100+60}{6000}}=\frac{6000}{160}$
$=37.5$
So, the correct option is (C) : 37.5 W