Question

Three bulbs of ratings 40 W, 60 W and 100 W are designed to work on 220V mains. The bulb which burn most brightly if they are connected in series-across 220 V mains is

• A. 40 W bulb
• B. 60 W bulb
• C. 100 W bulb
• D. All bulbs will burn equally brightly

Answer 1

Answer: (A)

40 W bulb

$R_1=\frac{(220{)}^2}{46}{R}_2=\frac{(220{)}^2}{60}{R}_3=\frac{(220{)}^2}{100}$
$\Rightarrow R_1>R_2>R_3$
$\Rightarrow I^2{R}_1>I^2{R}_2>I^2{R}_3$
$\Rightarrow$ Bulb of 40 W glows more brightly