Question

Two bulbs A and B are marked as ‘A’ 60W, 220V & ‘B’ 100W, 220V are connected in parallel to 220 V source. Which of the two will glow brighter?

• A. Bulb B
• B. Bulb A
• C. Same brightness
• D. ‘A’ glows but ‘B’ does not

Answer 1

The correct option is A

Bulb B

When voltage is constant then P = $\frac{V^2}{R}$
P $\propto$ $\frac{1}{R}$ so, when resistance increases, power decreases and viceversa
For Bulb A:
P = $\frac{V^2}{R}$ $\Rightarrow$ $R_A$ ​ = $\frac{V^2}{P}$ = $\frac{{220}^2}{60}$ = 806
For Bulb B:
P = $\frac{V^2}{R}$ $\Rightarrow$ $R_B$​ = $\frac{V^2}{P}$ = $\frac{{220}^2}{100}$ = 484
So, $R_B$ < $R_A$​​
$\Rightarrow$ $P_B$​ > $P_A$​
$\Rightarrow$ Bulb B will glow brightly