Question

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

• A. Bulb B.
• B. Bulb A.
• C. Both will glow with same brightness.
• D. Bulb A glows and B does not glow.

Answer 1

The correct option is (C) Bulb B.

Two bulbs are connected in parallel

When voltage is constant then P = $\frac{V^2}{R}$

We can say that

P $\propto$ $\frac{1}{R}$ so when R increases, power decreases and vice versa.
Bulb A: P = $\frac{V^2}{R}$ $\Rightarrow$ $R_A$ ​= $\frac{V^2}{P}$ = $\frac{{220}^2}{60}=806\Omega$

Bulb B: P = $\frac{V^2}{R}$ $\Rightarrow$ $R_B$​ = $\frac{V^2}{P}$ = $\frac{{220}^2}{100}=484\Omega$

So, $R_B$​ < $R_A$​

$\Rightarrow$ $P_B$​ > $P_A$​

$\Rightarrow$ Bulb B will glow brighter.