Question

In an electrical circuit, three incandescent bulbs A, B and C of rating 40 W, 60 W and 100 W respectively are connected in parallel to an electric source. Which of the following is likely to happen regarding their brightness?

• A. Brightness of all the bulbs will be the same
• B. Brightness of bulb A will be the maximum
• C. Brightness of bulb B will be more than that of A
• D. Brightness of bulb C will be less than that of B

Answer 1

The correct option is C Brightness of bulb B will be more than that of A

Given, power of bulb A = $P_A$ = 40W, power of bulb B = $P_B$ = 60W and power of bulb C = $P_C$ = 100W
let,V be the voltage, $R_A$ = resistance of Bulb A, $R_B$ = resistance of Bulb B and$R_C$ = resistance of Bulb C
Now, as we know,
Power P = $\frac{V^2}{R}$
$\Rightarrow$ R = $\frac{V^2}{P}$
$R_A=\frac{V^2}{40}$
$R_B=\frac{V^2}{60}$
$R_C=\frac{V^2}{100}$
$\Rightarrow R_A>R_B>R_C$
Since P = $\frac{V^2}{R}$ , and V is constant,
$\Rightarrow$ P is inversely proportional to R, and hence bulb with the lower resistance will glow brighter.