Question

In an electrical circuit, three incandescent bulbs A, B and C of rating 40 W, 60 W and 100 W 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

Answer: (C)

Brightness of bulb B will be more than that of A

Power rating of a bulb is defined at a fixed rated voltage (usually $220V$).

Using Joule's law,

$P_{rated}=\frac{V_{rated}^2}{R}$

Hence, $R_A>R_B>R_C..........\left(i\right)$

When connected in parallel, voltage is same across each bulb. Using Joule's Law again,

$P=\frac{V^2}{R}...........\left(ii\right)$

Using $\left(i\right)$ and $\left(ii\right)$, $P_C>P_B>P_A$. Hence, option C is correct.