Question

An electric bulb is marked 100 W, 230 V. If the supply voltage drops to 115 V, what is the heat and light energy produced by the bulb in 20 min?

• A. 10 kJ
• B. 15 kJ
• C. 20 kJ
• D. 30 kJ

Answer 1

The correct option is D 30 kJ

Here, power of the bulb, P = 100 W
Supply voltage, $\epsilon =230V$
Let R be the resistance of the bulb.
As $P=\frac{{\epsilon }^2}{R}\Rightarrow =\frac{{\epsilon }^2}{P}=\frac{(230{)}^2}{100}=529\Omega$
Changed supply voltage, ${\epsilon }^{\prime }=115V$
Heat and light energy produced by the bulb in 20 min.
$=\frac{{\epsilon }^{\prime }2t}{2}=\frac{{115}^2\times 20\times 60}{529}=30,000J=30kJ$