Question

A light bulb is rated $100W$ for $220V$ ac supply of $50Hz$. Calculate
(i) The resistance of the bulb;
(ii) The rms current through the bulb.

Answer 1

(i) Given $P=100W,V_{rms}=220V,f=50Hz$

by $P=\frac{V_{rms}^2}{R}$

or $R=\frac{V_{rms}^2}{P}=\frac{{220}^2}{100}=484\Omega$

(ii) Now $I_{rms}=\frac{V_{rms}}{R}=\frac{220}{484}=0.45A$