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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.

Solution:

(i) Given P=100W, Vrms=220V, f=50Hz
by P = V²rms/R or R = V²rms/P = 220²/100 = 484Ω
(ii) Now Irms = Vrms/R = 220/484 = 0.45A