Two lamps, one rated `100 W` at `220 V`, and the other `60 W` at `220 V`, are connected in parallel to the electric mains supply. What current is drawn from the line if the supply voltage is `220 V` ?

Explanation

Resistance of first lamp, `r_1 = (V^2)/(P) = ((220 V)^2)/(100 W) = 484 Omega`
Resistance of the second lamp, `r_2 = (V^2)/(P) = ((220 V)^2)/(60 W) = 806.7 Omega`
Since the two lamps are connected in parallel, the equivalent resistance is given by
`(1)/(R_p) = (1)/(r_1)+(1)/(r_2) = (r_2 + r_1)/(r_1 r_2)` or `R_p = (r_1 r_2)/(r_1 + r_2) = (484 xx 806.7)/(484 xx 806.7) Omega = (390442.8)/(1290.7) Omega = 302.6 Omega`
Current drawn from the line, i.e., `I = (V)/(R_p)= (220 V)/(302.6 Omega) = 0.73 A`.