Two lamps one rated 60 W, 220 V and the other 40 W, 220 V are connected in parallel to a 220 V electric supply mains. What is the total current drawn from the electric mains if the voltage of the electric supply is 220 V?
Relationship between power (P), voltage(V), and Resistance (R)
P=V2R
⇒R=V2P
Resistance of lamp of 60W,220V is R1,
R1=220260=806.67 Ω
Resistance of 40W,220V lamp be R2,
R2=220240=1210 Ω
These resistances are connected in parallel.
Equivalent resistance,
R=R1R2R1+R2
R=(806.67×1210)(806.67+1210)=484Ω
Current drawn from 220V,
I=220484=0.45A