Question

1. An electric iron has a rating of 750W, 220V. Calculate:
(a)current passing through it
(b)Its resistance when in use

2. Two lamps,one rated 100W,220V and other 40W, 220V are connected in parallel to 220V mains supply.
Calculate the current drawn from the supply line..

3. An electric lamp is marked 100W, 220V. Its used for 5hrs daily. Calculate:
(a)Its resistance when in use
(b)Energy consumed in kWh per day.

Answer 1

The equation for electrical power isP=VIwhere V is the voltage and I is the current. This can be rearranged to solve for I in 6(a). 6(b) can be solved with Ohm's LawV=IRor if you'd like, from power, after substituting Ohm's law in for IP=V2R For 7, realize that because they are in parallel, their voltages are the same. We can find the resistance of each lamp fromP=V2RThen the equivalent resistance as1R∗=1R1+1R2Then the total power asPt=V2R∗However, this will reveal that (with a bit of algebra)Pt=P1+P2 For 8, again the resistance can be found asP=V2RThe energy usage is simplyE=P⋅t