(a) Define Electric power. An electrial device of resistance R is connected across a source of voltage V and draws a current I. Derive an expression for power in terms of current and resistance.
(b) Tow electric bulbs rated 100W , 220V and 60W, 220V are connected in parallel to an electric mains of 220V . Find the current draw by the bulbs form the mains.

Explanation

(a) Electric Power . Electric Power is the rate at which work is done or enegy is transformed in an electric circuit.
We know that electric work done is ,
`W = V xx I xx t`
`P = (V xx I xxt)/(t) rArr P = VI`
As per Ohm's law , V= IR
Substituting (i) in the power formula .
We get .
`P = IR xx I = I^(2)R" "`.... where [I is the current , R is the resistance.
(b) Resistance of `1^(st)` bulb.
`R_(1) = (V^(2))/(P) = (220 xx 220)/(60) = 484 Omega = R_(1)`
Resistance of `2^(nd)` bulb,
`R_(2) = (V^(2))/(P) = (220 xx 200)/(60) = (2420)(3)Omega`
Total Resistance in parallel,
`(1)/(R) = (1)/(R_(1)) + (1)/(R_(2))`
`(1)/(R) = (1)/(484) + (3)/(2420) = (8)/(2420) rArr R = (2420)/(8)Omega` .
current darwn by the bulbs from the main line .
`I = (V)/(R) = (220 xx 8)/(2420) = (8)/(11) A or 0.73A`