Question

(a) Write two points of difference between electric energy and electric power. 

(b) Out of 60 W and 40 W lamps, which one has higher electrical resistance when in use. 

(c) What is the commercial unit of electric energy? Convert it into joules.

Answer 1

(a) Difference between electric energy and electric power:

Electrical energy	Electric power
(i) The work done or energy supplied by the source in maintaining the flow of electric current is called electrical energy. It appears in the form of heat given by  H = Vlt = V2t/R =l2RT	(i) The time rate at which electric energy is consumed or dissipated by an electrical device is called electric power and is given by p = Vl = V2/R = l2 R
(ii) It is equal to the product of power and time i.e. E = P × t	(ii) It is equal to the rate of doing work by an energy source i.e P =W/T
(iii) Its SI unit is joule (J) 1 J = 1 W × 1s	(iii) Its SI unit is watt (W) 1 W = 1 J s-1

(b) less the power of electrical device, higher is its electrical resistance. 

(c) Kilowatt hour – Commercial unit of electrical energy 1 kWh = 1000 Wh = 1000 J/S x 3600 sec = 3600000 J = 3.6 x10⁶ J