Question

Q. 8 (a) Write two points of difference between electric energy and electric power. (6) 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

Answer:

a) Electrical Energy

• It represents the overall work done on an electrical circuit.

• SI unit is Joule (J)

   Electrical Power

• Power defines the work done per unit time in an electrical circuit.
• SI unit is Watt(W)

b) The 40W has higher electrical resistance as resistance and power are inversely proportional to each other. Less the power, more the resistance and vice versa.

c) The commercial unit of energy is kilo-Watt-hour (kWh).

1 kWh =  3.6×10^6 J

Answer 2

Answer:

Answer. (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

2

H VIt V t I 2RT

R

  

(i) The time rate at which electric energy is

consumed or dissipated by an electrical device is

called electric power and is given by

2

P VI V I 2R

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

i.e. 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 x106J

Explanation: