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 a higher electrical resistance when in use.
(c) What is commercial unit of electric energy?
Convert it into Joules.

Answer 1

$\begin{array}{ccc}\text{Sr. No.}& \text{Electric Energy}& \text{Electric power}\\ \left(i\right)& \text{Electrical energy consumed by an}& \text{It is the rate of which}\\ & \text{electrical appliance is given by}& \text{electrical energy is}\\ & \text{the product of its power rating}& \text{consumed.}\\ & \text{and time for which it is used.}& \\ \left(ii\right)& \text{It is measured in kilowatt hour (kWh).}& \text{It is measured in watt (W) or}\\ & & \text{kilowatt (kW).}\end{array}$

(b) We know, power (P) $=\frac{V^2}{R}$
i.e. At same voltage, power (P) is inversely proportional to resistance (R).
$\therefore$ 40 W lamp has higher resistance.

(c) The commercial unit of electrical energy is kWh.
$1kWh=1000W\times 1h$ $=1000W\times 3600s$
$=36\times {10}^{5}Ws$
$=3.6\times {10}^{6}J.$