Question

# (a) Write two points of difference between electric energy and electric power (b) Out of $60\mathrm{w}$ and $40\mathrm{w}$ lamps which one has higher electrical resistance when in use?

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Solution

## (a) Difference between electric energy and electric power. Electric energyElectric power1The 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 $\mathrm{H}=\mathrm{VIt}\phantom{\rule{0ex}{0ex}}=\frac{{V}^{2}\mathrm{t}}{R}\phantom{\rule{0ex}{0ex}}={\mathrm{I}}^{2}\mathrm{RT}\phantom{\rule{0ex}{0ex}}$The time rate at which electric energy is consumed or dissipated by an electrical device is called electric power and is given by $\mathrm{P}=\mathrm{VI}\phantom{\rule{0ex}{0ex}}=\frac{{V}^{2}}{R}\phantom{\rule{0ex}{0ex}}={\mathrm{I}}^{2}\mathrm{R}$2It is equal to the product of power and time $\mathrm{E}=\mathrm{P}×\mathrm{t}$It is equal to the rate of doing work by an energy source. $\mathrm{P}=\frac{\mathrm{W}}{\mathrm{t}}$3Its SI unit is the joule $\left(\mathrm{J}\right)$.Its SI unit is a watt $\left(\mathrm{W}\right)$.(b) To find the lamps which one has higher electrical resistance when in useFor the same applied voltage,$\begin{array}{r}\mathrm{P}=\frac{{\mathrm{V}}^{2}}{\mathrm{R}}\\ \mathrm{P}\propto \frac{1}{\mathrm{R}}\end{array}$The less the power of an electrical device, the higher its electrical resistance. So $40\mathrm{W}$ lamp will have higher resistance.

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