Question

There are $5$ bulbs in a house written on them as $220volt$, $40$ watt. The bulb glow for $30$ days at the rate of $5$ hours per day. If the price of electric energy is Rs.$4$ per unit, calculate-
(i) equivalent resistance of the combination of bulbs
(ii) the number of electric units consumed.
(iii) the price of spent electric energy

Answer 1

Power used by 5 bulbs $=5\times 40watt=200watt$ and it is given by,

$P=\frac{V^2}{R}$ ....where $R$ is equivalent resistance of the combination of bulbs.

$\therefore 200=\frac{{220}^2}{R}$

$\therefore \left(i\right)R=\frac{{220}^2}{200}=242\Omega$.

Total electricity consumed $=T=P\times$number of hours per day$\times$number of days$=200\times 5\times 30=30000Wh$

Unit of electricity$=U=1000Wh$

$\therefore \left(ii\right)$ The number of electrical units consumed$=\frac{T}{U}=\frac{30000}{1000}=30units$

$\left(iii\right)$ Price of spent electricity=number of electrical units consumed$\times$price of electrical energy per unit

$=30\times 4=120Rs$