Question

a bulb is rated 220v 100w calculate its resistence six such bulbs operate for 5hours daily calculate the units of electrical energy consumed per day calculate cost of bulbs if the rate is rs 6 per unit

Answer 1

Given,
P=100 W
V=220 V
Using relation,
$$\begin{gathered} \mathrm{P}=\frac{{\mathrm{V}}^2}{\mathrm{R}} \\ \mathrm{Or}\mathrm{R}=\frac{{\mathrm{V}}^2}{\mathrm{P}} \\ =\frac{220\times 220}{100}=484\mathrm{\Omega } \\ \mathrm{Thus},\mathrm{resistance}\mathrm{of}\mathrm{bulb}\mathrm{is}484\mathrm{ohm}. \\ \mathrm{Now},\mathrm{if}\mathrm{six}\mathrm{such}\mathrm{bulbs}\mathrm{are}\mathrm{connected}, \\ \mathrm{Power},\mathrm{P}=100\mathrm{W}=0.1\mathrm{kW} \\ \mathrm{Time},\mathrm{t}=5\mathrm{hours} \\ \mathrm{Energy}\mathrm{consumed},\mathrm{E}=\mathrm{P}\times \mathrm{t}=0.1\times 5=0.5\mathrm{kWh}\mathrm{or}0.5\mathrm{unit} \\ \mathrm{For}\mathrm{six}\mathrm{bulbs},\mathrm{E}=6\times 0.5=3\mathrm{unit} \\ \mathrm{Cost}\mathrm{of}\mathrm{one}\mathrm{unit}=\mathrm{Rs}6 \\ \mathrm{Thus},\mathrm{cost}\mathrm{of}3\mathrm{unit}\mathrm{will}\mathrm{be}6\times 3=\mathrm{Rs}18\mathrm{per}\mathrm{day}. \end{gathered}$$