Q: A 1 kW heater is meant to operate at 200 V.

(a) What is its resistance ?

(b) How much power will it consume if the line voltage drops to 100 V?

(c) How many units of electrical energy will it consume in a month (of 30 days) if it operates 10 hr daily at the specified voltage ?

Sol. (a) The resistance of an electric appliance is given

by, R = (V_{s}^{2})/W

so, R = (200)^{2}/1000 = 40 Ω

(b) The ‘actual power’ consumed by an electric appliance is given by,

$\large P = (\frac{V_A}{V_S})^2 \times W $

$\large P = (\frac{100}{200})^2 \times 1000 $

P = 250 W

(c) The total electrical energy consumed by an electric appliance in a specified time is given by,

$\large E = \frac{\Sigma W_1 h_1}{1000} kWh $

$\large E = \frac{1000 \times (10 \times 30 )}{1000} kWh $

So, E = 300 kWh.