Question

A 1 kW heater is meant to operate at 200 V.
a. what is the 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 h daily at the specified voltage (200 V) ?

Answer 1

a. The resistance of electric appliance is given by
$$ R = \frac {V^2_s}{P_0} = \frac {(200)^2}{1000} = 40 \Omega $$
b. The 'actual power' consumed by an electric appliance is given by
$$ P = ( \frac {V_A}{V_5})^2 \times P_0 = ( \frac {100}{200})^2 \times 1000 = 250 W $$
c. The total electrical energy consumed by an electric appliance in a specified time is given bt
$$ E = \frac { \sum w_ih_i}{1000} kWh= \frac { 1000 \times ( 10 \times 30) }{ 1000 } = 300 kWh $$