An electric bulb of 40 W is connected to a power supply of 220 V. Calculate the resistance of the bulb and the units consumed if it is used for 24 hours.

Concept:

Power: The rate of work done by electric energy is called power. It is denoted by P.

\(P = VI = \frac {V^2}{R} = I^2 R\)

• The SI unit of power is the watt (W)
• 1 Kilo-watt = 1000 Watt

1 unit of electric energy: When a one-kilowatt load works for 1 hour then the energy consumed is called 1 unit of electricity.

• 1 Unit of electricity = 1 kWh = 1000 Watt-hour = 3.6 × 106 J

Calculation:

Given:

P = 40 W = 0.04 kW, V = 220 V, t = 24 hr

Using \(P = \frac {V^2}{R} \)

\(40 = \frac {220^2}{R} \)

R = 1210 Ω 

Now, the units consumed = P × t

= 0.04 × 24 

= 0.96 kWh = 0.96 Unit