WHAT IS ELECTRIC POWER?

WHAT IS ELECTRIC POWER?

You have studied in your earlier Class that the rate of doing work is power. This is also the rate of consumption of energy.

H = I² Rt                                              ...........  (12.21)

Equation (12.21) gives the rate at which electric energy is dissipated or consumed in an electric circuit. This is also termed as electric power. The power P is given by

P = V . I

Or    P  =  I²R  =  V²/R                            ...........  (12.22)

The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus,

1 W = 1 volt × 1 ampere = 1  V A           ...........  (12.23)

The unit ‘watt’ is very small. Therefore, in actual practice, we use a much larger unit called kilowatt. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt - hour (W-h). One watt-hour is the energy consumed when 1 watt of power is used for 1 hour. The commercial unit of electric energy is a kilowatt -hour (kW-hr), commonly known as ‘unit’.

1 kW h = 1000 watt × 3600 second

            = 3.6 × 10⁶ watt second

            = 3.6 × 10⁶ joule (J)

More to know

Many people think that electrons are consumed in an electric circuit. This is wrong! We pay the electricity board or electric company to provide energy to move electrons through electric gadgets like an electric bulb, fan, and engines. We pay for the energy that we use.

Illustration 12.12:

An electric bulb is connected to a 220 V generator. The current is 0.50 A. What is the power of the bulb?

Sol:

P   =  V . I

     = 220 V \(\times\) 0.50 A

     = 110 J/s

     = 110 W.

Illustration 12.13:

An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kW h?

Sol:

The total energy consumed by the refrigerator in 30 days would be 400 W \(\times\) 8.0 hour/day \(\times\) 30 days = 96000 W h = 96 kW h

Thus the cost of energy to operate the refrigerator for 30 days is 96 kW h \(\times\) Rs 3.00 per kW h = Rs 288.00

Questions

1.   What determines the rate at which energy is delivered by a current?

2.   An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.

Source: This topic is taken from NCERT TEXTBOOK