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Power:  The Rate of Doing Work

All of us do not work at the same rate. A stronger person will be able to do more work in a certain period of time than an ordinary person. We talk of cars and motor cycles in terms of power of the engines. What is power? It measures the speed with which work is done.

Power is defined as the rate of doing work or rate of transfer of energy.

If 'W' joule of work is done in `t’ seconds,

Power P = =

The unit of power is watt.

1 watt is the power of a machine or an agent that does work at the rate of 1 joule per second.

1000 watts = 1 kilowatt
1000 kilowatt = 1 megawatt

The practical unit of power in the FPS system in one horse power

1 H. P. = 746 watt.

The kilowatt hour (kWh) is a large practical unit of energy used in electricity.

1 kilowatt hour is the work done in one hour using 1000 watt of energy or 1000 joule per second.

1 kilowatt hour = 1000 watt x 60 x 60 second
                       = 36 x 105 watt second

                           = 3.6 x 106 joule

1 kilowatt = 1.34 horse power

The power of an agent is not constant. It may vary with time, doing work at different rates at different intervals of time. The average power (often simply called "power" when the context makes it clear) is the average amount of work done or energy transferred per unit time. The instantaneous power is the limiting value of the average power as the time interval 'Δt' approaches zero.

Commercial Unit of Energy
Joule is too small a unit to be used for commercial purposes. So a bigger unit called kilowatt hour (kWh) is used. 1 unit that is sold to customers (domestic and commercial) by the electricity board is actually 1 kWh.

Work and Energy

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