Power and its units

Power and its units

Machines may be classified by the speed with which they transfer energy or do work thps-there are motor-car engines of small “power”, as they are rated, or large power (Power is defined as the rate of transfer of energy, energy transferred
or average power = time taken

The SI unit of power is called the watt (W) and is a rate of transfer of energy of joule per second Thus, 1 W = I Jjs
Larger units used are the kilowatt (kW) and the megawatt (MW) 1 kW = 1000 W (or 103 W) \ J 1 MW = 1 000 000 W (or 106 W
Example. Calculate the power of a pump which can lift 200 kg of water through a vertical height of 6 m in 10 s. (Assume g = 10 mjs2.)
Force overcome Distance work done (energy transferred)
Time taken
energy transferred
Power = time taken
= 200 x 10
=6m
= 200 x 10 x 6 J
= 10 s
200 x 10 x 6
10
1200W
1.20 kW

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