Thus, work done in unit time or rate of doing work is defined as power.
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Power is denoted by the letter 'P'.
As energy is the capacity to do work, power can also be defined as energy consumed in unit time.
SI unit of power
SI unit of work is joule and time is second. Therefore, SI unit of power is joule/sec.
1 joule/sec = 1 watt
Thus, if one joule of work is done in one second by any agent then its power is said to be one watt.
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Kilowatt and megawatt are higher units of power.
1 kilowatt (KW) = 103 watts
1 Megawatt (MW) = 106 watts
Another unit of power is horse power (h.p)
1 h.p. = 746 watts
Numericals
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01. A man of mass 60 kg runs up a flight of 60 steps in 40 seconds. If each step is 20 cm high, calculate his power.
Solution:
Since the man runs up a flight of steps, the work done will be equal to the potential energy.
Height = height of one step x number of steps
Acceleration due to gravity (g)=9.8m/s2
Time = 40 sec
02. Calculate the power of an engine required to lift 10
5kg of coal per hour from a mine 100m deep. Given g=10m/s
2.
Solution:
The work done in lifting coal is nothing but gravitational potential energy.
Mass of coal = 105 kg
Time = 1 hour
Power of the engine = 27777.78 W
03. A man buys an electric motor of 2 h.p. What is its power in watts?
Solution:
Power of the electric motor = 2 h.p
Power of the electric motor = 2 x 746
