Power is the rate of doing work.
From the definition we can deduce the following equation
Power = Work done /Time taken
The unit of power is the Watt (symbol W) or the Joule/second (J/s)
Example 1
A boy pushes a box and as a result does 120 J of work in 10 s. What is the power developed by the boy?
Power = Work done / time taken
= 120 /10
= 12 W or J/s
There is another definition for power that is often used. It is
Power is the rate of dissipation of energy or the rate of change of energy conversion.
Power = Energy dissipated / Time taken
Example 2
A girl climbs a staircase gaining 500 J of gravitational potential energy in 10 s.
What is the power developed by the girl?
Power = Energy conversion / Time taken
= 500 / 10
= 50 W or J/s
Example 3
During the boiling of some water 4000 J of heat energy is dissipated in the kettle’s heater in a time of 8 s. What is the power of the heater?
Power = Energy dissipated/Time taken
= 4000/8
= 500 W or J/s
It is now time to do some questions. It will give these questions after a few of you have supplied your answers.
1. A trains of mass 50000 kg accelerated form rest and reaches a velocity of 50 ms-1 in 60 s.
(a) Calculate the kinetic energy gained by the train.
(b) Calculate the power of the train engine.
2. A lamp is rated 80 W. If it is switched on for two hours, how much light energy is dissipated.
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