Power,Units and Energy transformation

Suppose you being an young guy can do a work in 3o minutes.and an old man can do it in 1 hour.It means you have more energy than the old man, and your rate of doing work is higher than the old man.

Power is defined as the work done per unit time.

Power =WorkDone/TimeTaken


Also in terms of  energy consumed is power is  equal to  Energy Consumed/TimeTaken

Units Of Power is Joule/Second = Watt

1 Kilowatt = 1000 watt

1 Megawatt = 106 Watt

1 Horse power or 1 h.p. =  746 watt


A body does 2o joules of work in 10 seconds. what is its power.

Power = 20/10= 2 Watt

Example 2

What is the power of a pump which takes 20 seconds to lift 2oo kg of water to a tank situated at a height of 20 m.


we know that power =WorkDone/TimeTaken

Work done = w = m * g * h


now P= 40000/20=2000 Watt or 2 Kw.

Q. An Electric Bulb consumes 7.2 kJ of electrical energy in 2 minutes. What is the power of the Bulb. 

Energy consumed = 7200 J.

Time taken = 120 second

Power = work done / time taken

Power = 7200/120 = 60 watt.

Q. What is the power  of a pump which takes 30 seconds to lift 300 kg of water to a tank situated at a height 40 metre. 

Ans 4 KW,

Commercial unit of energy

  • You have seen electric meter at your house and you must have heard saying this time 60 units of energy consumed . so what is this unit.
  • This is actually means 1 Kilowatt Hour and it is the commercial unit of energy
  • So,1 Unit means amount of Kilowatt energy consumed in one 1 hour .
  • So 1 Kilowatt-hour = 3.6 X 106 Joule

A simple Problem

A TV set rated 80 watts runs for 10 hours per day for 30 days. If the cost of electricity per unit is 5 rupees, find the bill for that month.

So here total hours run = 10 *30 = 300

Power consumed= 80*300=24000 watt= 2.4 KWH

Cost of electricity per unit or per KwH = 5 rupees

Cost for 24 Unit = 24*5=120Rupees

Law of conservation of energy = Energy cannot be created nor be destroyed but can be transformed from one form into another.

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