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measuring work and energy


Energy is the ability to do work. This applies to humans as well as to objects. If you've got a lot of energy, you can go clubbing. If you've got no energy, you just feel like slumping down in a chair and watching TV.



Sometimes when energy is transferred, a force acts which moves something: lifting a book, cycling uphill, walking up stairs, kicking a football. The word 'work' in science has a special meaning. We say that work is done on an object when a force moves that object.


Getting to grips with the amount of energy.

Image of James Prescott Joule

James Joule

The unit in which energy is measured is called a joule (J). The amount of energy (in joules) transferred per second is measured in watts (W). One watt is therefore the transfer of one joule every second. Power stations output many megawatts (millions of watts).


Take a look at this table of energy and compare the different amounts of energy produced.

Image of James Watt

James Watt



Energy to lift a book from the floor to the table


Energy of a bullet leaving a gun


Average energy used by your body in a day


Energy provided by 1 gallon of petrol in a car


Energy from 1 ton of coal


Energy in a hurricane


Energy received on Earth from the Sun each second


Energy of a severe Earthquake


World energy consumption in a year


Energy received on Earth from the Sun each day



Image of the Sun taken by YOHKOHEvery second, the Sun radiates 4 x1026 J of energy.
That's 400,000,000,000,000,000,000,000,000 joules per second or

400 billion billion megawatts!


Only a small fraction of this energy actually reaches the Earth (about 2x1017 joules each second), but that's still a vast amount of energy. All the power stations in the world put together produce less than 0.4% of that!


[...did you know? - Every second, the Sun gives out more energy than the total amount of energy humans have used throughout history.]

[...did you know? - The contents of your average dustbin could be used to produce enough energy for you to watch TV for 5,000 hours!]




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