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Energy is a word with more than one meaning.

  • Mostly it is used in science to describe how much potential a physical system has to change. In physics, energy is a property of matter. It can be transferred between objects, and converted in form. It cannot be created or destroyed.
  • It may also be used in economics to describe the part of the market where energy itself is harnessed and sold to consumers.
  • In ordinary language, the word is used to describe someone acting or speaking in a lively and vigorous way.

Scientific energy

In science energy is something that can do work.

There are two basic forms of energy:

Conservation of Energy

Energy cannot be made or destroyed, it is changed from one form to another.[1] The amount of energy in a closed system is always the same.[1] This rule is called the "conservation law of energy".


Here is an example:

  1. the energy of a thing is measured to start with
  2. the energy changes from potential energy to kinetic energy and back again
  3. at the end the energy of the thing is measured again

The measurements of energy at the start and end will always be the same.

Conservation of matter and energy

Scientists now know that matter can be made into energy, and energy into matter, through processes like nuclear fission and nuclear fusion. The law of conservation of energy has therefore been extended to become the Law of conservation of matter and energy. Albert Einstein was the first to mathematically derive this in the formula E = mc2.


A stone is thrown upwards and falls to the ground.

  1. human throws the stone: energy store in muscles= chemical energy
  2. stone moves upwards= kinetic energy
  3. stone at the highest point= potential energy
  4. stone falls to ground= kinetic energy
  5. stone hits ground= thermal energy/sonic energy

Types of energy

Scientists have identified many types of energy, and found that they can be changed from one kind into another. For example:

Measuring energy

Energy can be measured. That is, the amount of energy a thing has can be given a number.

As in other kinds of measurements, there are measurement units. The units of measurement for measuring energy are used to make the numbers meaningful.

Some scientific units of measurement

The SI unit for both energy and work is the joule (J). It is named after James Prescott Joule. 1 joule is equal to 1 newton-metre. In terms of SI base units, 1 J is equal to 1 kg m2 s−2.

The energy unit of measurement for electricity is the kilowatt-hour (kW·h). One kW·h is equivalent to 3,600,000 J (3600 kJ or 3.6 MJ)

Related pages


  1. 1.0 1.1 Woolley, Steve. Edexcel IGCSE Physics Revision Guide. Pearson Education. p. 49. ISBN 9780435046736 .