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# Use of E = MC2

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## Use of E = MC2

The existence of antimatter came as a surprise to physicists. This strange idea, however, reveals a much more fundamental idea about our universe: if particles of matter and antimatter can meet and annihilate producing **energy**, then **matter** itself can be **changed** into energy and vice versa!

Some scientists like to think of matter as being a 'frozen' form of energy. Under the correct conditions this energy can be recovered by 'unfreezing' the energy.

Einstein's famous equation **E = mc ^{2}** summarises this idea. To find out how much energy is produced when a certain mass is changed to energy ('unfrozen!') simply multiply the mass by the speed of light squared (c

^{2}). As the speed of light is very large a

**tremendous**amount of energy is released when a very small amount of mass is released.

* For example:* if 1 Kg of mass is changed into energy then 9 x 10

^{16}J of energy is produced (that's about enough to satisfy the energy needs of a the UK for a whole year!)

As energy can be changed into mass and vice versa we need to modify the familiar idea of the conservation of energy. We now simply have the conservation of mass/energy.

When a particle and its corresponding antiparticle meet they annihilate one another perfectly illustrating the idea of mass/energy conservation. The combined mass is converted into pure energy in the form of photons.

If the rest mass of the electron and positron is 9.1 x 10^{-31}kg and the speed of light is 3.0 x 10^{8}ms^{-1} the minimum gamma ray energy is found using E = mc^{2}.

E = 2 x (9.1 x 10^{-31}) x (3.0 x 10^{8})^{2} = 1.638 x 10^{-12} J = 1.02 MeV