Question: What does E=mc2 actually mean and how do we use it?
Keywords: e=mc2
Comments

commented on 9 Jan 2017:
This answer is just plain wrong, the equation shows how much energy is produced from nuclear FUSION (I.E. what happens in the sun) NOT FISSION (what nuclear power plants do). If this answer was correct, we would LOSE energy from nuclear fission.

commented on 10 Jan 2017:
Both fission and fusion reactions result in a conversion of mass to energy. Fusion reactions start with light atoms and fuse them to heavier atoms where there is slightly less total mass in the products than the reactants. Fission reactions start with heavy atoms and split them into lighter atoms where the net mass of the products is also slightly less than the reactants. This is because the binding energy is highest for elements in the middle of the periodic table, see the nuclear binding energy curve as a function of the number protons an neutrons in an atom: https://en.wikipedia.org/wiki/Nuclear_binding_energy.

commented on 10 Jan 2017:
Hi Atto Allas,
For elements lighter than iron on the periodic table, nuclear fusion releases energy. For iron, and for all of the heavier elements, nuclear fusion consumes energy, but nuclear fission releases it.
However, both nuclear fission and nuclear fusion work on the principle of E=mc2 The give out energy by converting a small percent of the mass into energy. For instance: in nuclear fusion, if you take the mass of the fused atom and compare it to the 2 atoms you fused, the product will have fractionally less mass. In nuclear fission, the mass of the original atom will be slightly higher than the mass of all the products. In both cases this missing mass has been converted directly into energy.

Einstein’s most famous equation tells us that there is an equivalence between mass and energy. As you probably know E= energy, m= mass and c = the speed of light
As the speed of light is already a huge number, the speed of light squares (c^2) is mind bogglingly large = 89875517900000000 m2/s2
So in other words there is a huge amount of energy locked up in even a very small amount of mass (the energy equivalent of 1kg is 25 billion kWh!!)
Of course, we cannot access all of this energy, so what use is it??
Well, apart from being the subject of every Alevel physics exam paper for the last 50 years (!) it has a number of practical uses, including in nuclear power.
The nuclear reactors that we use to generate energy today are nuclear fission reactors – they take big atoms such as uranium and ‘split’ them, into much smaller atoms. When this happens there is a tiny mass change, and this mass is converted into energy according to E=mc2 – so the equation tells us how much energy we get out of our nuclear fission reactor (amongst other things!)
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