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    In the main text we talked about gravitational energy (gravitational fields). Similarly we can talk in terms of chemical energy (electromagnetic fields) and nuclear energy (nuclear fields).

    Chemical energy is released when the electrons round atoms are rearranged to form molecules thus binding the atoms more tightly; thus we get energy out of a burning bonfire or heat is released during a chemical reaction in a test tube.

    Nuclear energy is released when the particles within the nucleus move into a more stable state as happens when light elements fuse to form heavier elements (fusion), or when very heavy elements break up into less heavy elements (fission). Fusion occurred in the hot early Universe and continues to occur in the cores of stars converting hydrogen into helium and ultimately to the most stable nucleus, iron. Hence the long term future for the universe is for everything to end up as iron.

    In general nuclear forces are stronger than electromagnetic forces which are stronger than gravitational forces. Hence nuclear power stations are more efficient than coal or oil burning power stations. However the gravitational field close to a black holes in the Universe can be extremely strong and vast amounts of energy can be released by matter falling into the black hole (and so firmly attached it can never escape!).

    So, in general when matter is brought closer together in a more stable configuration, energy is released from the system. When matter is being forced apart to become less stable, energy has to be put into the system.

    Einstein's equation equates energy with mass (E=mc2). Would you expect that the mass of the system changes when there is a change in the binding energy?

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The University of Birmingham 

Physics and Astronomy Department, The University of Birmingham