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Mass-Energy


When a transmutation occurs, the total atomic number and the total atomic mass number of all the particles involved in the reaction do not change. It appears that both mass and electric charge are conserved. There is another aspect of a nuclear reaction that we have not yet considered-that of the conservation of energy.

In all chemical reactions, energy is either liberated or absorbed. For example, the burning of gasoline liberates energy, and the separation of water into hydrogen and oxygen by electrolysis absorbs energy. Nuclear reactions can also liberate or absorb energy, but we are primarily interested in those that liberate energy, because when that happens the quantity liberated can be enormous. So much energy can be released, in fact, that the Law of Conservation of Energy appears to be violated. Where does all this extra energy come from?

The A-Bomb in action In 1905, Albert Einstein published his Theory of Relativity. In this important document, he suggested that mass is another form of energy-that a decrease in mass of a system might show up as an increase in the energy of the system. His equation relating energy and mass was:

E=mc2

where
E is the energy liberated, in joules
m is the mass that disappears, in kilograms
c is the speed of light, in metres per second

In 1932 Cockroft and Walton, using their accelerator, bombarded lithium with protons, creating two alpha particles for each proton.

The mass of the two alpha particles was found to be slightly less than the mass of the proton and the lithium nucleus together. It was found that the kinetic energy of the two alpha particles far exceeded the initial kinetic energy of the proton. Careful calculations showed that the extra energy possessed by the alpha particles exactly corresponded to the loss in mass, in accordance with Einstein's equation. The Law of Conservation of Energy, as it had been known before Einstein, had been violated: energy appeared to have been created. In fact, energy had not been created, but mass had been converted into energy.

Since 1932, hundreds of similar nuclear experiments have been devised to check Einstein's mass-energy relationship, and in each case it was found to be valid. The energy, because it usually originates in the conversion of nuclear mass into energy, is called nuclear energy.

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Last updated: 8 June, 1998
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