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When an nucleus is split, what form of energy is released? All of the websites I have looked at say there is a lot of energy released when an atom is split, but it never says what form of energy it is in. Is it electrical energy, or is it just a lot of heat, or something else?

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A lot of different forms, but mostly kinetic energy.

A good table is given at Hyperphysics. The energy released from fission of uranium-235 is about 215 MeV. This is divided into:

  • Kinetic energy of fragments (heat): ~168 MeV
  • Assorted gamma rays: ~15-24 MeV
  • Beta particles (electrons/positrons) and their kinetic energy: ~8 MeV
  • Assorted neutrons and their kinetic energy: ~17 MeV
  • Neutrinos and their kinetic energy: ~?

Note that neutrons can, if left alone for long enough, decay, typically by this reaction: $$\text{n} \to \text{p} + \bar{\text{v}}_{\text{e}} + \text{e}^{-}$$ where $\text{n}$, $\text{p}$, $\bar{\text{v}}_{\text{e}}$, and $\text{e}^{-}$ represent neutrons, protons, anti-electron neutrinos, and electrons, respectively.

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    $\begingroup$ This list omits the neutrinos. That may be reasonable (if they are cataloging the energy available in a thermal pile, for instance, in which case neutrino energy escapes) or they may be folding that energy in with the charged leptons. $\endgroup$ – dmckee Jun 7 '15 at 23:38
  • $\begingroup$ @dmckee You're right; I missed that. $\endgroup$ – HDE 226868 Jun 7 '15 at 23:39
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    $\begingroup$ "A lot of different forms, but mostly kinetic energy" would also be a valid summary. Basically, the pieces go flying off in all directions. $\endgroup$ – hobbs Jun 8 '15 at 6:03
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    $\begingroup$ Worth explicitly noting, also, that the extreme kinetic energy imparted to the atomic fission products (ie: ~75% of the energy) can be generally considered as simply (and directly) heat. $\endgroup$ – J... Jun 8 '15 at 13:09
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    $\begingroup$ It is worth emphasizing that the 'energy' in beta particles and neutrons is also mostly kinetic energy. In the end, all of the energy will end up either as kinetic energy inside the sample, or as radiation coming out if it: beta if they get out, gamma most likely, and neutrinos. The kinetic energy of the fragments is not really "heat" as such - it is energy which is definitely not in thermal equilibrium. After the decay the sample will thermalize, by the fragments bouncing around other atoms, and this energy, once spread out, is (only) then what we think of as heat. $\endgroup$ – Emilio Pisanty Jun 8 '15 at 18:32
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The energy that is released when a the absorption of a neutron causes a heavy atom nucleus to fission into two daughter nuclei comes from the tighter binding energy of the two daughter nuclei compared to the weaker (smaller) binding energy of the original nucleus.

This extra energy is mostly released in the form of the kinetic energy of the two daughter nuclei. Therefore most of the energy is released as the equivalent of heat energy.

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The mass of an atom is always less than the sum of the masses of the particles that compose it. The lack of mass (or energy, from E = mc^2) is called binding energy and it is the energy expended by the particles to remain confined inside of the atom. When fission occurs, not more spending of energy to hold together the individual particles. So the energy that is released corresponds to the binding energy and it is distributed among the fission products in the form of kinetic energy.

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protected by Qmechanic Jun 8 '15 at 6:58

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