Binding energy, fusion and fission

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  • Thread starter Volta
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  • #1
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Hi.

My high school physics book doesn't elaborate the idea of binding energy and how it's related to fissions and fuisions adequately in a way that made me have wrong thoughts about these ideas.
What i understand after doing some research is that:
- Binding energy is the energy that has to be given for nucleons to separate them from each other.

- Nucleons in stable nucleus have negative energy considering the energy of a free static nucleon to be the reference energy.

My questions and confusions:

- in fissions, does the nucleus divide because of the collision between the neutron and the heavy nucleus, or because the nucleus would become unstable after the mass number has increased.

- this question is related to the above one ; neutrons are supposed to be the main factor of nucleus stability because it contributes in the strong force. why does adding a new neutron or more to any nucleus without changing the number of protons, make the nucleus unstable?

- My book mentions that when alpha decay happens ,a decrease in mass turns into kinetic energy gained by the products. does it mean the decrease of mass because of the lost neutrons and protons or what?

- the process of losing mass for energy and vice versas in fissions and fuisions, shouldn't the mass for the neutrons shot out of the mass increase and that's it? where would energy come from?
 

Answers and Replies

  • #2
PAllen
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Fission is possible because a nucleus may have negative binding energy relative separated nucleons, but there may exist smaller nuclei that have more (negative) bonding energy per nucleon. Thus, two smaller nuclei would have less total energy in their ground state then the on larger nucleus. More generally, nickel and iron have the highest binding energy per nucleon. Sufficiently smaller nuclei can exothermically fuse, and sufficiently larger nuclei can exothermically fission.
 
  • #3
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Sorry for late reply, but i think your answer didn't address my questions.
 
  • #4
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- in fissions, does the nucleus divide because of the collision between the neutron and the heavy nucleus, or because the nucleus would become unstable after the mass number has increased.
Both. Note that spontaneous fissions also exist. Nucleus has to release energy by fission, but it also has to get through barrier, whether by tunnelling or due to excitation.
- this question is related to the above one ; neutrons are supposed to be the main factor of nucleus stability because it contributes in the strong force. why does adding a new neutron or more to any nucleus without changing the number of protons, make the nucleus unstable?
Because it excites the nucleus.
Nucleus might also be excited by something else, like absorbing a photon or inelastic collision with a charged particle. But the nuclei often emit the energy by gamma without fission. If and after this has happened, spontaneous fission is still possible, but far rarer.
 

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