Why nuclear fusion results in energy?

Question

So every other kind of energy we can get is from splitting molecules (combustion) or splitting atoms (fission)
So how come we can get energy from fusing atoms together?

In: Physics

Answer 1

Potential energy and forces.

With splitting and combining molecules (which is how chemistry works) we are dealing with the *electromagnetic interaction*. Electrons want to be closer to protons, (and further away from other electrons) so if you split up and recombine molecules in a way that lets the electrons be happier (by being closer together) you get more energy out. You are lowering the potential energy of the electrons, and that energy has to go somewhere.

With splitting (fission) and combining (fusion) the nucleus of atoms we are dealing with the *strong interaction*. Protons and neutrons want to be close to each other (but not too close). So if we can take a bunch of protons and neutrons and find a way to combine them in a way that they are closer together than they were before, they will be happier, they will have less potential energy, so that energy can be freed up for other use.

To take the simplest example (a little over-simplified), if you have a single proton on its own, and a single neutron on its own (which you can’t really have, but never mind), and bring them close together, they will zoom together and stick to each other. Just like holding something and letting it fall to the ground. In both cases you get energy out because the system is moving to a lower energy state.

Fusion works when we take smaller nuclei and let them combine to form bigger ones where, on average, the protons and neutrons are closer together and so happier (lower energy state).

Fission works when we take bigger nuclei and let them split up into smaller ones where, on average, the protons and neutrons are closer together.

The details of how this works is pretty complicated (and quantum mechanics gets involved), but [this graph](https://en.m.wikipedia.org/wiki/File:Binding_energy_curve_-_common_isotopes.svg) gives a rough idea of how this works; on the horizontal axis you have the number of protons and neutrons in your nucleus, and on the vertical axis it tells you how happy (per proton and neutron) a particular nucleus is. So iron-56 is the most stable, happiest thing – it is just the right size. Anything bigger will want to get smaller, anything smaller will want to get combined into something bigger.