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
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Combustion is the combination of oxygen with carbon or hydrogen. That is not splitting – splitting is usually a necessary step but the splitting *costs* energy.
This is generally true for chemistry. Splitting costs energy, combining releases it.
In the nucleus, there are two forces. One that pulls together, one that pushes apart. Whichever one is stronger (on average) can give us energy, but we have to defeat the weaker one first. In fission the stronger force is electrostatics – the pushing apart. In fusion the stronger force is the nuclear strong force – the pulling together.
Atoms in the middle experience the same strength from both, and so we can’t get energy from them at all.
Your assumption is wrong. We get energy by going from a higher energy state to a lower energy state.
Combustion isn’t splitting molecules, but rather rearranging them: if you burn methane you go from CH4 + O2 to CO2 + H2O. The resulting molecules “contain” less energy than the second ones, the difference gets released.
For fission and fusion it’s the same. You start from something (a heavy atom or two hydrogens) and end up with something else that contains less energy in total, the difference gets released.
It all depends on what you’re splitting of fusing. In an atomic nucleus the particles they consist of are bound together by the Strong Nuclear Force. This force can only act on things that are very close together. With a small nucleus, all of the particles are close enough to each other that the Strong Nuclear Force will act on all of them at once. In a larger nucleus though, some of the particles are far enough apart that the Strong Nuclear Force on one side of the mass won’t affect those on the other side. Other forces, such as electrostatic repulsion and the Weak Force, begin to have an effect instead.
So, when you fuse two deuterium nuclei (hydrogen with a proton and neutron), you get some energy out of it because their particles are bound more closely. In something like Uranium-236, the particles are so weakly bound that they’ll quickly fly apart into Barium and Krypton and a bunch of free neutrons, again releasing a lot of energy.
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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.
Atoms and molecules want* to be at their lowest energy state. For atoms this means that every atom wants to be an Iron-55 atom (Iron always has 26 protons, Iron-55 has 29 neutrons).
Atoms that are lighter than this (like hydrogen and helium) can release energy through fusion. Atoms that are heavier than this can release energy through fission.
*They don’t really want anything, but the laws of the universe leads to this.
P.S: Combustion is not about splitting molecules. It’s about recombining molecules into a combined molecule that has a lower energy state. Burning gasoline for example means that the bond between carbon and hydrogen requires more energy than the bond between Carbon Dioxide. So by introducing enough heat to break the bonds and then allowing them to recombine with oxygen you get more heat.
No one has actually bothered to answer your question..
The reason why is because of the famous equation that we’ve all heard of – E=mc^2. You may of heard of that equation but what does it actually mean?
Well what it means is that energy is equal to mass multiplied by the speed of light squared. Ok… so what does *that* mean? What that means is that there is an equivalence between energy and mass, where one can be converted into the other. Not only can mass be converted into energy, but because of the conversion rate even the smallest bits of mass can be converted into a *fuckton* of energy. To give you a sense of what I mean, if you were to convert a paperclip into pure energy it would result in an explosion larger than the bomb dropped on Hiroshima or 20x more powerful then the blast that took out Beirut in 2020.
What’s happening with fusion specifically is that you’re essentially taking two little things and squishing them into one big thing, but that one big thing has less mass than the sum of the two individual little things. That left over mass gets converted into energy and because of the reasons explained above even if those two little things are a pair of hydrogen atoms that itty bitty difference in mass results in an enormous release of energy.
You know how some chemical reactions are endothermic and some are exothermic? You can burn gasoline and it releases lots of heat, but if you react vinegar with baking soda it gets cold.
It depends on how much energy is holding the reactants together, and how much energy is holding the products together. The difference is exchanged with the environment. You can’t really predict which way it will go unless you’re familiar with the particular reaction or you have knowledge of the energies involved.
It’s the same thing with atomic nuclei. They start small, and from there it’s energetically favorable for them to combine. By the time they get as big as iron, their size outweighs the short-range forces holding them together, and above that size it’s energetically favorable for them to split up.
For fission and fusion, iron is the line where it switches. Breaking apart atoms heavier than iron release energy, but fusing atoms smaller than iron also releases energy. Combustion and such are all chemical processes and involve electric bonds, whereas the nuclei are a different force altogether.
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