How can energy be extracted from both these processes?

In the case of fission, we are splitting atoms and the energy released from the bond that was broken can be extracted. In other words, the energy it took to smash two H atoms together to form He can be recovered. That part seems okay to me.

But for fusion, how can the opposite be true? How do you gain energy from smashing to He atoms together?

I must be missing something basic.


Fusion is smashing hydrogen together to get helium, not smashing helium. When hydrogen is fused to make helium, a small amount of the original hydrogen mass is converted to energy, not helium.

In fission, breaking down one element into a lesser one, the energy comes not from their bonds, but in the loss of mass into energy by the transformation.

In each case, the conversion of one form of matter into another is accompanied by the conversion of a small amount of said matter into energy.

Doubtless a SDMB physicist/physical scientist will clarify soon.

Well, there are four fundamental forces: gravity, strong nuclear, weak nuclear, and electro-magnetic. Normally electronic forces would keep you from smashing the two hydrogen nuclei together, but once there is enough pressure and energy (in various forms… like in the sun) to smash the nuclei really close together, the strong nuclear force actually makes them want to stick together. I’m not sure how much you know about physics… but it’s kind of like in chemistry. When an electron drops to a lower energy level in an atom it gives off a photon. Similarly, when the nuclei join together their potential energy is turned into some other form of energy. This works up until about Iron, when the nuclei start to get so big that it actually takes more energy to smash them together than you get back when they stick together. Thats why we use fision on heavy elements like uranium, and fusion is done with hydrogen and helium.

Just to add on, Qadgop is correct too. Whenever energy is exchanged a small amount of mass is too. Hence E=mc2. I remember once hearing that a large portion of the mass of atoms if contained in the enegry inside them. Like, if you weighed them, and didn’t account for energy of internal vibrations, etc, it would be less than your typical experimental value. I’m sorry, I described that poorly… but it was a hijack anyway.

I so mangled that OP I feel really dumb now. I know what I meant, I just didn’t type it well.

Fission: Separate He to two H

Fusion: Join two H to form He

My question, which was touched on in the answers given:

IT WAS MY UNDERSTANDING (NOW I KNOW IT WAS WRONG) that you could separate a He atom into two H atoms and recover some energy. Conversely, you could join to H atoms and form a He atom and recover some energy.

This seemed illogical to me, as it seemed you could keep splitting/fusing He/H indefinitely.

RyanD004 explained the part I was missing. Basically, whether you can gain energy from fusion or fission depends on the elements and the nuclear binding energy.

Do I have that a little better now?

Fission does not involve Hydrogen or Helium. You smash apart very large atoms, namely Uranium or Plutonium. There has to be a net gain of neutrons, i.e. one neutron hitting a Uranium nucleus frees up two or more other neutrons which hit other nuclei etc. That’s a sustained chain reaction. Its controlled in a nuclear reactor, its uncontrolled in a nuclear bomb.

Um,well. If you separate the He atom into two H, you do recover some energy. But it is less than it took to pull them apart, so basically you lose energy.

Okay, fix this in your mind: everything wants to be iron.

Isaac Asimov described iron as the “dead-end middle” in his essay of the same name. This element (in its most common form) has 26 protons and 29 neutrons, and is the most “relaxed” configuration known. You cannot force iron into a fusion or fission reaction witout expending more energy than you gain.

Elements that are lighter than iron can be fused and elements that are heavier than iron can be split, producing more energy than was required to start the reaction. Naturally, the elements at the extremes like hydrogen (the lightest) and Plutonium (the heaviest) are the best potential fuels. The fusion of hydrogen is the most energetic reaction known (not counting matter/antimatter). You could even fuse the resultant helium into heavier elements, but the energy you get back is much less, and although you can fuse elements together to create matter more massive than iron (i.e. gold or uranium), you lose a lot of energy in the process. Usually you require a supernova in a double star system to manage it.

Anyhoo, after cooking along for a few billion years, a star can squish together quite a bit of hydrogen into iron. When the star goes nova (which it does if it’s big enough), it creates a vast cloud called a “nebula”. Through simple gravity, much of this mass will condense in the center and as it does, its temperature will slowly rise. Eventually, that core temperature gets so high that hydrogen, already eager to “go iron” starts to undergo fusion and a star is born.

The “rocky” and “metallic” elements in the nebula that are not captured by the nascent sun also condense through gravity. In our solar system, the inner planets formed out of pockets of (mostly) iron and other massive elements. Lighter elements, like hydrogen, were kept in gaseous form by the sun’s heat. Only much further away could gasses stay frozen solidly enough to form planets, whence the “gas giants”.

The reason by fission and fusion work, and why Earth’s core is mostly iron, is the same reason televison and movies frequently suck: everyone is striving for mediocrity. So there is a cosmic significance to Charlie’s Angels and Veronica’s Closet after all.

Isaac Azimov’s book Understanding Physics Vol. III, Chapter 11 on Nuclear Structure clearly explains the stability of nuclei and fission/fusion for the lay reader.

Note: You can fission He to get H . Atom smashers do it all the time. But it is not energy efficient. Ditto people build up trans-iron elements all the time to get heavier elements via fusion (as mentioned).

What elements are involved in fission/fusion is immaterial. (Except you obviously can’t fission H, etc.) The “Curve of Binding Energy” (the OP should Google on that phrase) does tell you whether you are getting a net gain/loss of energy though.