Basically, from mass.
Okay, this gets kind of complicated. In nuclear fusion you have two light atomic nuclei slamming together to create a heavier nucleus. Energy and other by-products are released in the process. But how does it work?
If you weigh* the individual protons and neutrons needed to make a particular atom, you'll find that their total mass is actually greater than the atom itself. When protons and neutrons combine to form atomic nuclei, they lose some mass! But where does that mass go?
The "missing" mass is called the mass defect, and it turns into energy. This energy is called the binding energy. If you wanted to separate an atomic nucleus into its parts, you'd have to give that energy back (after which it would turn into mass and make the protons and neutrons collectively a bit heavier than the nucleus they came from.
When hydrogen nuclei combine to form helium (as happens in the center of the Sun), the resulting nucleus is a bit lighter than the nuclei that combined to form it. The missing mass is converted to energy, and that's why we have sunlight!
What about nuclear fission? Well, as it turns out, you only get energy from fusing nuclei if the nuclei are lighter than iron. Fusing iron, or any element heavier than iron, actually ABSORBS energy, because the resulting nucleus is a bit HEAVIER than the sum of the two individual nuclei. However, you CAN release energy by splitting the nucleus. The principle works the same as it does in nuclear fusion, but in reverse; since the daughter nuclei are collectively lighter than the parent nucleus, some of the missing mass turns into energy.
So there you go: fusing nuclei lighter than iron or splitting nuclei heavier than iron releases energy. Trying to do the opposite in either case will absorb energy. I hope that helps. Good luck!
* I realize that a few pedants will complain about my answer because I've used "mass" and "weigh" interchangeably. They are correct: mass and weight are NOT the same thing. You know that, I know that, they know that. Everybody knows that...right?
Okay, this gets kind of complicated. In nuclear fusion you have two light atomic nuclei slamming together to create a heavier nucleus. Energy and other by-products are released in the process. But how does it work?
If you weigh* the individual protons and neutrons needed to make a particular atom, you'll find that their total mass is actually greater than the atom itself. When protons and neutrons combine to form atomic nuclei, they lose some mass! But where does that mass go?
The "missing" mass is called the mass defect, and it turns into energy. This energy is called the binding energy. If you wanted to separate an atomic nucleus into its parts, you'd have to give that energy back (after which it would turn into mass and make the protons and neutrons collectively a bit heavier than the nucleus they came from.
When hydrogen nuclei combine to form helium (as happens in the center of the Sun), the resulting nucleus is a bit lighter than the nuclei that combined to form it. The missing mass is converted to energy, and that's why we have sunlight!
What about nuclear fission? Well, as it turns out, you only get energy from fusing nuclei if the nuclei are lighter than iron. Fusing iron, or any element heavier than iron, actually ABSORBS energy, because the resulting nucleus is a bit HEAVIER than the sum of the two individual nuclei. However, you CAN release energy by splitting the nucleus. The principle works the same as it does in nuclear fusion, but in reverse; since the daughter nuclei are collectively lighter than the parent nucleus, some of the missing mass turns into energy.
So there you go: fusing nuclei lighter than iron or splitting nuclei heavier than iron releases energy. Trying to do the opposite in either case will absorb energy. I hope that helps. Good luck!
* I realize that a few pedants will complain about my answer because I've used "mass" and "weigh" interchangeably. They are correct: mass and weight are NOT the same thing. You know that, I know that, they know that. Everybody knows that...right?
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