Energy Released in Fission and Fusion
Splitting a heavy nucleus and joining two light ones both move the products up the binding energy per nucleon curve, and the mass lost turns up as energy. The size of the release follows straight from E = mc squared.
What a learner can do afterwards
- Calculates the energy released from the mass difference across a stated reaction
- Explains from the curve why fission suits heavy nuclei and fusion suits light ones
- Says why fusion needs extreme temperature, in terms of electrostatic repulsion
1 · Read
Fission splits a heavy nucleus into lighter fragments, while fusion joins light nuclei into a heavier one. Both moves shift the products up the binding energy curve. Weigh the starting nuclei and the products: the lost mass times c squared is the energy set free.
Picture a uranium nucleus struck by a neutron. It wobbles, splits into two lighter fragments, and throws off extra neutrons. Add up both sides and the fragments weigh less in total. That missing mass appears as the fragments' motion plus radiation, through E equals m c squared.
Fusion demands extreme temperature for one reason: charge. Both nuclei are positive, so they repel each other electrically. Only very high speeds can push them close enough for the short range attraction to grab hold. Stars manage it with crushing heat; labs must copy those conditions.
Let the curve pick the technology. Fission suits heavy nuclei because splitting climbs toward the iron peak. Fusion suits light nuclei because joining climbs the same peak from below. Past iron in either direction slides downhill and would swallow energy instead.
Split heavy nuclei or join light ones, and the climb toward iron pays out the lost mass as energy.
More to read
22.4 Nuclear Fission and Fusion
Both fission and fusion pay out energy, and the binding energy curve explains why. Split a heavy nucleus and the fragments land higher on the curve, meaning tighter binding. Join two light nuclei and the merged nucleus also lands higher. Either way the difference in binding shows up as released energy plus fast moving products. To put a number on it, weigh everything before and after the reaction. The total mass always drops a little, and that missing mass times c squared is the energy the reaction sets free. Fission breaks heavy nuclei apart because that is the direction that climbs the curve, while fusion builds light nuclei up for the same reason. Fusion has one extra hurdle: the nuclei are all positively charged and push each other away. Only at extreme temperatures do they move fast enough to slam together before repulsion wins, close enough for the short range strong force to grab hold.
32.6 Fission
A heavy nucleus struck by a slow neutron can wobble, stretch, and snap in two. The two fragments fly apart carrying most of the released energy as motion, and a few extra neutrons spray out as well. Those spare neutrons can each split another nucleus, which is the chain reaction behind both reactors and weapons. Control rods soak up just enough neutrons to keep the chain steady at one split per split. Without control, the chain doubles and redoubles and the energy escapes all at once. The mass of the fragments plus the freed neutrons is always slightly less than the starting mass, and the gap becomes the energy output.
10.6 Nuclear Fusion
Stars run on fusion. Deep inside the Sun, protons crash together and build helium step by step, and each step turns a little mass into light and heat. The pace is set by the hardest step, getting two repelling protons to touch, which is why the Sun burns slowly enough to last billions of years. Heavier stars burn faster and climb further, fusing carbon, oxygen, and more, up to iron. Past iron, fusion would swallow energy instead of freeing it, so the heaviest elements are born in the violence of dying stars instead. Human fusion machines chase the same fire with heavy hydrogen, still fighting to hold the hot plasma steady long enough to win back more energy than heating costs.
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Where it sits
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.