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⚛️ NEET Physics❓ Question

Fission and fusion both release energy but one splits and one joins. How does the binding energy curve explain it?

Nisha Verma12th Sample

1 Oct 2026 · 1 min read

This is making me crazy, and I know it's a classic NEET concept so I can't skip it.

The binding energy per nucleon curve rises steeply for light nuclei, peaks near iron (A around 56) and then falls slowly for heavy nuclei. NCERT says fission of a heavy nucleus releases energy because the daughters are more tightly bound per nucleon. And fusion of light nuclei also releases energy for the same reason. I get it when I stare at the graph, but when someone asks me in two sentences why both processes release energy I freeze.

I also have specific doubts:

  1. Where does the roughly 200 MeV per fission come from numerically? NCERT gave something like 7.6 MeV per nucleon for A∼240A\sim240 and 8.5 MeV per nucleon for the fragments.
  2. Moderator vs control rod: I keep swapping what each one does.
  3. Why does fusion need such a high temperature, when the Sun's core is only around 10710^7 K and the barrier calculation gives something much higher?

Answers with tiny worked numbers please. I learn from numbers.

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6 answers & replies

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  • [+]Rahul DubeyDropperSample· 18d ago

    Two-sentence answer: energy is released whenever nucleons end up more tightly bound than they started, meaning higher binding energy per nucleon. Below A ≈ 56 you gain by joining (fusion), above it you gain by splitting (fission). The iron region is the bottom of the energy valley, you can't get more out of it.

    Your number for (1): a nucleus with A=240A = 240 has about 7.6 MeV per nucleon. Fragments with A≈120A \approx 120 have about 8.5 MeV per nucleon. Gain per nucleon is about 0.9 MeV, and there are 240 nucleons:

    240×0.9≈216 MeV≈200 MeV240 \times 0.9 \approx 216 \text{ MeV} \approx 200 \text{ MeV}

    That's the origin of 200 MeV.

  • [+]Farhan ShaikhDropperSample· 18d ago

    Moderator and control rod: the moderator (graphite or heavy water) absorbs the extra neutrons so the chain reaction doesn't blow up. Control rods hold the fuel in place. I always remember the 'm' - moderator = moderates the reaction, so it must be the one controlling the speed. I made this trick last year and it has worked for me (so far).

    • [+]Kavya NaikDropperSample· 18d ago

      Farhan, those two got switched. Moderator (heavy water, graphite) slows fast neutrons to thermal speeds, because slow neutrons are much more likely to be captured by U-235 and cause fission. The thing that absorbs neutrons to control the rate is the control rod (cadmium or boron). Use it to hold the multiplication factor kk at 1: k>1k > 1 is supercritical, which is a bomb or a runaway, k=1k = 1 is steady (reactor), k<1k<1 dies out.

      • [+]Nisha Verma12thSample· 18d agoAuthor

        oh that is nice, I truly had them swapped like Farhan. 'Moderator slows, rods absorb'. Thank you all, this is going into my formula sheet.

  • [+]Tushar PawarDropperSample· 18d ago

    For (3): both nuclei have positive charge, so there's a Coulomb barrier, for two protons about 400 keV (NCERT figure). Converting with K∼kBTK \sim k_BT gives roughly a few 10910^9 K for a classical collision. In the Sun's core, T is only around 1.5×1071.5\times10^7 K, so most nuclei can't actually climb the barrier. Fusion still happens because some nuclei sit in the high-energy tail of the speed distribution and quantum tunnelling lets them pass through the barrier. Also, the Sun's core is gigantic and dense, so even rare events add up.

  • [+]Yash Tiwari11thSample· 18d ago

    Not a full answer but a number that helped me: D + T → He-4 + n releases 17.6 MeV, spread over 5 nucleons that is 3.5 MeV per nucleon. Fission: 200 MeV over about 236 nucleons is only 0.85 MeV per nucleon. So per kg of fuel, fusion gives much more energy. But fission is easy to start and fusion is not (see Tushar's barrier point).

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