E.4 · IB Physics
Binding energy curve
Iron sits on the summit. Fusion climbs towards it from the light side and fission slides towards it from the heavy side, and both release the difference in binding energy. Drag along the curve or use the slider.
Predict first
Press 'Iron-56 peak', then 'Uranium-235'. What happens to the binding energy per nucleon and to the total binding energy?
Iron-56 has about 8.79 MeV per nucleon and uranium-235 about 7.64 MeV, but 235 nucleons × 7.64 MeV (about 1800 MeV) exceeds 56 × 8.79 MeV (about 492 MeV).
Check it in the model: Press the 'Iron-56 peak' button, note the readouts, then press 'Uranium-235' and compare BE per nucleon and Total binding energy.
binding energy per nucleon Fe-56 peak · drag on the graph to move the marker
Total BE = (BE/A) × A
Fusion of two A/2 nuclei: Q = A × [BE/A(A) − BE/A(A/2)]
Fission into two A/2: Q = A × [BE/A(A/2) − BE/A(A)]
Try this
- Press 'Helium-4' and then move A to 2 and to 12. Why is helium-4 so far above its neighbours on the curve?
- Press 'Uranium-235' and read the fission energy released per nucleus. Compare it with the energy per nucleon released by fusion near A = 4.
- Move the marker from A = 20 to A = 56 and then to A = 238 and describe how BE per nucleon changes on each side of the peak.
Common error: a higher binding energy per nucleon means a more stable nucleus, not a less stable one, and energy is released when the products sit higher on the curve than the reactants.