Back to the full dot-point answer
NSWPhysicsQuick questions
Module 8: From the Universe to the Atom
Quick questions on Nuclear fission, fusion and binding energy: HSC Physics Module 8
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is mass defect and binding energy?Show answer
The mass of a nucleus is always slightly less than the sum of the masses of its constituent protons and neutrons. The difference is the mass defect:
What is binding energy per nucleon?Show answer
The bound state of a nucleus is more meaningful when normalised by the number of nucleons:
What is rule for releasing energy?Show answer
A nuclear process releases energy if the products have higher binding energy per nucleon than the reactants. Geometrically, this means the reaction moves the nucleons "uphill" on the binding-energy-per-nucleon curve, toward the iron peak.
What is nuclear fission?Show answer
A heavy unstable nucleus splits into two medium-mass fragments and a few neutrons. Example, induced fission of uranium-235 by a thermal neutron:
What is nuclear fusion?Show answer
Two light nuclei combine into a heavier one. Example, deuterium-tritium fusion (the easiest practical fusion reaction):
What is worked example?Show answer
Calculate the energy released. Atomic masses: $m(^2$H$) = 2.01410$ u, $m(^3$H$) = 3.01605$ u, $m(^4$He$) = 4.00260$ u, $m(n) = 1.00867$ u.
What is treating mass defect as a loss of nucleons?Show answer
All nucleons are still present after the reaction; the mass loss is in the bound system as a whole, converted to kinetic energy and radiation.
What is mixing up fission and fusion?Show answer
Fission = heavy nucleus splits; fusion = light nuclei combine. Both move nucleons toward the iron peak and both release energy.
What is using non-relativistic kinetic energy at nuclear scales?Show answer
Fission fragments and fusion products often have several MeV of kinetic energy, well within the non-relativistic regime for the heavy fragments, but the relativistic energy budget $E = m c^2$ is the consistent accounting tool.
What is saying fusion takes "no fuel"?Show answer
Fusion fuel (deuterium, tritium, lithium) is needed; tritium in particular must be bred in the reactor blanket from neutron capture on lithium.