nuclear fusion
Summary
Nuclear fusion is the joining of two light nuclei into a heavier nucleus, releasing energy because the product has a higher binding energy per nucleon. It powers stars and needs extreme temperature to overcome the repulsion between positive nuclei.
Detail
Meaning: Fusion combines light nuclei, such as hydrogen isotopes, into a heavier nucleus. The product is more tightly bound per nucleon than the reactants, so the total mass drops slightly (the mass defect) and that mass appears as released energy. This holds for nuclei lighter than roughly iron (mass number near 56); fusing much heavier nuclei absorbs energy instead. Fusion needs very high temperature and pressure because nuclei are positively charged and repel each other. They must get close enough for the short-range strong nuclear force to take over. Stars fuse hydrogen into helium.
Equation: E = Δm c², where E is the energy released (J), Δm is the mass of the reactants minus the mass of the products (kg), and c = 3.00 × 10⁸ m/s. Example: ²H + ³H → ⁴He + n + energy.
On the MCAT: A passage gives a fusion reaction and particle masses, and you find Δm and then E. Or you read the binding energy per nucleon curve: fusion releases energy for light nuclei on the rising left side, and fission releases energy for heavy nuclei on the right side of the peak. You may also balance mass number and atomic number in a reaction.
Common confusion: Students say mass number is lost in fusion. Nucleon count (mass number) and charge are conserved; only the rest mass, a tiny amount, decreases. Also, do not mix up fusion with fission, which splits a heavy nucleus.
Sources
- Physics (Cutnell and Johnson)
- Physics (Giancoli)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.