Physics · Ch 13 — Nuclei
Nuclear Fusion
Nuclear Fusion
NUCLEAR FUSION is the process, in a sense the mirror image of fission, in which two light nuclei combine (fuse) together to form a single heavier nucleus, again releasing a very large amount of energy -- because, exactly as the curve of Section 8.7 shows, the single heavier product nucleus is MORE tightly bound per nucleon than the two separate light nuclei were before they fused.
Fusion, however, is far harder to bring about than fission, for a very direct physical reason: every nucleus carries a POSITIVE electric charge, so two nuclei approaching each other feel a strong, repulsive Coulomb (electrostatic) force pushing them apart, long before they get anywhere near close enough for the short-range, attractive strong nuclear force to take over and bind them together. Overcoming this Coulomb repulsion requires giving the nuclei an enormous amount of kinetic energy -- which, in bulk matter, means raising the material to an EXTREMELY HIGH TEMPERATURE, typically of order to , at which nuclei move fast enough, on average, to occasionally get close enough to fuse despite their mutual repulsion. Because such enormous temperatures are needed to trigger it, fusion is often called a THERMONUCLEAR reaction. …
What this figure shows. A schematic diagram of the Sun's core shown as a large circle filled with a bright, mottled texture suggesting extreme heat and density, with a small labelled inset zoomed in on a tiny region of it. Inside the inset, several small filled circles labelled 'p' (protons/hydrogen nuclei) are shown converging from different directions and colliding, with short curved arrows indicating a multi-step sequence: two protons fuse first (releasing a positron and a neutrino, each drawn as a tiny labelled dot flying away), the resulting nucleus fuses with a further proton, and finally two of the intermediate nuclei produced this way combine, with the whole chain's NET effect summarised in a boxed equation beneath the inset: '' (four hydrogen nuclei combining, net, into one helium nucleus, releasing energy). A wavy-line arrow labelled 'energy (light and heat)' leads outward from the core region to the edge of the large Sun circle, representing this fusion energy eventually radiating outward from the Sun's surface as the …