Physics · nuclei · NEET
Fusion happens with LIGHT nuclei (small mass number A, like hydrogen and its isotopes). Light nuclei sit on the left, low side of the binding-energy-per-nucleon curve. When they fuse, the product moves toward the peak (around iron), so binding energy per nucleon goes up and energy is released. Heavy nuclei do the opposite: they split (fission). Remember: light join, heavy split.
Both nuclei are positively charged, so they push each other away by Coulomb repulsion. To fuse, they must get very close (within the short range of the nuclear force). Only very fast nuclei can push through this Coulomb barrier. High temperature means high kinetic energy, so the nuclei move fast enough to touch. NCERT states the fuel must reach about 10^8 K, where matter becomes a plasma of ions and electrons.
The Coulomb barrier is the electric potential energy hill two positive nuclei must climb before the attractive nuclear force can take over and pull them together. Its height depends on the charges and radii of the two nuclei. Nuclei overcome it by having enough kinetic energy, which comes from very high temperature. This is why fusion is called a thermonuclear reaction — heat drives it.
In stars, the net proton-proton cycle turns four hydrogen nuclei into one helium-4 nucleus: 4 (1-H) to (4-He) + 2 neutrinos + gamma rays + 26.7 MeV. So about 26.7 MeV is released per helium-4 formed. This slow, self-sustaining fusion is the energy source of the Sun and all stars.
To copy the Sun we must heat fuel to about 10^8 K, where it becomes a plasma (ions plus electrons). No solid container can hold matter at that temperature. The hard part is confining the hot plasma long enough for fusion to happen, using magnetic fields or other methods. If solved, fusion reactors could give almost unlimited clean power — this is the goal of controlled thermonuclear fusion.
It is the joining of two light nuclei into one heavier nucleus with the release of energy, because the heavier nucleus is more tightly bound.
Very high temperature (about 10^8 K) and high density, so nuclei have enough kinetic energy to overcome the Coulomb repulsion and come within the short range of the nuclear force.
The proton-proton cycle. Its net effect is four hydrogen nuclei fusing to form one helium-4 nucleus, releasing about 26.7 MeV of energy.
Because the product nucleus has a higher binding energy per nucleon than the light starting nuclei. The extra binding energy appears as released energy, following E = mc^2.
An uncontrolled fusion reaction powers the hydrogen bomb. A controlled, steady fusion reaction is what scientists want in fusion reactors for peaceful power, which is still a major challenge.
