Nuclear Fusion

Nuclear fusion, described in NCERT, is the joining of light nuclei into a heavier nucleus, releasing energy because the product sits higher on the binding-energy-per-nucleon curve. However, both fusing nuclei are positively charged and strongly repel each other through the Coulomb force at large separations. Only if they collide with very high kinetic energy can they approach closely enough for the short-range attractive nuclear force to take over, so extremely high temperatures, giving nuclei enormous thermal speeds, are required. The option about ionisation is wrong because at stellar core temperatures matter is already fully ionised; that is not the barrier to fusion. The option that heat raises binding energy is wrong because binding energy is fixed by nuclear structure, not temperature. The option about creating neutrons is wrong because fusion of protons proceeds without needing pre-made neutrons; instead protons themselves convert during the reaction chain. As stated in NCERT Class 12, Chapter 13 (Nuclei), such thermonuclear fusion powers the Sun and stars through the proton-proton cycle, releasing energy as light nuclei climb toward the binding-energy peak. A plausibility check: the millions of kelvin in the solar core supply exactly the thermal energy needed to breach the Coulomb barrier, confirming that high temperature, and not ionisation or neutron supply, is the true requirement.