The Physics of the Atomic Bomb (Level 4)

Level 4: Nuclear Fusion

The binding energy of a nucleus is the energy needed to separate a nucleus into its respective individual nucleons and is proportional to the stability of the nucleus. For atoms lighter than Nickel-62, binding energy and also the stability of the nucleus increases as atomic mass increases. Energy given off by hydrogen bombs arise from nuclear fusion in which lighter isotopes such as deuterium and tritium ‘combine’ into a much stable element such as helium. The energy released during nuclear fusion is the difference between the binding energies of the isotope used in the reaction and the fission products. The energy released can be calculated using Einstein’s mass-energy equivalence E = Δmc², where Δm is the difference in mass between the start and end nucleus and c is the speed of light (3 x 10⁸ m/s).

However, nuclei are positively charged due to the presence of positively charged protons. Extremely high temperatures are required for the positively charged nuclei to overcome their mutual electrostatic repulsion and gain enough kinetic energy to fuse.

This diagram compares examples of nuclear fission and nuclear fusion.