The Conditions for Fusion (Edexcel IGCSE Physics): Revision Note

Fusion reactions in stars

• Stars are huge balls of (mostly) hydrogen gas

• In the centre of a star, hydrogen nuclei undergo nuclear fusion to form helium nuclei

• An equation for a possible fusion reaction is:

• Where (deuterium) and (tritium) are both isotopes of hydrogen

  • These are formed through other fusion reactions in the star

• Fusion reactions release a huge amount of energy

• The heat from fusion provides a pressure that prevents the star from collapsing under its own gravity

Forces acting on a stable star

The outward and inward forces within a star are in equilibrium. The central red circle represents the star's core, and the orange circle represents the star's outer layers

• In larger stars where the temperature gets hot enough, helium nuclei can fuse into heavier elements

Conditions for nuclear fusion

• Nuclear fusion can only occur when two nuclei get extremely close together

• The two conditions required for nuclear fusion are:

  • extremely high temperatures

  • extremely high pressures

• These conditions are required because of the electrostatic repulsion between protons

  • Since protons are positively charged, they repel each other

  • To overcome this repulsion and allow the protons to get close enough to fuse, they must be moving very fast — this means they need very high kinetic energy

Electrostatic repulsion between protons

Hydrogen nuclei are positively charged protons which repel one another, making it difficult to achieve fusion under normal conditions

• For hydrogen nuclei (protons) to travel fast enough to fuse, the gas has to be heated to millions of degrees

  • Such high temperatures are usually only achievable in the cores of stars

  • The higher the temperature, the faster the nuclei move, and the more energy they have to overcome electrostatic repulsion, and the closer together they can get

• In regular conditions, such as on Earth, where temperatures and pressures are low, the possibility of collisions between nuclei which result in fusion is significantly lower

  • To increase the number of collisions (and hence fusion reactions) that occur between nuclei, high densities (and hence pressures) are also needed

  • The higher the pressure, the smaller the space the nuclei are forced into, so the more likely they are to collide