Edexcel IGCSE Physics: Double Science 复习笔记:7.3.5 The Conditions for Fusion

Edexcel IGCSE Physics: Double Science 复习笔记:7.3.5 The Conditions for Fusion

Fusion in Stars


  • Stars are huge balls of (mostly) hydrogen gas
  • In the centre of star, hydrogen atoms undergo nuclear fusion to form helium
  • The equation for the reaction is shown here:

Deuterium and Tritium are both isotopes of hydrogen. They can be formed through other fission reactions in the star



  • huge amount of energy is released in the reaction
  • This provides a pressure that prevents the star from collapsing under its gravity


The fusion of deuterium and tritium to form helium with the release of energy


  • In larger stars the helium that has been produced can then be fused to form heavier elements



The Conditions for Fusion


  • Within stars the atoms are ionised
  • This means that the electron that is normally on a hydrogen atom would be lost
  • A hydrogen ion, or a proton, is therefore positively charged
  • There exists a repulsive force between protons that has to be overcome for hydrogen fusion to happen


Hydrogen ions are protons, and their positive charge makes them repel one another




  • In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds
  • In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star
  • In regular conditions, ie. on Earth, the possibility of collisions between nuclei which result in fusion is very low
    • In order to increase the number of collisions (and hence fusions) that occur between nuclei, high densities (and hence pressures) are also needed


  • The difficulties in achieving and maintaining such high temperatures and pressures are the main reasons why fusion is not currently used as a source of power on Earth
  • The conditions for fusion therefore are:
    • Very high temperature of fuel
    • Very high kinetic energy / speed of nuclei to overcome repulsion
    • Very high density / pressure to increase the possibility of suitable collisions



Exam Tip

It is useful to remember that hydrogen is the fuel within stars, but the details of the reaction between deuterium and tritium is not required at this level