Edexcel IGCSE Physics: Double Science 复习笔记:7.3.4 Nuclear Fusion

Edexcel IGCSE Physics: Double Science 复习笔记:7.3.4 Nuclear Fusion

Nuclear Fusion


  • Small nuclei can react to release energy in a process called nuclear fusion
  • Nuclear fusion is defined as:


When two light nuclei join to form a heavier nucleus

  • This process requires extremely high temperatures to maintain
    • This is why nuclear fusion has proven very hard to reproduce on Earth


  • Stars, including the Sun, use nuclear fusion to produce energy
    • Therefore, fusion reactions are very important to life on Earth


  • In most stars, hydrogen atoms are fused together to form helium and produce lots of energy


Two hydrogen nuclei are fusing to form a helium nuclei




  • The energy produced during nuclear fusion comes from a very small amount of the particle’s mass being converted into energy
  • Albert Einstein described the mass-energy equivalence with his famous equation:


E = mc2

  • Where:
    • E = energy released from fusion in Joules (J)
    • m = mass converted into energy in kilograms (kg)
    • c = the speed of light in metres per second (m/s)


  • The amount of energy released during nuclear fusion is huge:
    • The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal





Conditions for Fusion


  • Since protons have a positive charge, they repel each other
    • 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 conditions for fusion 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





Energy from Fusion

  • The main reasons why fusion is not currently used as a source of power on Earth are the difficulties in achieving (and maintaining)
    • High temperatures
    • High pressures


  • Whilst physicists have been able to attain the temperatures and pressure needed, there are difficulties in containing them, which inevitably means that only a small amount of fusion can take place
    • Such a small rate of fusion is not useful for current energy needs


  • Creating the temperatures needed for fusion requires a great deal of energy
    • Hence, physicists are still a long way from the point where they will produce more energy from fusion than the energy needed to start it



Worked Example

An example of a hydrogen fusion reaction which takes place in stars is shown here.

Which of the following is a valid reason as to why hydrogen fusion is not currently possible on Earth?A   Hydrogen fusion produces dangerous radioactive wasteB   Hydrogen nuclei require very high temperature to fuse togetherC   Hydrogen is a rare element that would be difficult to get large amounts ofD   Hydrogen fusion does not produce enough energy to be commercially viable



    • Hydrogen nuclei have positive charges
    • So two hydrogen nuclei would have a repulsive force between them
    • High temperatures are required to give the nuclei enough energy to overcome the repulsive force
    • The answer is not A because the products of the hydrogen fusion shown in the reaction is helium<
      • Helium is an inert gas


    • The answer is not C because hydrogen is a very abundant element
      • It is the most common element in the universe


    • The answer is not D because hydrogen fusion would produce a huge amount of energy




Fusion vs Fission


  • Fusion and fission reactions should not be confused
  • We are able to use fission reactions on Earth as a source of energy for producing electricity
  • However, the temperatures required to maintain fusion reactions means that it is not yet a commercially viable method for generating electricity
  • If however it were possible, then fusion would have a number of advantages over fission:
    • Fusion reactions would be capable of producing even more energy than fission reactions
    • The fuels required for fusion reactions are more plentiful than the Uranium and Plutonium used in fission reactors
    • The products of nuclear fusion are much safer, with very little long term radioactive waste produced, and the risk of a dangerous accident at a fusion reactor is much lower than in a fission reactor





  • The following table summarises some of the key differences between fusion and fission:


Comparison of Nuclear Fusion and Fission Table