Nuclear Fusion (Oxford AQA IGCSE Physics): Revision Note

What is Nuclear Fusion?

• Small nuclei can react to release energy from their nuclear store in a process called nuclear fusion

• Nuclear fusion is defined as:

When two light nuclei join to form a heavier nucleus

• Nuclear fusion occurs in stars

• Hydrogen nuclei fuse together to make helium nuclei

• Massive amounts of energy are released in the process

Fusion of two nuclei

Nuclear fusion equations

• Equations can be used to represent nuclear fusion reactions

• For example, the fusion of a proton and a nuclei to form a helium nucleus can be written as:

  

• Where represents a proton

  • The mass number of a proton is 1 and the atomic number of a proton is 1

  • This is the same as a hydrogen nuclei, so a proton can also be represented by

Mass & Energy in Fusion

• The energy released 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:

• 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 (several times greater than nuclear fission):

  • The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal

Conditions Required for Fusion

• For two nuclei to fuse, both nuclei must have high kinetic energy

  • This is because the nuclei are positively charged, and when they come close to each other, they repel one another

  • If the nuclei have very high kinetic energy, the nuclei can travel towards each other at very high speeds, overcome the force of repulsion and the nuclei can fuse together

• The conditions for fusion are:

  • Very high temperature of fuel

    • This causes very high kinetic energy

    • Therefore the nuclei travel at very high speeds

    • Which enables them to overcome repulsion

  • Very high pressure

    • This causes high density of particles

    • Increasing the possibility of suitable collisions

Fusion of two protons

• 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 global energy demands

• 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

Fusion in stars

• Stars use nuclear fusion to release energy

• The temperature inside their core is so high that the electrons have been stripped away from their atoms, leaving the bare nuclei which are able to collide

• Stars are mainly comprised of hydrogen and helium

  • Hydrogen nuclei fuse together to form helium nuclei and release lots of energy