6.45P Explain why nuclear fusion does not happen at low temperatures and…

The idea

Fusion joins light nuclei, for example two hydrogen-1 nuclei forming deuterium, but each nucleus carries positive protons. Because like charges repel, the two nuclei feel an electrostatic force pushing them apart, and this force grows strongly as they get closer. Nuclei must approach within about the range of the strong nuclear force, roughly 10⁻¹⁵ m, before that attractive force can overcome the repulsion and bind them. Getting that close needs the nuclei to collide with high kinetic energy, which in practice means very high temperature and high pressure or density. At low temperature the nuclei move too slowly to overcome the electrostatic barrier, and at low pressure they rarely meet, so fusion does not occur.

What earns the marks

• States that protons are positively charged and therefore repel each other by electrostatic force.
• Explains that nuclei must be brought very close together, within the short range of the strong nuclear force, before attraction can dominate.
• Links low temperature to low kinetic energy of nuclei, so they cannot overcome the electrostatic repulsion.
• Links low pressure to a low collision rate between nuclei, so fusion events are rare.

Common mistakes and how to put them right

• Claims that fusion needs no energy input because it releases energy.

  Put it right by noting that energy must first be supplied to overcome repulsion.

• Says gravity alone pulls nuclei together in a laboratory.

  Put it right by identifying the strong nuclear force as the short-range attraction that must win.

• Confuses fusion with fission by describing a heavy nucleus splitting.

  Put it right by describing light nuclei joining.

• States that low pressure increases repulsion.

  Put it right by linking pressure to collision frequency rather than to the electrostatic force.

Exam tips

• Name the force explicitly as electrostatic repulsion between protons, not just 'repulsion'.
• Use the phrase 'high kinetic energy' when explaining why high temperature is needed.
• Keep the answer to the two conditions named in the question: temperature and pressure.
• Check that you have explained why fusion fails at low values, not just why it works at high values.