Joining light nuclei
Definition
Nuclear fusion
The joining of two light nuclei to form a single heavier nucleus, releasing energy.
Definition
Electrostatic repulsion
The force that pushes two objects carrying the same type of charge apart, such as two positively charged nuclei.
- Nuclear fusion is the joining of two light nuclei to make a single heavier nucleus.
- The most common fusion reactions join nuclei of hydrogen isotopes to form helium.
- Every nucleus contains protons, so every nucleus is positively charged.
- Two nuclei therefore push each other apart by electrostatic repulsion, and this force gets stronger the closer they get.
- Fusion only happens if the nuclei get close enough to join, so they must collide fast enough to overcome this repulsion.
- At extremely high temperatures of millions of degrees Celsius, nuclei have enough kinetic energy to collide at the speeds needed.
- A very high density is also needed, so that collisions between nuclei happen often enough for fusion to keep going.
- At these temperatures atoms lose all of their electrons, so the material is a mixture of bare nuclei and free electrons.
Mass and energy in fusion
- The mass of the particles produced in fusion is slightly less than the total mass of the nuclei that joined.
- This missing mass has not disappeared, because it has been converted into energy.
- The energy is released as radiation and as kinetic energy of the particles produced.
- Only a tiny fraction of the mass is converted in each fusion, but a small amount of mass is equivalent to an enormous amount of energy.
- Fusion releases more energy per kilogram of fuel than fission does.
- This conversion of mass into the energy of radiation is the source of the light and heat given out by every star.
Example
Fusion of deuterium and tritium
- Deuterium and tritium are isotopes of hydrogen, with one and two neutrons in their nuclei.
- A deuterium nucleus and a tritium nucleus fuse to form a helium-4 nucleus and release a neutron.
- Add the mass numbers on each side to check that the number of nucleons is unchanged.
- Add the atomic numbers on each side to check that the charge is unchanged.
- The helium nucleus and the neutron together have about 0.4% less mass than the deuterium and tritium nuclei that fused.
- This missing mass has been converted into energy, which is carried away mainly by the fast-moving neutron.
Fusion in stars
- The Sun and other stars are powered by fusion in their cores, where the temperature and density are high enough.
- In the Sun, hydrogen nuclei fuse in a series of steps, and the overall result is that four hydrogen nuclei become one helium nucleus.
- The Sun converts around four million tonnes of mass into energy every second.
- This energy travels outwards through the Sun and leaves its surface as electromagnetic radiation, which reaches the Earth as light and infrared.
Common Mistake
- Do not describe fusion as atoms sticking together, because it is the nuclei that join, not whole atoms.
- Do not say that a high temperature is needed to melt the nuclei together, because it gives the nuclei enough kinetic energy to overcome their repulsion.
- Do not mix up fusion and fission, because fusion joins light nuclei while fission splits a heavy nucleus.
Exam technique
Describing nuclear fusion
- State that two light nuclei join to form one heavier nucleus, using the word nuclei rather than atoms or particles.
- Explain why a high temperature is needed by linking it to kinetic energy and to overcoming the repulsion between positive nuclei.
- State that some mass is converted into energy, and that this energy is released as radiation.
- Give an example where one is asked for, such as hydrogen nuclei fusing to form helium in the Sun.
Self review
- What is nuclear fusion?
- Why do two nuclei repel each other?
- Explain why fusion needs a very high temperature.
- Where does the energy released by fusion come from?
- Name the element formed when hydrogen nuclei fuse in the Sun.
