Nuclear fusion is the process in which light atomic nuclei combine to form a heavier nucleus and release energy. It powers the Sun and most stars, making it one of the most important energy processes in the universe. Fusion matters because a tiny amount of mass can become a huge amount of energy according to E = mc^2.
Understanding fusion connects nuclear physics, astronomy, and the search for cleaner energy sources on Earth.
In stars like the Sun, the main fusion pathway is the proton-proton chain, where hydrogen nuclei gradually combine to form helium. The process requires extremely high temperature and pressure because positively charged nuclei repel each other through the electric force. If nuclei get close enough, the strong nuclear force binds them together, and the final products have slightly less mass than the starting particles.
That missing mass is released as energy in photons, neutrinos, and particle motion.
Key Facts
- Fusion combines light nuclei into heavier nuclei, such as hydrogen forming helium.
- Mass-energy relation: E = mc^2.
- Energy released in a reaction is Q = Δm c^2, where Δm is the mass lost.
- In the Sun, the net proton-proton chain reaction is 4 1H -> 4He + 2 e+ + 2 neutrinos + energy.
- Fusion needs very high temperature because nuclei must overcome electric repulsion, also called the Coulomb barrier.
- On Earth, common fusion fuel candidates are deuterium and tritium: 2H + 3H -> 4He + n + 17.6 MeV.
Vocabulary
- Nuclear fusion
- Nuclear fusion is a reaction in which light atomic nuclei join to form a heavier nucleus and release energy.
- Plasma
- Plasma is a hot ionized gas made of free electrons and nuclei that can respond strongly to electric and magnetic fields.
- Proton-proton chain
- The proton-proton chain is the set of fusion reactions that converts hydrogen into helium in stars like the Sun.
- Coulomb barrier
- The Coulomb barrier is the energy barrier caused by electric repulsion between positively charged nuclei.
- Mass defect
- Mass defect is the difference between the mass of the starting nuclei and the smaller mass of the final products in a nuclear reaction.
Common Mistakes to Avoid
- Confusing fusion with fission, which is wrong because fusion joins light nuclei while fission splits heavy nuclei.
- Forgetting that nuclei repel each other, which is wrong because the positive charges create a Coulomb barrier that fusion must overcome.
- Assuming all lost mass disappears, which is wrong because the mass defect is converted into energy by E = mc^2.
- Treating fusion power as easy once fuel is available, which is wrong because the plasma must be heated, confined, and kept stable long enough for net energy gain.
Practice Questions
- 1 A fusion reaction has a mass defect of 3.1 x 10^-29 kg. Using c = 3.00 x 10^8 m/s, calculate the energy released in joules.
- 2 One deuterium-tritium fusion reaction releases 17.6 MeV. If 1 MeV = 1.60 x 10^-13 J, how much energy is released by 1.0 x 10^20 reactions?
- 3 Explain why fusion can release energy even though two positively charged nuclei repel each other before they fuse.