Mass & Energy

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Mass and Energy

The nucleus of an atom is made up of protons and neutrons. When protons and neutrons combine to make an atom energy is released.

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Binding energy

  • A nucleus is made up of protons and neutrons.
  • The mass of a nucleus is less than the mass of the individual protons and neutrons.
  • This is because when the protons and neutrons combine they release energy.
  • The energy released is called the binding energy.
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Energy with mass

  • Energy and mass can be linked through a very famous equation. The equation linking energy and mass is:
    • Energy = mass x (the speed of light)2^2
    • E=mc2E=mc^2
  • If the change in mass between the individual particles and the mass of the nucleus is known, we can find the binding energy using this equation.


Nuclear fission is a reaction in which a nucleus is split (or fissured).

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  • Energy is released when heavy nuclei are split apart.
  • The new nuclei (daughter nuclei) have a larger binding energy per nucleon than the parent.
  • This means that a large amount of energy is released (about 100 times the energy of a normal nuclear decay).
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Chain reaction

  • Fission occurs when an unstable atom splits, we can force this split by making the atom absorb a neutron.
  • The fission reaction will produce daughter nuclei but also release energy and some neutrons.
  • These neutrons will go on to cause fission reactions themselves, which will produce more neutrons.
  • This is now a chain reaction.


Nuclear fusion is a reaction in which two nuclei are combined, or fused, to form a larger nucleus.

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  • We know that all nuclei have less mass than the sum of the masses of the protons and neutrons.
  • When an atom is formed it releases binding energy - the greater the binding energy, the greater the missing mass.
  • The binding energy per nucleon is a maximum at iron.
  • This means that if two low-mass nuclei can be fused together to form a larger nucleus, energy can be released.
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Making fusion work

  • Fusion would release a huge amount of energy but there are many issues which need to be overcome before it will work on earth.
  • The repulsion between positive nuclei is very strong and can only be overcome at very high temperatures or pressures.
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Energy from fusion

  • We can use the energy and mass equation:
    • E=mc2E=mc^2
  • If we know the mass of the nuclei before and the mass of the large nucleus afterwards we can find the change in mass.
  • If we know the change in mass we can find the energy released.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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