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Mass & Energy

Physics: AQA A Level, topic 8.1.9

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.

Binding energy

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.
Energy with mass

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.

Fission

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

Fission

Fission

  • 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).
Chain reaction

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.

Fusion

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

Fusion

Fusion

  • 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.
Making fusion work

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.
Energy from fusion

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.

Test yourself on Mass & Energy

  • What is the equation which links energy and mass?

  • Which of these statements are true?

  • When neutrons are produced in fission they cause more fission reactions, what is this called?

  • Binding energy per nucleon is highest at which element?

  • How can we find the energy released in fusion?

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