Particles, Antiparticles & Photons

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Antimatter is made up of antiparticles. Every particle has an antiparticle. Antiparticles have the same mass and rest energy but are oppositely charged. They are usually labelled with a line over the top of their symbol.

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Proton and antiproton

  • The antiparticle of the proton is the antiproton.
    • It has the same mass (1.67×10-27 kg) and rest energy (938 MeV) as a proton.
    • It has a relative charge of -1, the opposite to a proton.
    • Its symbol is p\overline{p}.
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Electron and positron

  • The antiparticle of the electron is the positron.
    • It has the same mass (9.11×10-31 kg) and rest energy (0.51 MeV) as an electron.
    • It has a relative charge of +1, the opposite to an electron.
    • Its symbol is e+.
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Neutron and antineutron

  • The antiparticle of the neutron is the antineutron.
    • It has the same mass (1.67×10-27 kg) and rest energy (940 MeV) as a neutron.
    • It has a relative charge of 0 (the opposite of 0 is also 0).
    • Its symbol is n\overline{n}.
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Neutrino and antineutrino

  • The antiparticle of the neutrino is the antineutrino.
    • It has a mass of 0, the same as a neutrino.
    • It has a relative charge of 0 (the opposite of 0 is also 0).
    • Its symbol is νe\overline{\nu}_e.

Photon Model of Electromagnetic Radiation

We can think of electromagnetic radiation (light) as little packets of energy. We call these packets photons.

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Frequency and energy

  • The energy of a photon is directly proportional to the frequency of the radiation.
    • E=hfE = hf
  • The letter 'h' is the Planck constant.
    • It is a fundamental constant of nature and is equal to 6.63×10-34 Js.
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Energy of different photons

  • Gamma photons have the highest frequency and so carry the most energy.
  • Radio photons have the lowest frequency and so carry the least energy.

Annihilation and Pair Production

We know from Einstein's famous equation, E = mc2, that mass and energy are interchangeable. This is shown in annihilation and pair production.

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  • When a particle and its corresponding antiparticle collide, they annihilate each other.
  • Their masses are converted into pure energy, producing a pair of gamma photons.
  • The energy carried away by the gamma photons must equal the total energy of the particles to begin with (kinetic energy plus rest mass energy).
  • So each gamma photon must carry away at least the rest mass energy of one particle.
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Pair production

  • Pair production is the opposite of annihilation.
  • Pair production is when one high energy photon spontaneously turns into a particle-antiparticle pair.
  • The energy of the photon must be at least the total rest mass energy of the particle-antiparticle pair it creates.

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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