Comparison of particle and antiparticle masses, charge and rest energy in…

The idea

A particle and its antiparticle have identical rest masses and therefore identical rest energies, but opposite electric charges and opposite signs for quantum numbers such as baryon number, lepton number and strangeness. Rest energy is the energy equivalent of rest mass, calculated using E = mc², and in particle physics it is usually quoted in megaelectronvolts, MeV. For example, the electron and positron each have a rest energy of about 0.511 MeV, while the proton and antiproton each have a rest energy of about 938 MeV. When comparing a particle with its antiparticle, you should state that mass and rest energy are the same, while charge sign is reversed. This comparison is used in calculations of annihilation energy and in interpreting particle tracks.

What earns the marks

• States that a particle and its antiparticle have the same rest mass.
• States that a particle and its antiparticle have the same rest energy in MeV.
• States that a particle and its antiparticle have opposite electric charge, for example electron e⁻ and positron e⁺.
• Uses E = mc² to convert between rest mass and rest energy, or recalls standard rest energies such as 0.511 MeV for the electron and positron.
• Applies the comparison to calculations, for example finding the total energy released when a particle and antiparticle annihilate.
• Recognises that other quantum numbers such as baryon number and lepton number are opposite in sign for the antiparticle.

Common mistakes and how to put them right

• Stating that the antiparticle has a different mass from the particle.

  Put it right: The rest masses are identical, so the rest energies in MeV are also identical.

• Confusing rest energy with kinetic energy.

  Put it right: Rest energy is the energy equivalent of the mass when the particle is at rest, calculated using E = mc², and is separate from any kinetic energy the particle may have.

• Using the wrong sign for charge on an antiparticle.

  Put it right: The antiparticle has the opposite sign of charge to the particle, so the positron is e⁺ while the electron is e⁻.

• Forgetting to convert between joules and MeV when using E = mc².

  Put it right: Use the conversion 1 MeV = 1.60 × 10⁻¹³ J, or work consistently in MeV using the appropriate mass–energy conversion.

Exam tips

• When comparing a particle and its antiparticle, present your answer in a clear table or list covering mass, charge and rest energy in MeV.
• Quote rest energies to a sensible number of significant figures, for example 0.511 MeV for the electron and positron, and 938 MeV for the proton and antiproton.
• In annihilation calculations, remember that the total rest energy of the particle and antiparticle is converted into the total energy of the photons produced.
• Check that any equation you write is dimensionally consistent and that you have not mixed joules with MeV without converting.