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# AS Physics - Annihilation watch

1. This is probably a very simple annihilation question but still...

The rest energy of an electron is 0.511MeV

A positron has 0.158MeV of kinetic energy. It collides with an electron at rest, creating 2 photons of equal energies as a result of annihilation.

Calculate the total enery of the positron and the electron.

I thought it was a very simple add the energy of the positron to the energy of the electron which would give 0.669MeV. But the answer was 1.180MeV which seems to be the energy of 2 electrons plus the energy of the positron... could anyone tell me why???

Thankies
2. The positron has rest energy and kinetic energy. The listed 0.158 is its kinetic, you need to include its rest as well.
3. It might be due to the fact that the positron's energy is kinetic energy, while the electron's energy is rest energy.

so basically, the kinetic energy of the positron plus its rest energy (which should be the same as the electron's) + 0.511 (the rest energy of the electron)

so, (0.158 + 0.511) + 0.511 = 1.180
4. The positron has both rest energy and the additional kinetic energy. Therefore the positron has 0.669MeV (0.511+0.158) of energy, which when added to the 0.511MeV of the electron results in 1.180MeV total.
5. (Original post by Hudzy)
The positron has both rest energy and the additional kinetic energy. Therefore the positron has 0.669MeV (0.511+0.158) of energy, which when added to the 0.511MeV of the electron results in 1.180MeV total.
This. All particles have rest mass-energy by virtue of their existence, which becomes available when they are destroyed (as here: electron + positron --> 2 photons) Even if this is not explicitly stated in a question like this you need to consider it.
6. oooohhhhh makes much more sense now....

Thank you everyone!!

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