### Question

What is the amount of energy given off when an electron and a positron anhilalate?

Asked by: Bill Devenport

### Answer

Using 9 x 10^{-31} Kg. as the mass of each particle, and knowing that their combined mass
will be totally converted to energy, you can use Einstein's famous E = mc^{2} (where c = the
speed of light in vacuum = 3 x 10^{8} m/sec) to determine the total energy output of the electron/positron annihilation:

E = (mass of electron + mass of positron) x (speed of light in m/sec)^{2}

E = 2 x (9 x 10^{-31})kg x (9 x 10^{16})m^{2}/sec^{2}

E = 1.6 x 10^{-13} kg m^{2}/sec^{2} = 1.6 x 10^{-13} Joules

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor