# Homework Help: Internal Energy of an Electron

1. Dec 7, 2015

### Barry Melby

1. The problem statement, all variables and given/known data
What is the internal energy of an electron moving at 0.750c0 in the Earth reference frame?

2. Relevant equations

3. The attempt at a solution
E = mc^2
E = (9.11 * 10^-31)(.750)(3*10^8)

However, this appears incorrect. What have I done wrong or what am i missing?

2. Dec 7, 2015

### DuckAmuck

What do you mean "internal" energy? It's just one particle, so the options are "rest energy", "kinetic energy" and "total energy".
If m is the mass of an electron, c is light speed, and v is the speed of the electron, then:

Rest energy is $$E=mc^2$$
Total energy is $$E_T = \frac{mc^2}{ \sqrt{1 - \frac{v^2}{c^2}}}$$
Kinetic energy is $$KE = E_T - E$$