100 keV electron energy

1. Jun 25, 2015

blue_leaf77

This may be a naive question but this has been bothering me. I read that electrons having energy above 100 keV are considered as relativistic, on the other hand electron's rest mass is 511 keV. How can a free electron have energy less than its rest mass? What kind of energy implied in saying that 100 keV? Is it $\sqrt{p^2c^2+m_e^2c^4}-m_ec^2$?

2. Jun 25, 2015

Staff: Mentor

Yes, they are surely referring to the kinetic energy of the electron, not the total energy. I think this is common practice, because the kinetic energy relates directly to how you produce the electrons in the first place, i.e. effectively accelerate them through a potential difference of 100 kV.

3. Jun 25, 2015

Thanks!