# Q & A: electron inertia

Q:
If in a Vacuum, we apply a force to accelerate an electron to a constant velocity and then stop the force, does the electron move with that constant speed in the vacuum or little by little its speed is going to be lesser and finally it stops? thank you
- Abouzar (age 33)
A:

The electron will never slow down in the ideal case. There are several ways to see that result. One is by Newton's First Law, which says that something's velocity doesn't change unless a force acts on it. It sounds like there's no force in the vacuum you're thinking about. The other is by relativity, whether in Galileo's version of Einstein's. From the electron's point of view, it's at rest. That's a perfectly valid point of view. The electron sees us moving, but unless there's some force from us on it, that's not a reason for it to start moving too.

If, however, the electron is near some conductor, say a sheet of metal, its motion will stir up some eddy currents in the metal. These will drain away some energy, which must come from the electron's motion. So then the electron would gradually slow down. From the point of view of someone initially at rest with respect to the electron, the nearby moving piece of metal exerted a force on the electron.

Mike W.

(published on 11/02/2014)