# Two electrons, one moving towards the other.

pdeco1

## Homework Statement

Two electrons, one moving towards the other that is in a fixed position. Determine the final speed of the mobile electron when it is far away from the fixed one.

Initial Velocity 10^5m/s directly at the fixed electron.
Mass electron = 9 x 10^-31kg
distance between them 10^-8m

## Homework Equations

W=Fd
Work initial = Work final

## The Attempt at a Solution

I first attempted to find the distance r between the two electrons, when KE final is zero. I set Wfinal = Winitial and solved for r. r = 5.8 x 10^-8, which is larger than the original d= 10^-8.

The Force on q at r= 10^-8m is also much larger than the KE of the moving electron. Does Electric Field Force > KE mean that the particle should instantly reverse directions and be repelled away?

If my assumption is correct I can then find the final velocity by equating Fd = KE and solving for v. Where d = "far away" = 1m

Thanks!

## Answers and Replies

Mentor
You'll have to show more of your work. For example, in your first attempt, how did you go about calculating the value of r?

Also, Energy is not force (different things entirely) so you can't directly compare them and draw any conclusions without knowing more about the things involved. In physics, almost nothing happens "instantly" when it comes to motion of objects with mass.

You might think about the fact that the electric field is a conservative one, so that you can expect the moving electron's return path from its "close encounter" with the fixed electron to be a mirror image of its inbound trajectory. This might save you some work, so to speak.