# Homework Help: Electrical Energy Electron Question

1. Dec 5, 2012

### BlueCardBird

1. The problem statement, all variables and given/known data
Two electrons are fired at 3.5x10^6 m/s directly at each other

a) calculate the smallest possible distance between the 2 electrons

me=9.1x10^-31 Ve1=3.5x10^6 m/s qe=1.6x10^-19 C

2. Relevant equations

Em1=Em2

3. The attempt at a solution

Tried using Em1=Em2 which 1/2meve1^2+kqeqe/r1=1/2meve2^2+kqeqe/r2

I cant seem to use that formula because both electrons are moving, instead of one being stationary.

2. Dec 5, 2012

### haruspex

By symmetry, you only need to consider the energy for one electron. What is its initial KE, and what is its PE when a distance x from the midpoint between them?

3. Dec 5, 2012

### BlueCardBird

So how would i calculate the distance between them if i could consider one electron?

4. Dec 5, 2012

### haruspex

As I said, work relative to the midpoint between them, which, by symmetry, is stationary.

5. Dec 5, 2012

### BlueCardBird

that kind of confuses me, sorry but could you elaborate on the symmetry?

6. Dec 5, 2012

### haruspex

Let C be the midpoint. When one electron is distance x from it and moving towards it at speed v, the other will be distance x on the other side, also moving towards it at speed v. Each separately satisfies conservation of energy. The only thing to watch is that when you calculate the PE of one you measure distance from the other electron (2x), not from C.

7. Dec 5, 2012

### BlueCardBird

Thanks sir! The help is much appreciated, with all the formulas in this unit, questions are quite confusing.