# An electron with known speed and distance from a proton moving away?

1. Sep 16, 2009

### zoomrocket88

1. The problem statement, all variables and given/known data
Suppose that an electron is set down 10e-10 m away from a proton that is "pinned down" so that the proton cannot move. The electron has a velocity V0= 1.6e6 m/s pointing away from the proton, along the line between the proton and the electron. What is the force on this electron? Explain its motion in terms of energy conservation. How far away will it get from the proton? Where will it end up?

2. Relevant equations
Not sure

3. The attempt at a solution
I am stumped at how to go about this. I know that you should turn the changing kinetic energy into potential energy, and that eventually the electron will stop and come back to the proton if the attractive force is grater than the initial velocity of the electron, but I have no idea how to go about it.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 16, 2009

### turin

You need Coulomb's Law. You need the kinetic energy equation. Force and velocity are not comparable quantities.

3. Sep 16, 2009

### zoomrocket88

Alright so I can find the values for kinetic energy and the force from those equations, but how do I relate the two to find out how far the electron will go?

I have 1.16e-18 J and 2.30e-8 C so far...

4. Sep 16, 2009

### zoomrocket88

I suppose you could divide the two and get the Electric Potential but I don't think that would help...

5. Sep 16, 2009

### zoomrocket88

I found an equation that states that an electron will escape the pull of a proton if:
V=sqrt (2kQe/mr) but I cant tell if Qe is one value or two. Any help?

6. Sep 16, 2009

### turin

Please state Coulomb's Law for us. Please give your math in "complete sentences" (use equations, not just numbers or expressions).

7. Sep 16, 2009

### ideasrule

While you're at it, also state the equation for electric potential energy.

8. Sep 16, 2009

### zoomrocket88

Coulomb's law states that the force of attraction or repulsion between two charged particles is directly proportional to the product of the charges and inversely proportional to the distance between them.

9. Sep 16, 2009

### zoomrocket88

The equation for potential energy is PE= kQq/r

10. Sep 17, 2009

### turin

Excellent. And, these days, we know the constant of proportionality, so when you look up Coulomb's law in a text book, for instance, you will see this constant, and the text book will tell you it's value. Use this to answer the first question (What is the force).