# Calculate the speed of a particle?

1. Apr 25, 2013

### x86

1. The problem statement, all variables and given/known data
If you have an electron in the following electrostatic field:

------------------------- (+)
(-) B 2m from origin

(-) A 1m from origin

------------------------- (-) Origin

Okay, so; the force field is pushing the electron from point A to point B.

It has more potential energy at point A than point B, meaning point B is moving faster than point A.

So I have to find the velocity of the electron at point A assuming that its HALF of the speed at point B

2. Relevant equations
F = kq1q2 / r^2
E = kq1q2 / r

Ek = 1/2 m v^2
Et1 = Ek + Ep
Et1 = Et2

3. The attempt at a solution
If we move an electron from point A to point B, we do negative work on it. How is this possible? I don't understand this at all.

Also, if it gains 10 J of potential energy from point B to A, and we have to calculate its speed.. How is this possible?

We have the equation

Ek ---> transfer to ----> Ep

So our change in kinetic energy is now negative.

But we can't take the square root of a negative value, so we can't solve for velocity

Last edited: Apr 25, 2013
2. Apr 26, 2013

### Simon Bridge

Who is this "we" that you speak of?
You have worked out that the electron has negative work done on it - by what?

If you physically pushed the electron from B to A, you would have to do positive work on it right?

How about doing it the other way around?

Note - the negative KE is a negative change in KE.
It just means you end up with less than you started - which is find because you know that the speed at A is less than the speed at B.