Calculating Electron Speed Between Two Point Charges

[weathercheck]

Homework Statement

Two stationary point charges + 3.00 {\rm nC} and + 2.00 {\rm nC} are separated by a distance of 50.0 \rm cm. An electron is released from rest at a point midway between the two charges and moves along the line connecting the two charges

Homework Equations

What is the speed of the electron when it is 10.0 \rm cm from the + 3.00- {\rm nC} charge?

The Attempt at a Solution

Dont know where i am going wrong and need to inpout this like now!

using

v = sqrt(2Q(Va-Vb)/m)

Got Va-Vb = 225

then surely i just chuck in the value for Q of an Electron and the Mass!

But no work!

 

[quantumdude]

using
v = sqrt(2Q(Va-Vb)/m)

Got Va-Vb = 225

How the heck did you get that? You don't know what v is!

I don't know where you pulled that equation from, but it's definitely not the one you want to be using here. Treat this as a problem in conservation of energy. What is the energy of the electron when it is midway between the two charges? What is the energy once it has moved 50cm?

 

[weathercheck]

i said 0 + q0Va = 1/2mv^2 + q0Vb

 

[quantumdude]

That's true, but you're skipping over the important details. You have a formula for calculating the electric potentials of point charges, right?