Electrostatics and infinately charged plate question driving me nuts!

Homework Statement

An electron is released from rest 2.0 cm from an infinite charged plane. It accelerates toward the plane and collides with a speed of 1.0x10^7m/s. What is the surface charge density of the plane?The answer is 2.52×10−7 C/m^2 !

Homework Equations

v=d/t, d=0.5at^2, E=n/2*8.85x10^-12, F=ma=Eq

The Attempt at a Solution

I don't know what to do with the velocity. I tried relating it to energy but got stuck.

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Doc Al
Mentor
I don't know what to do with the velocity.
Use that and the distance to find the electron's acceleration.

using v=d/t t is 2E-9
using a=v/t a is 5E15
E=ma/q E=28469
E=n/(2 epsilon not).. n=5.03E-7

which is 2 times too big, but isn't E=n/(2 epsilon not) the correct equation, with the 2?

Doc Al
Mentor
using v=d/t t is 2E-9
That equation applies for constant velocity, not accelerated motion. But you can use it if you replace v (the final velocity) with the average velocity. What's the average velocity?

You can also make use of additional kinematic relationships.

ahh so I guess you mean Vf^2=V0^2 + 2ad because that works haha

Thanks!

Doc Al
Mentor
That's the one.