Electrons in uniform electric field

[herbert1]

Homework Statement

http://dl.dropbox.com/u/305636/yeephysics.jpg

If that doesn't work, the problem states:

A pair of parallel metal plates, 100mm long and a distance of 20mm apart are set up in an evacuated tube as shown. A potential difference is applied to the plates, creating an electric field of 56 NC down 9the page). A beam of cathode rays, where all the particles are traveling at 4x10^6 m/s, enters the region between the plates from the left, at a point midway between the plates. (Neglect effects due to gravity).

I need help with part D

Which asks:

What is the increase in the kinetic energy of the cathode ray particle in traveling the length of the plates.

Homework Equations

E = F/q
F = ma
v^2 = u^2 + 2as (where v = final vel. u = initial, s = displacement)

The Attempt at a Solution

I thought about using Work = Vq, as work has to be the change in kinetic energy? But the length of the places threw me off.

So I used F = Eq into Eq = ma, a = Eq/m

Then v^2 = u^2 + 2(Eq/m)*0.1 to find the velocity of the particle at the end. Then i found the kinetic energy of the particle at the start and end using 1/2mv^2 and subtracted them and got 4.4856x10^-19 J. But I am not sure if this is right?

Thanks for your time.

 

[ehild]

The electric field accelerates the electrons in the direction perpendicular to the plates. The normal velocity component gained is proportional to the time the electrons spend between the plates.

ehild