Deflection of electron beam in oscilloscope

[Blehs]

Homework Statement

Calculate the deflection of an electron beam as it passes between the plates of a CRT tube.
In the picture, the parallel plates create an electric field, with the positive plate on top and the negative plate on bottom, causing the electron's path to be deflected upwards as it travels to the right.

Homework Equations

Kinetic energy of electrons: K = 3.2 x 10^-16 J
Electric field between plates: E = 1.2 x 10^4 N/C
Distance along plates = 15mm

The Attempt at a Solution

I know the equation F=Eq is involved. I've been told that the force is centripetal, so that the equation for centripetal force is also involved, but I am not entirely convinced. This is because centripetal force is always perpendicular to the velocity, but in this example that can't be the case.
So I am stuck and need help.

[edit] and the answer is 0.34mm

 

[ehild]

Homework Statement

Calculate the deflection of an electron beam as it passes between the plates of a CRT tube.
In the picture, the parallel plates create an electric field, with the positive plate on top and the negative plate on bottom, causing the electron's path to be deflected upwards as it travels to the right.

Homework Equations

Kinetic energy of electrons: K = 3.2 x 10^-16 J
Electric field between plates: E = 1.2 x 10^4 N/C
Distance along plates = 15mm

The Attempt at a Solution

I know the equation F=Eq is involved. I've been told that the force is centripetal, so that the equation for centripetal force is also involved, but I am not entirely convinced. This is because centripetal force is always perpendicular to the velocity, but in this example that can't be the case.
So I am stuck and need help.

[edit] and the answer is 0.34mm

A constant force F=qE acts on the electron in the vertical direction while its initial velocity is horizontal. The problem is analogous with projectile motion, the trajectory is parabolic. You can decompose the motion into horizontal and vertical direction.

ehild