# Homework Help: Electric field between two deflection plates

1. Sep 12, 2008

### demonhunter19

1. The problem statement, all variables and given/known data
In a cathode-ray tube (CRT), an electron travels in a vacuum and enters a region between two "deflection" plates which have equal and opposite charges. The dimensions of each plate are 12 cm by 4 cm, and the gap between them is 2.5 mm (not to scale in the diagram).

During a 0.001 s interval while it is between the plates, the change of the momentum of the electron P=< 0,4.80e-17,0 > kg m/s.

What is the electric field between the plates?
Hint: remember the Momentum Principle (the relationship between Impulse and change in momentum.)

What is the charge (both magnitude and sign) of the upper plate?

q =

2. Relevant equations
Momentum Principle: Change of P = Fnet(t)
Electric Force, F=qE (q is charge, E is electric field)
Electric field of a plate? (I have no idea. Probably something like E=(Q/A)/Epsilon_0 where A is the area of the plate)

3. The attempt at a solution
First I find the Fnet acting on the electron by dividing <0,4.80e-17,0> by .001 which in turn gives me <0,4.8e-14,0>N. I then plug it into F=qE, and find that E=-300000, (because q = -1.6e-19) But webassign tells me that is wrong. I also don't know how to find the charge of one of the plates.