Homework Help: Archived Displacement and Momentum, particle in cathode ray tube with variables

1. Feb 7, 2012

physics1311

1. The problem statement, all variables and given/known data
In a cathode ray tube (CRT) used in older television sets and oscilloscopes, a beam of electrons is steered to different placed on a phosphor screen, which glows at locations hit by electrons. The CRT is evacuated, so there are few gas molecules present for the electrons to collide with. Electric forces are used to accelerate electrons of mass m to a speed v0 << c, after which they pass between positively and negatively charged metal plates which deflect the electron in the vertical direction (upward in the diagram, or downward if the sign of the charges on the plates is reversed).
CRT deflection
While an electron is between the plates, it experiences a uniform vertical force F, but when the electron is outside the plates there is negligible force on it. The gravitational force on the electron is also negligibly small in this situation. The length of the metal plates is d, and the phosphor screen is a distance L from the metal plates. Where does the electron hit the screen? (That is, what is yf?)

2. Relevant equations
displacement=V(avg)Δt
Δp=FnetΔt

3. The attempt at a solution
Using the displacement equation over L
L=(v0+v0)/2*Δt rearanged Δt=L/v0
I substituted this Δt into Yf=(v-final+v-initial)/2*Δt
V(fina)l should be the same as v(initial) over L because there are no other forces as states, right?, so we have Yf=v(final)*L/v0

then to find vf I used the momentum principle assumng v(initial)=0
m*v(final)=FΔt (different Δt here)
and to find this Δt i used the displacement equation for x over d,
d=v0/2*Δt
My final answer, which was wrong, was 2*F*d*L/((v0)^2*m

2. Mar 22, 2017

Keith Gellar

Hope the attached outline helps.

The only difference I can see is that I didn't apply the force over the length from ray gun to screen. The force is only applied while between the plates.

3. Mar 22, 2017

Staff: Mentor

Hi Keith G.,

If you're keen to pursue this problem and work out the full solution, here's a diagram you might be able to make use of:

$\Delta y$ is what is sought.