Week 1: Discrete Charge and the Electrostatic Field 55
Problem 4.
−e E^0l LCathode Ray Tube (CRT)electron source deflector platesphosphorescent screen∆yAn electron moves to the right with speedvalong the axis of a cathode ray tube. There is
an electric fieldE~=E 0 ˆjin the region between the deflection plates, which are of lengthl, and
everywhere elseE~= 0. The flat screen is a distanceLfrom the end of the plates. Assume that
the electron is moving fast enough that it will not “fall” or hit the deflection plates while crossing
the deflection zone (ignore effect of the gravitional force on the electron as it is negligible across the
entire distance).
Find ∆y, the deflection from the center point where the electron hits the screen. You might want
to break the problem up into two parts as the figure hints.
Problem 5.
A ball of known chargeqand unknown massm, initially at rest, falls freely from a heighthin
a uniform electric fieldE~that is directed vertically downward. The ball hits the ground at a speed
v= 2
√
gh.
Findmin terms ofE,qandg.