In: Physics
An electron is projected with an initial speed v0 = 5.05
Let the distance between the plates = d
Let the length of the plate = L
The electron enters the field, experiences a uniform acceleration
in the vertical direction
= (e*E/m) (where e and m are the charge & mass of an electron
and E is the field strength)....(1)
Since the field is purely vertical, there is no force in the
horizontal direction and so the horizontal component of the
velocity remains constant while traversing the field = u and
therefore the time of travel = t =
(L/u)......................................
In order for the electron to barely miss the upper plate, the
distance travelled in the vertical direction = (d/2) =
(1/2)*f*t^2................................
where t = time and f = acceleration. (d/2since the electron enters
midway between the plates)
the figure in the link mentions that d = 1cm; L =
2cm........................................
Substituting the value of t obtained from (2) into (3) and then
applying (4) you get
L/4 = (1/2)*E*(e/m)*[(L/u)^2]....................
After simplification, this yields E = (u^2/2)*(m/e)*(1/L) =
(3.55*E+3) N/C (newtons per coulomb).