In a vacuum chamber, a positron (particle with same mass as the electron but opp

Question

In a vacuum chamber, a positron (particle with same mass as the electron but opposite charge) is initially kept at rest at the center of a square metal plate, a distance s=0.06 m away from a second, parallel square metal plate. At t=0, the potential of the plate where the positron is kept is brought to 4.2 kV above that of the second plate, causing the positron to travel from the first plate to the second plate.

a) At what time ta does the positron arrive at the second metal plate?
b) How far from the arrival plate is the positron at t=ta/2?

Explanation / Answer

V=Es
4200 V = E x 0.06
E = 70000 N/C
--> neglect gravity
qE = ma
a = qE/m = (1.6e-19)(70000)/(9.1e-31) = 1.23e16 m/s2
s=ut+0.5at2 -->   0.06 = 0.5(1.23e16)t2

(a)    t= 3.12e-9 s

(b) s at t=3.12e-9/2 = 1.56e-9 s

   s = 0.015 m

Distance away from arrival plate = 0.06-0.015 = 0.045 m

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