Cathode-ray tubes (CRTs) are often found in oscilloscopes and computer monitors.

Question

Cathode-ray tubes (CRTs) are often found in oscilloscopes and computer monitors. In the figure (Figure 1) an electron with an initial speed of 6.90

Cathode-ray tubes (CRTs) are often found in oscilloscopes and computer monitors. In the figure (Figure 1) an electron with an initial speed of 6.90 times 106m/s is projected along the axis midway between the deflection plates of a cathode-ray tube. The potential difference between the two plates is 20.0V and the lower plate is the one at higher potential. a)What is the magnitude of the force on the electron when it is between the plates? b)How far below the axis has the electron moved when it reaches the end of the plates? C)At what angle with the axis is it moving as it leaves the plates?

Explanation / Answer

a) E = v/d = 20/0.02 = 1000 N/c

F = e*E = 1.6*10^-19*1000 = 1.6*10^-16 N

b) a = F/m = 1.6*10^-16/9.1*10^-31 = 1.758*10^14 m/s^2

time taken, t = d/v = 0.06/6.9*10^6 = 8.7*10^-9 s

y = 0.5*a*t^2

= 0.5*1.758*10^14*8.7^2*10^-18
= 6.65*10^-3 m

c) vy = a*t = 1.758*10^14*8.7*10^-9 = 1.529*10^6 m/s

theta = tan^-1(vy/vo) = 12.5 degrees with +x axis

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