In a television picture tube, electrons are boiled out of a very hot metal filam

Question

In a television picture tube, electrons are boiled out of a very hot metal filament placed near a negative metal plate. These electrons start out nearly at rest and are accelerated toward a positive metal plate. They pass through hole in the positive plate on their way toward the picture screen, as shown on the diagram below. If the high-voltage supply in the television set maintains a potential difference of 15,000 volts: Calculate the electric field between the two plates Calculate the change in potential from the negative plate to the positive plate. Calculate the change in electric potential energy for the electron when it reaches the hole. Calculate the change in kinetic energy of the electron. Calculate the maximum speed of the electron.

Explanation / Answer

a)

electric field = E = V/L = 15000/(0.01*10^-3) = 15*10^8 V/m

b)

dV = 15000 v

c)

dU = dV*e = 15000*1.6*10^-19 = 2.4*10^-15 J

d)

work energy theorem

work = change in KE

change in KE = 2.4*10-15 J

e)

dKE = K2-K1

k1 = 0

k2 = 0.5*m*v^2

0.5*9.11*10^-31*v^2 = 2.4*10^-15

v = 7.26*10^7 m/s

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