Early black-and-white television sets used an electron beam to draw a pictureon
ID: 1388242 • Letter: E
Question
Early black-and-white television sets used an electron beam to draw a pictureon the screen. The electrons in the beam were accelerated by a voltage of 3.5kV; the beam was then steered to different points on the screen by coils ofwire that produced a magnetic ?eld of up to 0.75 T.
a. What is the speed of the electrons in the beam?
b. If the electrons were to complete a full circular orbit, what would be the !! diameter?
c. A magnetic ?eld can be used to redirect the beam, but the electrons are !! brought to high speed by an electric ?eld. Why can
Explanation / Answer
Workdone on electron = q*delta_V
According work-energy theorm
Workdone = change in kinetic energy
q*delta_V = 0.5*m*v^2
==> v = sqrt(2*q*delta_V/m)
= sqrt(2*1.6*10^-19*3500/(9.1*10^-31))
= 3.51*10^7 m/s <<<<<<<<<<<<<--------------Answer
b)
When elelctrons enetedred magnetic filed, the magnetic field exert force on electrons perpendicular to
velocity. so the electrons revolve in a circular orbit.
Apply, m*v^2/r = q*v*B
r = m*v/(B*q)
= 9.1*10^-31*3.51*10^7/(0.75*1.6*10^-19)
= 2.7*10^-4 m or 0.27 mm <<<<<<<<<<<<<--------------Answer
c) magnetic force always act on elelctron perpenicular to instantanataneous velocity.
so, electrons speed remains constant. but the direction always changes.
It means they move in a curcular path.
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