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Approximate Maximum Deflection Of Beam

  1. Mar 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Estimate the approximate maximum deflection of the electron beam near the center of a CRT television screen due to the Earth's field. Assume the screen is 20 from the electron gun, where the electrons are accelerated by the following voltages. Note that in color TV sets, the beam must be directed accurately to within less than 1 in order to strike the correct phosphor. Because the Earth's field is significant here, mu-metal shields are used to reduce the Earth's field in the CRT.

    By 2.2 kV

    2. Relevant equations

    We know that

    eV=Work
    KE=(1/2)mv^2
    and |q|vB=(mv^2)/R

    3. The attempt at a solution

    First I found the speed of the traveling electron(s):

    (1.60*10^-19)(2.2*10^3)=(1/2)(9.1*10^-3)(v^2)
    v=2.78*10^7

    And then when I plug it into the equation

    |q|vB=(mv^2)/r

    I get a value of deflection to be 3.16 meters, which is not the write answer. I'm not too sure where I went wrong. If anyone could help me out that would be great!
     
  2. jcsd
  3. Mar 22, 2009 #2

    Delphi51

    User Avatar
    Homework Helper

    The 2.2 kV is very low - could it be 22 kV? Is that 20 cm from the screen?
    What value did you use for B?

    The radius you found is not the answer to the question. This is the situation:
    CRT.jpg
    The blue arc is the path of the electron. You are trying to find the red deflection distance marked dx.

    If you can write an equation for the circle it will be easy to use it to find dx.
     
    Last edited: Mar 22, 2009
  4. Mar 22, 2009 #3
    Ohh I see :)

    I'll be sure to post the answer soon, just need to get down on paper first!

    Melqarthos
     
  5. Mar 27, 2009 #4
    whats the answer
     
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