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An electron shot from the back of a T.V. help?

  1. Apr 2, 2009 #1
    1. The problem statement, all variables and given/known data

    If I shoot an electron from the back of a TV at a screen towards the front at .5c (e= speed of light) What magnetic field must be in the coils around the cathode ray tube (picture tube) if the electron would have made a circle radius 30m if it hadn't hit the screen and made it glow. (your TV show)

    2. Relevant equations

    r= (mv)/qB

    3. The attempt at a solution

    Using the equation rearranged to solve for B:


    mass of an electron = 9.1 x 10^-31
    charge of an electron = 1.602 x 10^-19
    .5c=....well it's half of the spd of light so I assume : 1.5 x 10^8 since the spd of light is 3x10^8 if I remember correctly.
    and radius = 30m


    B= (9.1x10^-31)(1.5x10^8)/(1.602x10^-19)(30)
    B= .026?

    Could someone check if I did this correctly, did I forget something?
  2. jcsd
  3. Apr 2, 2009 #2


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    Homework Helper

    Your calculation is wrong. Check it.
  4. Apr 2, 2009 #3

    Now I'm getting

    B = 3.06x10^-5

    Is that correct?

    I tried taking baby steps to see what I did incorrectly but I'm not sure if it's helping.
  5. Apr 3, 2009 #4


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    Homework Helper

    Yes. It is correct.
  6. Apr 3, 2009 #5
    Thanks, now I know those baby steps helped.
  7. Apr 3, 2009 #6
    Have checked those calculations and got 2.8402x10^-5... :uhh:
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