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Homework Help: Yet another question

  1. Apr 8, 2007 #1
    1. The problem statement, all variables and given/known data

    Photo 6.jpg
    In the oscilloscope shown in the picture, an electron beam is deflected by charged metal plates AD and BC. In the region ABCD, each electron experiances a uniform downward electric force of 3.20E-15 N. Each electron enters the electric field along the illustrated axis, halfway between A and B, with a velocity of 2.25E7 m/s parallel to tge plates. The electric force is zero outside ABCD. The mass of an electron is 9.11E-31 kg. The gravitational force can be neglected during the short interval an electron travels to the fluorescent screen, S. Determine how far an electon is below the axis of entry when it hits the screen.
    I took a picture with the iCamera thing in my laptop and it comes out backwards, but I can't seem to save it flipped over. Basically it shows that the distance between AD and BC, the plates, is 3 cm and the distace from the plates to the screen is 13cm.

    The variables I pulled from this were:
    V = 2.25E7 m/s
    d = 3cm = 0.03m
    Fe = 3.20E-15 N
    V - d/t, so t = 1.33E-9 s
    m = 9.11E-11 kg

    3. The attempt at a solution
    Fnet = Fe
    ma = Fe
    (9.11E-31)a = (3.20E-15)
    a = 3.51E15
    a = distance/time^2
    3.51E15*(1.33E-9)^2 = d
    d = 6.245E-3 m
    The distance below the axis is 6.245E-3 m
    My textbook gives the answer as 3.02E-2 m - however I cannot see where I have gone wrong.
    Last edited: Apr 8, 2007
  2. jcsd
  3. Apr 8, 2007 #2


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    Homework Helper

    Could you maybe type out the question? Your pic is backwards and blurry!
  4. Apr 8, 2007 #3
    oh, sorry i hadnt realized
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