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TV Elctron Path

  1. Mar 7, 2008 #1
    1. The problem statement, all variables and given/known data
    In the oscilloscope shown, an elctron beam is deflected by an electric force produced by charged metal plates AD and BC. In the region ABCD, each elctron experiences a uniform downward electric force of 3.2x10^-15N. Each electron enters the electric field along the illustrated axis, halfway between A and B, with a velocity of 2.25 x 10^7 m/s parrallel to the plates. The electric force is zero outside ABCD. The mass of an electron is
    9.11 x 10^-31 kg. The gravitational force can be neglected during the short time interval an electron travels to the fluorescent screen, S. Determine how far en electron is below the axis of entry when it hits the screen.


    2. Relevant equations

    3. The attempt at a solution
    I really have no idea how to go about this, my classmates and I have been working on it all day, and we don't even know how to go about doing it. Any help is wonderful. And sorry for my paint skills.
  2. jcsd
  3. Mar 7, 2008 #2


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    Take an account of what you have been given:

    - initial velocity
    - force (downward)
    - horizontal distance to be travelled
    - width of plates

    Now, while the electron is between the plates, this is somewhat similar to a projectile motion problem in the sense that there is a downward force that only affects the vertical component of the electron's velocity (which is initially zero). The horizontal velocity remains constant....which means that you can use that and the distance to the screen to figure out:

    - how much time it takes the electron to hit the screen
    - how much of this time is spent in the between the plates (i.e. how long does it take for the electron to move past them).

    From this you can ask the question...how far does the electron move in the downward direction DURING that period of time? The problem can be broken up into two regimes...the first part is when the electrons are in between the plates under the influence of the field and therefore accelerating downwards. The second regime occurs after they have moved beyond the plates and move with a constant speed.
  4. Mar 7, 2008 #3
    Ok, I will try this and if I have any problems I will post again. Thank you, you are a gentleman and a scholar.
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