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Homework Help: Electron Speed

  1. Dec 10, 2003 #1
    Two parallel plates A and B are separated by 5.0cm. The electric potential between the plates is 200V. An electron starts from rest at time t_A and reaches plate B at time t_B. Then the electron goes through an opening and reches point C at time t_C.
    What are the speeds at t_A , t_B and t_C?

    + -
    | |
    | |
    | |
    | |
    + -
  2. jcsd
  3. Dec 10, 2003 #2
    If i use the equation

    v=sqr root of [2qV]/m


    I can find velocity. but this means that the velocity will remain the same throughout the whole trip.

    Will the electric potential or charge change during the whole trip?
    Does the 5cm that the plates are separates affect anything?
  4. Dec 10, 2003 #3
    You are applying the Work Energy Theorem which considers final and initial velocity

    Moreover why should e move from + plate to - plate starting from rest all the forces are against the motion
  5. Dec 11, 2003 #4
    Sorry, you're right, the diagram sould look like this

    - +
    | |
    | |
    | |
    | |
    - +
  6. Dec 11, 2003 #5
    What is the work energy theorem?

    What equations can i use to find the initial and final velocities?
    Or should i automatically asume that those velocities are zero?(b/c thier potential energies are 0) and that the velocity in the middle (plate B) is 8.4*10^6m/s?

    Once the electron passes through the positive plate, would it be attracted back to the plate?

    How does the 5.0 cm that the plates are separated affect anything?
  7. Dec 12, 2003 #6
    Work Energy Theorem States that

    [tex] \Delta K = W_{conservative forces}+W_{non conservative forces}[/tex]

    [tex] \Delta K = K_f-K_i [/tex]
    Last edited: Dec 12, 2003
  8. Dec 12, 2003 #7
    With the small distance separating them signifies that the electric field lines are concentrated and are almost parallel to it. Which helps in maintaining uniformity.
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