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Homework Help: Parallel electric plate question

  1. Jun 16, 2006 #1
    1. If an alpha particle (q=+2e, m = 6.64*10^-27kg) is released from the positive plate (Vab=120V and the distance separating the plates is 12cm), with what velocity will it hit the negative plate?

    2. An electron is fired from a negative plate toward a hole in a positive plate. It then passes between two parallel plates that are 3cm long and 2cm apart with a voltage of 550Volts across them. If the accelerating voltage across the two original plates is 900 volts what is the final velocity of the eletron?

    Ooooh my head spins. I haven't the slightest clue where to start. Could someone give me a hint?
  2. jcsd
  3. Jun 16, 2006 #2


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    Staff: Mentor

    -1- What is the magnitude of the electric field in that region? What is the equation for the force on a charged particle due to an E field? What equation relates force and acceleration. What equation relates acceleration and velocity?

    -2- Draw the geometry, and draw the E field lines. Use the equations from -1-.
  4. Jun 17, 2006 #3
    Quick Hint:[itex]q\Delta V = [/itex]Change in Kinetic Energy
    Last edited: Jun 17, 2006
  5. Jun 17, 2006 #4
    Okay, so I got the first one... but the 2nd one boggles my mind. Could you give me a more.... obvious hint? I tried doing vectors but that confused me even more...
  6. Jun 17, 2006 #5


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    Using conservation of energy, you should be able to calculate the speed of the electron when it comes out of the original plates (this is the same as question 1).

    Now let's pretend that the electron is moving along the x direction (with a v_x given by the speed you found just above) when it enters the second pair of plates (which are horizontally aligned so that the electric field between them is vertical).

    the steps are:

    a) figure out the E field between those two plates.
    b) Figure out the electric force on the electron (which is a vertical force)
    c) Find the vertical acceleration of the electron
    d) using the velocity along the x direction, figure out how long it takes ti get across the plates

    and then there is a last step that should be clear.

  7. Jun 22, 2006 #6
    Ah, yes. Thank you so much.
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