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Eletric field

  1. Sep 8, 2008 #1
    A uniform electric field of magnitude 640 N/C exists between 2 parallel plates that are 4.0 cm apart. A proton is released from the positive plate at the same instant an electron is released from the negative plate.

    a) Determine the distance from the positive plate at which the two pass each other (ignore the elecric attraction between the proton and electron) b) What if? Repeat part a for a Sodium ion (Na+) and a chloride ion (CI-).
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    I find the acceleration for both particles but I don't know where to go from there!! help, please!!
     
  2. jcsd
  3. Sep 8, 2008 #2

    Doc Al

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    Once you've found the acceleration this becomes a kinematics problem. Find expressions for the position of each particle (measured from the positive plate) as a function of time. Then you can solve for the time (and position) where they meet (and thus have the same position).

    Give it a try.
     
  4. Sep 8, 2008 #3
    I'm not sure what you're trying to say. xf=(1/2)at^2 for the proton and electron set equal to each other. find time and plug into find x?
     
  5. Sep 8, 2008 #4

    Doc Al

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    There are several ways to approach this, but that equation is the key. One way is to write two equations:
    (1) The distance the positive particle moves from the positive plate in time t; that would be:
    x1 = (1/2)(a1)t^2.
    (2) The distance the negative particle moves from the negative plate in time t; similar, but call it x2.

    When the particles meet, what must x1 + x2 equal? Use that to solve for the time, and then x1.

    Make sense?
     
  6. Sep 8, 2008 #5
    I little bit.
    x1 +x2=0, so.. (1/2)a1t^2+(1/2)a2t^2=0....t= sqr root (1/2)(a1=a2)
     
  7. Sep 9, 2008 #6

    Doc Al

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    Not exactly. x1 + x2 = the total distance between the plates.

    Say you live 10 miles from your friend. You both take off toward each other at different rates along the same path. When you meet you've traveled x1 miles and your friend has traveled x2 miles. What must x1 + x2 equal?
     
  8. Sep 10, 2008 #7
    x1 +x2= the total distance travels ?????
     
  9. Sep 10, 2008 #8

    Doc Al

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    Well, sure, but what does that equal? Give me an actual number.
     
  10. Sep 10, 2008 #9
    4.0 cm!!
     
  11. Sep 11, 2008 #10

    Doc Al

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    Exactly! Now set up the equations and solve for t and then x_1.

    Be sure to use standard units: 0.04 m.
     
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