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Homework Help: Quick physics question

  1. Jan 14, 2007 #1
    1. The problem statement, all variables and given/known data

    A uniform electric field of magnitude 678 N/C exists between two parallel plates that are 4.02 cm apart. A proton is released from the positive plate at the same instant that an electron is released from the negative plate. Determine the distance from the positive plate at which the two pass each other. (Ignore the electrical attraction between the proton and electron.)

    2. Relevant equations
    no relevant questions I can think of.

    3. The attempt at a solution

    Honestly I'm not even sure how to approach this question in terms of acceleration and velocity of these masses due to an electric field. I dont want somebody to do my homework for me, I just need some guidance of what equations to use and how to set them up.

    Any help would be appreciated. Thanks!
  2. jcsd
  3. Jan 14, 2007 #2


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    Homework Helper

    several ideas I can think of immediately are (ie. may not be completely reliable):
    parallel plates -> uniform field -> constant acceleration while particles are moving. Force and field strength are related in a simple manner (check your text).. with a bit of luck they should meet at the midpoint between the plates(that's only a guess) It remains for you to work out an expression for the field strength as function of distance away from the plates (or would this be an overkill? nice to check anyway....)
  4. Jan 14, 2007 #3
    the plates are pretty close and the field is uniform so the prof told us that the field strength would not weeken. He also mentioned that midway is not the correct answer...
  5. Jan 14, 2007 #4


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    Gold Member

    Well here is a hint. The acceleration of a particle in an electric field is proportional to the strength of the field and charge (q) of the particle; and inversely proportional to the mass of the particle. I basically gave you a formula in words.
  6. Jan 14, 2007 #5


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    Homework Helper

    ah yes, forgot about the difference in mass, my apology
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