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Homework Help: Electricity/magnetism problem

  1. Dec 5, 2011 #1
    Hi, I was given following problem to solve


    Now, I am not looking for solution, but i would appreciate if someone explain what exactly they are asking me to do (is this about finding final charge vector? or is this about gaussian surface, I am new at electricity and magnetism so I am pretty confused about this problem) and give me some revision materials relevant for this problem (or even better, examples on how to solve similar problems)

    Best regards.
  2. jcsd
  3. Dec 5, 2011 #2


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    Homework Helper

    Welcome to PF!
    I can't help much until you show your work, but I should be able to get you started.
    It is not a Gaussian problem. Just an E = kqQ/d² one. Part 1 asks you to find the dE at M due to a charge dq at P. No work needed; just write the answer. Of course it will be all symbols since you aren't given numbers for any of the quantities.

    Ultimately, in part 6, you will find the formula for the E field at M due to the charge on an infinitely long wire. You can look that up if you like to know where you are heading in advance.

    This is a very useful technique and an important problem; I hope you enjoy it!

    By the way, if you need some symbols like ² or λ (often used for the charge/meter density along a line charge), you can copy and paste from
    https://www.physicsforums.com/blog.php?b=346 [Broken]
    I have a few in my signature, too.

    If you have a diagram to show, the best way is to upload it to one of the free photo sites (I use photobucket.com), then paste an IMG link into your post here.
    Last edited by a moderator: May 5, 2017
  4. Dec 5, 2011 #3


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    Homework Helper

    Darn, I made a mistake and the forum isn't showing me the usual edit button!
    Of course there should be only one Q in the formula I wrote for E.
  5. Dec 7, 2011 #4
    Thax for reply,
    so, all I need to do is to find final vector considering both positive and negative charge of segment? In that case I assume that charge vector should point directly downwardat point x, is that correct?

    Also, dE mean electric field vith certain direction (vector) and not derivative? It may sound like stupid question but I really just started on electricity.

    Also, about formula for infinitely long wire, where do I supposed to find it?

    Best regards.
  6. Dec 7, 2011 #5


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    Homework Helper

    It doesn't say anything about two charges of opposite sign. "Uniform charge density" implies the same charge below OM as above, so the E field at M will be horizontal.

    dE is an infinitesimal bit of electric field. Start with a dE = f(y)*dy, change variable y to theta and integrate dE to get E.

    Google it if your text doesn't have it.
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