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Gauss's Law yet again.

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    [tex]\Phi = \int{\vec E \cdot d\vec A}= \frac{q_{enc}}{\epsilon_0}[/tex]

    3. The attempt at a solution

    The positive plate is drawn to the left of the negative plate. If we draw a Gaussian surface around the inner edge of of the positive plate, we can see that E and A are both pointing in the +x direction. Therefore: [itex]\epsilon_0EA=q_{enc}[/itex]

    However, I have no idea where to go from here. Certainly I can't just solve for Qenc if they give me distances between plates in the problem.

    How do I continue?
  2. jcsd
  3. Sep 7, 2009 #2

    Doc Al

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    Staff: Mentor

    Says who? :wink:
  4. Sep 7, 2009 #3
    This isn't just a teacher's problem tho. Comes from a textbook. Why would they give you EXTRA information? I've rarely ever come across something like that.

    What purpose does giving the distance serve? I know that the Efield is independent of distance when talking about infinite plates. And since we're negating the edging in this problem, that's what we're dealing with. But giving extraneous numbers leads to confusion.

    So if I just plopped in:

    (8.85e-12)(60)(1.2) I should get the right answer.
  5. Sep 7, 2009 #4

    Doc Al

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    Staff: Mentor

    Sure, it's a bit odd to give extraneous information, but it's not out of the question. It's up to you to realize that it's extraneous. (Are there other parts to this problem?)

    The distance is not entirely extraneous. What if the distance was 4.8 m instead of 4.8 cm?
  6. Sep 7, 2009 #5
    Understood about the distances between the plates. There are no other parts to this question. I don't believe textbooks are out there to swindle us, or cause us pain. Normally, at least in all textbooks that I've encountered, the problem gives you exactly what you need to solve it. Here's A, find B. In this way, they teach along constant lines. Certainly, there are multifaceted questions that often ask "does this make sense?" and "what does X rely on?" But when problems typically ask for absolute answers, they normally feed you what you need, and nothing more.

    So far, I'm really really unimpressed with the problem sets in this book.

    Fundamentals of Physics, 8th Ed, Jearl Walker (halliday/resnick)
  7. Sep 7, 2009 #6


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    Gold Member

    Try the "few" (compared to Halliday's) exercises of Purcell's book (Berkeley Physics course, volume 2).
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