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Hey asume i have conected up parallel plates but of different areas

  1. Mar 28, 2010 #1
    Hey asume i have conected up parallel plates but of different areas with a given potential difference ie battery, what hapens 2 the plates actualy

    i expect a uniform field, but how can we obtain that since we would require same potential on each plate!

    and secondly i expect both plates to have same potential which is also imposible as we would require same charge density on each plate note they are of different surface area!
  2. jcsd
  3. Mar 29, 2010 #2
    Re: Capacitor

    The whole plates will be on the same potential if you connect them to the battery. That's the whole idea of the potential in electrostatics. The larger plate will not be homogeneously charged (why should it be?) There will be more charge in the area facing the second plate. The field will be approximately homogeneous between the plates if they are close enough and very small outside.
  4. Mar 29, 2010 #3
    Re: Capacitor

    Yes, they have same charge density; but not the same charge.

    In order to conceptualize this, it might be easier to think of one set of parallel plates, then break it in two capacitors of equal or unequal areas.
  5. Mar 29, 2010 #4
    Re: Capacitor

    Phrak, I don't believe that statement is true. If the capacitor is neutral at first, a voltage source can only pump charge from one capacitor plate to the other, and it cannot create charge.
    The capacitor that is split unequally, will have three capacity terms. One for each capacitor and one for the capacitance between the capacitors, since both have a net charge that is not 0.

    Here is an example of a charge density calculation, which uses a sphere and a point charge:
    http://www.robinhoodsolver.com/The_Robin_Hood_Solver_-_Example_1.html [Broken]

    If you have a larger plate and a smaller one in the center of it with a gap in between. Then the smaller one will draw the charges of the larger one towards the center, like the point charge pulls the charges on the sphere to one side.
    Last edited by a moderator: May 4, 2017
  6. Mar 29, 2010 #5
    Re: Capacitor

    Ox, I don't think we are considering the same set up. What the OP had in mind is a bit unclear. I am considering two parallel conductive sheets uniformly separated by a dielectric.

    When the separation is small compared to the area, each plate has a uniform charge distribution. Cut the area into pieces and the capacitance remains the same.
  7. Mar 31, 2010 #6
    Re: Capacitor

    Hey ox! Got you well! This means that the capacitance of the system is that due to the smaller area, we can call it the common area, actual area! Is that ox?
  8. Aug 11, 2011 #7
    Re: Capacitor

    Hey!! I come up with other confusion..... When I went through the derivation of capacitance of different capacitors I find them considering the field strength between the plates as the field strength due to one of the plates normally the positive plate

    Does it mean the negative plate does not increase the field strength between the plates??
    Contrary to the basics from coulombs law,....
  9. Aug 11, 2011 #8
    Re: Capacitor

    Hallow ox!! I love the statement but may I challenge it this way... How must should the plates acquire equal charge meaning equal potential... while the battery rods themselves are at different potentials ah??
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