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I Electric field inside a sphere

  1. Dec 16, 2016 #1
    I'm confused with the electric field inside a sphere.
    The book said that E=keQr/a^3
    While one of the properities of electrostatic equilibrium mentioned that the E-field is zero everywhere inside the conductor.
    Are there any exceptional cases?

    Thanks in advance.
     
  2. jcsd
  3. Dec 16, 2016 #2

    Nugatory

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    You'll have to tell us more about the problem: Conducting ot non-conducting sphere? What if any electrical charges are present, and where are they?
     
  4. Dec 16, 2016 #3

    Doc Al

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    Are you sure that your book was describing the field within a conducting sphere?
     
  5. Dec 16, 2016 #4

    Doc Al

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    That looks to me like the field within a uniform sphere of charge Q and radius a at a distance r from the center (where r < a). It's not a conducting sphere.
     
  6. Dec 16, 2016 #5
    Thanks for answering my question.
    I think I lack of consideration on whether the sphere is conducting or insulating.
    The conducting sphere will have E=0 while the insulating sphere will have E=keQr/a^3
     
  7. Dec 16, 2016 #6

    ZapperZ

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    This is still incomplete!

    You need to go back to the book, and re-read from the beginning the derivation that produced such an electric field. Pay attention to the charge distribution or charge density inside the sphere. Is it a uniform distribution? Or does it have a radial dependence, etc......?

    If you still do not understand this, then copy the exact scenario from your text into here. Otherwise, we are looking at this problem through what you think you understood, not what it actually is.

    Zz.
     
  8. Dec 16, 2016 #7

    Doc Al

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    In case I wasn't clear: By "uniform sphere of charge Q" I mean a sphere of radius "a" filled with a uniform charge density such that the total charge is Q.
     
  9. Dec 17, 2016 #8
    You are right!
    and the charge is uniformly distributed.
     
  10. Dec 17, 2016 #9

    jtbell

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    Then the sphere is not a conductor, and the following statement does not apply.
     
  11. Dec 18, 2016 #10
    I think both conditions are not quite the same.

    For the sphere, if you have charges distributed equally on the surface of the sphere, then the net electric field inside the sphere is zero.
    You can use Gauss law. This link talks about it: http://farside.ph.utexas.edu/teaching/302l/lectures/node25.html

    For the conductor, the reason why there is no field inside a conductor is a bit different. Because a conductor has so much free electrons,
    a field cannot penetrate inside a conductor. But if you have a thin enough conductor, and a strong enough field, then sure the field can
    penetrate through the thin conductor and so you can have a field there.

    You may have to try to visualize how a field is created. A field is created by having a net charge whether positive or negative. Supposed
    you have a metal wire of your choice which obviously has net charge of zero. Although this wire has a lot of free electrons moving around but they
    are balanced out by the same positive charge of the nucleus. In order to created a field within a conductor, you need to somehow
    "move" the electrons at least temporarily to one side locally which exposes a net positive charges. So now you have a net charge
    which will give you an electric field. But since the wire or any typical conductor has so much free electrons that as soon as you "move" a
    group of electrons to one side, that empty space will immediately replaced by a bunch of free electrons from nearby since there are
    so much available electrons. Think of trying to move water in a bucket with your hand. As soon as you move the water, that temporary
    void will be replaced with water nearby. That is why people say there is no field within a conductor since it's very difficult to get an electric
    field strong enough to penetrate a conductor. But on the surface of a conductor, a field doesn't have to be strong enough to temporarily
    move electrons to create a temporary net charge.
    On the other hand, if you have a non-conductor which is the opposite of conductor that there are very little free electrons, therefore
    it's easier to create a net charge since there are not enough free electrons to fill the gap.
     
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