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A conductor shields its inside from its outside but not vice-versa

  1. Feb 19, 2015 #1
    A point charge outside a hollow conducting sphere will produce no field in the hollow interior: the metal shields the interior. But a point charge inside the hollow interior will produce a field outside: the same metal ceases to act as a shield. Why is this?

    (In particular, aren’t “inside” and “outside” just matters of perspective? Why can’t I think of the sphere as enclosing the rest of the Universe, instead of the small spherical region?)
  2. jcsd
  3. Feb 20, 2015 #2


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    Staff: Mentor

    The field "far away" is always given by the total charge of your system - shielding cannot change this total charge, and far away is not a matter of perspective, you can measure it.
    Shielding inside works as this is just a local change in the field geometry, something that can happen via local rearrangements of charges.

    (In a hypothetical closed universe (with a finite volume), shielding the outside region could work if you want to wait hundreds of billions of years. Most experiments don't wait that long.)
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