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Homework Help: Couple of quick q's

  1. Jan 28, 2009 #1
    ok if you could just tell em if this sounds alright.

    (i) A large horizontal sheet of surface charge density [itex]\sigma[/itex] has a hole of radius b cout out of it. Find the electric field directly above the centre of the hole.

    I already have expressions for the electric field due to the sheet and the disc of radius b on their own - can i just use the principle of superposition to subtract the field of the disc of radius b from the field of the sheet to get the total electric field?

    (ii) By applying a Gaussian pillbox to an infintely large charged sheet of surface charge density [itex]\sigma[/itex], I obtained [itex]\vec{E(z)} = \frac{\sigma}{2 \epsilon_0} \vec{z}[/itex] for the electric field. I am now asked why this field would double if the same charge were placed on the surface of a conductor.

    I have a feeling this is because in the conductor the charges inside arrange themselves to give a net internal field of 0 and then this arrangement of the charges would cause a field to be generated outside the conductor equal to the initial external field, hence doubling it as a result of the superposition principle.

    How do these ideas sound?
  2. jcsd
  3. Jan 28, 2009 #2


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    Homework Helper

    Of course you can.
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