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Homework Help: Conducting spherical shells

  1. Apr 18, 2009 #1
    1. The problem statement, all variables and given/known data
    I am a bit stuck with how to approach this questions;
    "A thin conducting spherical shell of radius a carries a charge q. Concentric with this is another spherical shell of radius b>a carrying a net charge Q. Describe how the charge is distributed between the inner and outer surfaces of the shells. What is the capacitance of the two shells?"
    Is the answer...just that all the charge lie on the surface of the two spheres? The electric field inside a conducting sphere is zero..so how does that work..there would be charge lying on the surface of the inner sphere which itself is inside the larger sphere so there would then be an electric field inside the larger sphere. Sorry I'm really a bit confused and would really appreciate some help with this question.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 18, 2009 #2
    i reckon that the inner sphere, being a conductor has the charge q on its surface. but if this is the same question i looked at the other day then reading the question, it doesn't specifically say that the outer sphere is a conductor does it???

    so the Q which is a net charge on big sphere is induced by the q on the litle sphere. therefore Q is negative ( and possibly Q=-q but im not sure).

    anyway i reckon field lines going from the inner sphere to the outer sphere and then field lines from inifinity to the outer sphere.

    actually Q must be -q in order for it to be a capacitor. so then its just [itex]C=\frac{Q}{V}[/itex]

    where [itex]V=(b-a)E[/itex] and you can get E by pillboxing either surface
  4. Apr 18, 2009 #3
    here's the route to your answer.

    1. use gauss law and figure out the flux between the 2 concentric conducting spheres.
    2. figure out the electric field between the 2 concentric spheres.
    3. Use the E field to figure out the potential as a function of distance between the spheres.
    4. find the Capacitance of the conducting spheres by using [tex] C = \frac {q}_{V} [/tex]
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