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A electric charge question for two spheres

  1. Aug 12, 2014 #1
    1. The problem statement, all variables and given/known data

    A metallic sphere A of radius 1.00 cm is several centimetres away from a metallic spherical shell B of radius 2.00 cm. Charge 450 nC is placed on A, with no charge on B or anywhere nearby. Next, the two objects are joined by a long, thin, metallic wire, and finally the wire is removed. How is the charge shared between A and B?
    a. 225 nC on A and 225 nC on B.
    b. 90.0 nC on A and 360 nC on B, with equal surface charge densities.
    c. 0 on A, 450 nC on B.
    d. 150 nC on A and 300 nC on B.
    e. 450 nC on A and 0 on B.

    2. Relevant equations



    3. The attempt at a solution
    The answer given is D..
    My attempt was to determine the charge density when the two spheres are connected. Since they are both conudctors I took the fact that all the charge would be on the surface of both spheres. So dividing the charge Q by the total surface area of both spheres, gives a ratio of 1/4 of surface are for sphere A to sphere B. So 90/360, missing something here. thanks
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 12, 2014 #2
    I think i worked this one out, when they are connected by the wire the potentials are equal so V1=V2, simplifys to
    [itex]\frac{Q1}{Q2}[/itex]=[itex]\frac{r1}{r2}[/itex]=[itex]\frac{σ2}{σ1}[/itex]
    Leading to the correct answer..
     
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