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Homework Help: Find capacitance and charge of two aluminum electrodes

  1. May 13, 2017 #1
    1. The problem statement, all variables and given/known data
    Two 3.0-cm-diameter aluminum electrodes are spaced 0.50 mm apart. The electrodes are connected to a 200 V battery. Find the capacitance of one of the spheres.

    2. Relevant equations


    3. The attempt at a solution

    I'm really not sure how to approach this problem. I tried solving the problem for the area of the sphere inserted into the formula, and when that turned out wrong, I then tried the volume, but the answers are not correct. I know that the formula C=ε0A/d is for a parallel plate capacitor, so I'm pretty sure I can't use it as is because I'm dealing with spheres, but I don't know where to start.
    Last edited: May 13, 2017
  2. jcsd
  3. May 13, 2017 #2
    does it have something to do with the electric field of a sphere E=Q/4πε0?
  4. May 14, 2017 #3

    rude man

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    If there are two spherical shells of practically equal radii R then you can argue that this is essentially a parallel-plate capacitor with plate area = 4πR2 and separation 0.5 mm. In fact, the answer is very accurate since there are no fringing effects as in parallel-plate capacitors.

    The problem is poorly worded so I'm guessing as to the two shells. Capacitance is for both shells, not just one ...
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