Why can two spherical capacitors in parallel model this system?

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Homework Statement



Handwritten sketch of three concentric spheres with radii a, b and c, the inner and outer shells marked as earthed, plus the handwritten result C = 4πε0 ab/(b-a).


Homework Equations





The Attempt at a Solution



Working backwards I found that adding C1 (of radius a and b) and C2 (of radius b and c) in parallel gives the answer. Not sure why they can be modeled as capacitors in parallel though..
 
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Consider the inner sphere and the inner surface of the middle shell as one capacitor, (blue) and the outer surface of the middle shell and the outer shell as the other one (yellow). A and G are the terminals of the resultant capacitor.
How are the capacitors connected? ehild
 
Attachments
  • Concentric spherical capacitor diagram: a white inner sphere, a blue shell, a black middle shell and a yellow outer shell, with terminal A wired to the outer shell and terminal G grounded via a ground symbol wired to the inner sphere and the middle shell.
    sphcap.JPG
    11.9 KB · Views: 511

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