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Question about capacity calculation in two spheres example.

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There is great example in Comsol blog: "How to Calculate a Capacitance Matrix in COMSOL Multiphysics" (https://www.comsol.com/blogs/how-to-calculate-a-capacitance-matrix-in-comsol-multiphysics/).

All seems ok beside one point, i will be very thankful if someone can make it clear.

The question is: – According to equation for total charge of 1st conductor (Q1 = Cm11V1 + Cm12(V1 – V2) + ...), if we remove all conductors from the model beside 1st one, we obtain simple equation for capacity of single conductor Q1 = Cm11*V1.

– Consequently self-capacity of the first sphere Cm11 should be equal capacity of isolated sphere, that according to C = 4piEps0*r equals 11.3pF. But in example it equals 5.4pF (form mutual matrix calculation).

– Can someone tell please what the reason of this difference? Why Cm11 don’t equals capacity of single (isolated) sphere? Is there а something I am misunderstood?

Thanks for response.

0 Replies Last Post May 17, 2019, 9:40 AM EDT
COMSOL Moderator

Hello Gleb Kuman

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