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Capacitance calculated doesn't match expectation

Posted 3 févr. 2017, 02:06 UTC−5 Low-Frequency Electromagnetics Version 4.3 3 Replies

Kurian Pol
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Hi,

Question: Capacitance calculated doesn't match expectation

I am evaluating the comsol model for the parallel plate capacitance which i downloaded from here (www.comsol.co.in/community/exchange/140/)

I find discrepancy in the computed capacitance by COMSOl and the calculated value (hand calculated using basic parallel plate capacitance equation).

COMSOL gives a capacitance value as 39.95pF. But when i hand calculate using the equation AEoEr/d, i get around 26pF. I have taken Er=1, d=1mm, A=3mm*1meter (Assuming the depth of the geometry is 1m). I have read somewhere that COMSOL takes depth of the geometry in 2D simulations as 1m.



3 Replies Last Post 7 févr. 2017, 10:25 UTC−5
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 3 févr. 2017, 08:32 UTC−5
The formula you cite is for infinite parallel plates. If the gap is not extremely small compared to the width of the plates, that formula gives a poor approximation because it does not capture what happens at the edges. This is precisely why you have to perform numerical simulations: your COMSOL model does not make such simplifications.
Jeff
The formula you cite is for infinite parallel plates. If the gap is not extremely small compared to the width of the plates, that formula gives a poor approximation because it does not capture what happens at the edges. This is precisely why you have to perform numerical simulations: your COMSOL model does not make such simplifications. Jeff
Kurian Pol
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Posted: 8 years ago 6 févr. 2017, 23:55 UTC−5
Got it thanks for the reply.

Just for confirmation, in 2D simulations depth is taken as 1meter right ? I mean, the capacitance value reported by tool is F/meter right ?
Got it thanks for the reply. Just for confirmation, in 2D simulations depth is taken as 1meter right ? I mean, the capacitance value reported by tool is F/meter right ?
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 7 févr. 2017, 10:25 UTC−5
The out-of-plane thickness is set on the top node of the Electrostatics branch in the Model Builder. By default, I believe that it is set at 1[m], but please check how it is set in your model.
Best,
Jeff
The out-of-plane thickness is set on the top node of the Electrostatics branch in the Model Builder. By default, I believe that it is set at 1[m], but please check how it is set in your model. Best, Jeff

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