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Modeling Capacitor with Electric Currents

Posted Apr 5, 2013, 9:13 p.m. EDT Low-Frequency Electromagnetics Version 4.3a 4 Replies

Jeffrey Fong
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Hi,

So I am trying to model a parallel plate capacitor with a variable overlap between the plates.
Going through the ACDC models in COMSOL, I don't really understand how using electric currents can be accurate.

I have air between the two plates and reading from the forums, everyone says to set the conductivity of it between 1 and 1E-4 but when I am trying to figure out the current traveling between the two plates using a set potential on one side and a ground on the other, the current created is just due to V=IR?

Thanks,
Jeffrey
4 Replies Last Post Apr 6, 2013, 5:28 a.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago Apr 6, 2013, 3:47 a.m. EDT

Hi,

you are probably running a stationary study. So what the model shows you is the charged up capacitor and the current flow is just determined by the conductivity of the domain between the plates.

If you are interested in the reactivity of the capacitor you must do a time dependent or frequency domain study.

Cheers
Edgar

--
Edgar J. Kaiser
www.emphys.com
Hi, you are probably running a stationary study. So what the model shows you is the charged up capacitor and the current flow is just determined by the conductivity of the domain between the plates. If you are interested in the reactivity of the capacitor you must do a time dependent or frequency domain study. Cheers Edgar -- Edgar J. Kaiser http://www.emphys.com
Jeffrey Fong
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Posted: 1 decade ago Apr 6, 2013, 4:38 a.m. EDT
Hi Edgar,

I am actually doing a time dependent analysis but that is only to change the amount of overlap between the plates. So I guess that's true, the capacitor is most likely charged up.

I tried copying the capacitor_transient model which couples EC with CIR and added an additional ALE to change the plate overlap. I think the problem may be that the oscillation time (1/81Hz) is much too long in comparison to the time constant of the RC circuit so it ends up acting like a stationary study and using the conductivity of the air.

Does this mean that EC physics is not appropriate for this type of application or is there something I am missing?

Thanks,
Jeffrey
Hi Edgar, I am actually doing a time dependent analysis but that is only to change the amount of overlap between the plates. So I guess that's true, the capacitor is most likely charged up. I tried copying the capacitor_transient model which couples EC with CIR and added an additional ALE to change the plate overlap. I think the problem may be that the oscillation time (1/81Hz) is much too long in comparison to the time constant of the RC circuit so it ends up acting like a stationary study and using the conductivity of the air. Does this mean that EC physics is not appropriate for this type of application or is there something I am missing? Thanks, Jeffrey
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 6, 2013, 5:15 a.m. EDT
Hi

specifiy if its in EC or ES,
for EC you solve for current this is based on the hypothesis that a little current can flow everywhere, and little is limited by the resultion of binary number max min ratio in conductivity should not exceed 6-8 orders of magnitude (in reality its probably the double, but we cannot represent that correctly with binary number without going to log scale and changing all physics equations accordingly. Anyhow will it really bring us any thing more ? probably not, at least not for >90% of our models

--
Good luck
Ivar
Hi specifiy if its in EC or ES, for EC you solve for current this is based on the hypothesis that a little current can flow everywhere, and little is limited by the resultion of binary number max min ratio in conductivity should not exceed 6-8 orders of magnitude (in reality its probably the double, but we cannot represent that correctly with binary number without going to log scale and changing all physics equations accordingly. Anyhow will it really bring us any thing more ? probably not, at least not for >90% of our models -- Good luck Ivar
Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago Apr 6, 2013, 5:28 a.m. EDT

Does this mean that EC physics is not appropriate for this type of application or is there something I am missing?


It is not clear what your application and what your 'question' to the model is. The beauty of computer models is that you can adapt everything, the frequency, the circuit, whatever ....

Cheers
Edgar


--
Edgar J. Kaiser
www.emphys.com
[QUOTE] Does this mean that EC physics is not appropriate for this type of application or is there something I am missing? [/QUOTE] It is not clear what your application and what your 'question' to the model is. The beauty of computer models is that you can adapt everything, the frequency, the circuit, whatever .... Cheers Edgar -- Edgar J. Kaiser http://www.emphys.com

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