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Combining Electric currents and Electrostatics

Posted Apr 30, 2020, 10:38 a.m. EDT Electromagnetics, Low-Frequency Electromagnetics Version 4.2 2 Replies

Lukas Herrmann
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My Problem:

I have a condensator with a small non-conducting dielectric wire in between.

I try to get out how a conductive shell around the wire shields the electric field in the wire.

I tried electric current model, but the results don't make much sense, for example the epsilon (permittivity) of 12 in the wire is totally ignored. Also the ratio between conductivity of air (cant be set to 0) and the shell is important.

So I tried to combine electric current AND electrostatic model by giving the same name for the both potentials (V). It seemed to work, but if the conductivity gets around 1e-10 S/m it doesn't make sense any more. Suddenly the field in this shell is higher than outside and for smaller conductivities smaller again. Then it is the same result as in an electrostatic model only.

So my question: Is my thought of combining the two physics generally wrong or do I just use it in the wrong way?

Thanks for answers!

PS: My version is 4.2. Sorry, in my model some things are german.

2 Replies Last Post May 4, 2020, 8:50 a.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 5 years ago Apr 30, 2020, 1:55 p.m. EDT
Updated: 5 years ago Apr 30, 2020, 1:59 p.m. EDT

Lukas,

it looks like your model is completely static. With zero air conductivity no currents flow anywhere. In case the dielectric shell has some conductivity, (even if very small) it will completely screen the inside, also its own domain. That is why ec ignores the permittivity, there is no static E-field inside a conductor even if it is a bad conductor. I don't think it is possible to have two different physics with the same dependent variable.

Of course everthing changes as soon as you consider AC or some non-zero current flow.

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Lukas, it looks like your model is completely static. With zero air conductivity no currents flow anywhere. In case the dielectric shell has some conductivity, (even if very small) it will completely screen the inside, also its own domain. That is why ec ignores the permittivity, there is no static E-field inside a conductor even if it is a bad conductor. I don't think it is possible to have two different physics with the same dependent variable. Of course everthing changes as soon as you consider AC or some non-zero current flow. Cheers Edgar
Lukas Herrmann
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Posted: 5 years ago May 4, 2020, 8:50 a.m. EDT

Thanks for your answer, I think my idea of combination of the two physics here is not the right way, because actually we don't allow current to flow.

My new way is to estimate the induced surface charges on the wire by own calculations and assumptions and use this for the boundary condition "floating potential" in electrostatics (without electric currents).

Lukas

Thanks for your answer, I think my idea of combination of the two physics here is not the right way, because actually we don't allow current to flow. My new way is to estimate the induced surface charges on the wire by own calculations and assumptions and use this for the boundary condition "floating potential" in electrostatics (without electric currents). Lukas

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