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Time dependent Boundary Conditions in electrical potential.

Posted Feb 23, 2018, 2:28 p.m. EST Electromagnetics, Low-Frequency Electromagnetics Version 5.3a 1 Reply

Yiqi Wang
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Hi, I am trying to model a electrical potential in a metalic cavity with 4 electrodes inserting into the cavity. Now I want to try to vary the potential of electrodes with time, i.e., changing Dirichlet bcs. I am new to COMSOL, I cant find proper option for my condition. Should I change physics from static electircal field to something else?

 Best,
 Yiqi
1 Reply Last Post Feb 24, 2018, 10:08 a.m. EST
Robert Koslover Certified Consultant
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Posted: 6 years ago Feb 24, 2018, 10:08 a.m. EST
Updated: 6 years ago Feb 24, 2018, 10:09 a.m. EST

It depends on the time scale involved. If your potentials are changing relatively slowly (and from your choice of words, I suspect they are) , then your solutions are simply a sequence of electrostatic solutions, since there are no significant time-changing magnetic fields being generated. In this case, you are not really doing "time dependent" physics at all. Rather, you are just exploring a parameter range of static potential values on your electrodes (which, for example, you might "vary with time" by turning a knob on your power supply). For example, if your cavity is small enough to sit on your desk and you are varying potentials at 60 Hz, this would probably be the case. At the other extreme, if you are varying your potentials rapidly (and potential would not even be strictly defined if changes occurred rapidly enough with time) at, for example, microwave frequencies, then you have an RF problem, not a time varying electrostatics problem. And that would be totally different than your statics problem. Then, you would also likely be more interested in E & H fields (and perhaps charges and currents) than in potentials anyway. In between, you have what Comsol calls "quasi-static" and uses a lot in their AC/DC module. This basically applies when there are time changing magnetic fields that induce currents -- it's good enough for modeling transformers and motors, but generally not for antennas, waveguides, or microwave cavities. So, in summary, you really have to decide what physics regime you are working in, before you set up your model.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
It depends on the time scale involved. If your potentials are changing relatively slowly (and from your choice of words, I suspect they are) , then your solutions are simply a sequence of electrostatic solutions, since there are no significant time-changing magnetic fields being generated. In this case, you are not really doing "time dependent" physics at all. Rather, you are just exploring a parameter range of static potential values on your electrodes (which, for example, you might "vary with time" by turning a knob on your power supply). For example, if your cavity is small enough to sit on your desk and you are varying potentials at 60 Hz, this would probably be the case. At the other extreme, if you are varying your potentials rapidly (and potential would not even be strictly defined if changes occurred rapidly enough with time) at, for example, microwave frequencies, then you have an RF problem, not a time varying electrostatics problem. And that would be totally different than your statics problem. Then, you would also likely be more interested in E & H fields (and perhaps charges and currents) than in potentials anyway. In between, you have what Comsol calls "quasi-static" and uses a lot in their AC/DC module. This basically applies when there are time changing magnetic fields that induce currents -- it's good enough for modeling transformers and motors, but generally not for antennas, waveguides, or microwave cavities. So, in summary, you really have to decide what physics regime you are working in, before you set up your model.

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