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Wrong result while calculating field in dielectric
Posted Jun 14, 2015, 9:48 a.m. EDT Low-Frequency Electromagnetics Version 4.4, Version 5.0, Version 5.1 2 Replies
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I tried to calculate the electric field in a parallel plate with both a dielectricum and an air gap. I rougly followed the tutorial over here: www.comsol.co.in/model/download/216461/models.acdc.capacitor_dc.pdf.
When i try to calculate the electric and potential fields without an air gap evertything works fine, the field are what i expect them to be and the capacitance corresponds to reality. However when i include the air gap things start going wrong. Screenshot of the results are attached to this message.
In this picture the large piece of material between the plates is the dielectric and the small piece is the air gap. As you can see the electric field is in the dielectric and not in the gap, where it supposed to be. I checked it using Femm, see attached files, and it also makes sense since E = D/er/e0.
I only started using compsol this weekend, so I suppose i'm doing something wrong, but i have no idea what. I hoped maybe someone on this forum has an idea?
Greetings Martijn
Hello Martijn Schouten
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Today I helped someone having the same problem that I faced in this post and I thought that it may be helpfull to describe here also what is the problem with comsols "Computing Capacitance" tutorial.
The problem is that comsol doesn't handle overlapping geometry properly. If you do what the tutorial tells you, you define the air as a block around your capacitor and by doing so you create geometry overlapping with your plates and dielectric.
The solution is to copy your electrodes and dielectric via geometry->transform->copy and then to substract the copy from your block of air via a geometry->booleans and partitions->difference geometry. This makes that everywhere there is only one geometry, since the block of air now has holes on the place of your electrodes and dielectric.
Greetings Martijn
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There does appear to be a misunderstanding here. The tutorial you are referring to is correct, and COMSOL does handle overlapping geometry objects correctly. When you create two overlapping objects COMSOL will automatically resolve the overlapping objects into different domains when using the default Form Union geometry finalization method. It may be easier to understand if you first look at a 2D example such as:
www.comsol.com/model/dielectric-shielding-comparison-12625
Or, for a simpler 3D case, see:
www.comsol.com/model/electric-sensor-211
These examples will guide you towards a correct understanding of how COMSOL's geometry operations work.