E&M fields simulated and visualized in COMSOL, is that how they 'look' IRL?

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Summary:

E&M fields simulated and visualized, is that how they 'look' In Real Life ?

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Hi

I made a simulation (comsol) of a piece of aluminum wire (r=1mm h=10mm) excited by 1A DC inside a sphere (r=10mm) of air. Here are the 3D plots:

Screenshot from 2020-02-14 13-29-37.png
Screenshot from 2020-02-14 13-30-36.png


Since I ever only have 'seen' E&M fields depicted in text books and never in real life, I'm a bit surprised by the simulation result, I would at least have expected the M field to be more uniform like the E field.

I would appreciate if the experienced experts could weigh in here, is that really how real E&M fields would look like? Or maybe my simulation is flawed somehow, that's always a possibility.

Thank you.
KZ
 

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  • #2
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Summary:: E&M fields simulated and visualized, is that how they 'look' In Real Life ?

Or maybe my simulation is flawed somehow, that's always a possibility.
it looks pretty flawed. Your source is axisymmetric, so your fields should have axisymmetry also.
 
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  • #3
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My thought exactly, I now believe somehow E&M fields may not be fully coupled in the chosen physics interface Magnetic and Electric Fields (mef) despite the documentation stating otherwise. So I split the physics up in two physics interfaces Electric Current (ec) and Magnetic Field (mf), and now the plot looks like in the text books:
Screenshot from 2020-02-14 19-42-25.png
Screenshot from 2020-02-14 19-42-50.png


The only slight drawback of two physics interfaces is that the wire needs to be excited for each physics interface i.e. two different ways but resulting in the same.

Now I wonder how I calculate and plot the Poynting vector(?) Probably in the Derived Values in the Results tree, but how.
 
  • #4
ZapperZ
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First of all, how do you know that you've set this up correctly? COMSOL requires many tweaking, and just because you are able to produce a picture, it doesn't mean that this has any resemblance to reality. For example, how do you know the step-size is accurate enough, or if your mesh is fine enough that you haven't lost the detail?

Have you used COMSOL before and benchmarked it to something that you know the exact results for?

Zz.
 
  • #5
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First of all, how do you know that you've set this up correctly?
I don't.

COMSOL requires many tweaking
I agree.

just because you are able to produce a picture, it doesn't mean that this has any resemblance to reality.
I agree. But I was aiming for text book resemblance p.t.

how do you know the step-size is accurate enough, or if your mesh is fine enough that you haven't lost the detail?
I don't. But when I find something that I know the exact results for, then this tweaking step is on the to-do list.

Have you used COMSOL before and benchmarked it to something that you know the exact results for?
No.
 
  • #6
ZapperZ
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Then the problem here isn't physics, but rather the use and correct setup of the COMSOL routine.

Zz.
 
  • #7
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Then the problem here isn't physics, but rather the use and correct setup of the COMSOL routine.
Yes I agree, and I thought that was exactly what I said. But I see now that my mentioning of "physics interface" could be construed ambiguously.

"physics interface" is comsol terminology for software libraries that each solves certain/different types of physics problems.

I had initially chosen the physics interface Magnetic and Electric Fields (mef) which turned out to be the wrong choice for this kind of static physics problem i.e. a comsol setup mistake on my behalf.

I was not inferring that "physics" (as defined e.g. by Encyclopædia Britannica ) was the problem.
 
  • #8
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I'm just answering my own rhetorical question about how to calculate and plot the Poynting vector S when E and H is known. Maybe this has some interest from other comsol beginners like me, or maybe not.

It was actually simpler than I thought, you can simply calculate the cross product directly in the form field where you pick the values you want to plot.

Screenshot from 2020-02-15 17-15-50.png


And then the plot looks like below (all vectors are normalized). I note there are slight problems with vectors whose source point is on axis with the wire, but I believe this is a matter of a bit further tweaking.
Screenshot from 2020-02-15 17-16-39.png
 
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