# Discussion Forum

## Wire Loop Produces No Magnetic Field

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Frederick Gertz

April 14, 2012 11:15pm UTC

Wire Loop Produces No Magnetic Field

Greetings All,
I am a new user to COMSOL, and I have been trying to model a wire loop made of copper with current running through it. I have to wires that make a small loop and I was hoping (and expected) to see the magnitude of the B-Field greatest in the center of the loop. However, what COMSOL shows is a B-Field greatest at the wire. Unfortunately as well, when I try to plot a graph of the field across the wire loop I am unable to get a smooth graph (picture attached), which means I can't take the gradient of the field to try and calculate the force.

I have tried both 2D and 3D models. The 3D model is too intensive for my computer, taking in excess of an hour to run (using a very fine mesh to produce a smoother curve), and then frequently requiring a restart of the computer afterwords. I would like to do 2 things, first I would like my model to show a result of B^2 (magnetic pressure), preferably with the maximum B-Field in the middle of the loop (unless of course someone knows some physical reason why it wouldn't be), and I would like my model to produce a smooth curve when I take a "slice" of the data across the x-axis without the sharp peaks.

My thoughts were to produce the model in 2D (realizing that COMSOL will assume infinite thickness), so as to make the computations less intensive, and giving me the ability to make an incredibly fine mesh, and then just use the "Out-of-Plane" thickness option to define the thickness as d = 0.00001m, since the copper wire is only 10 microns thick. However, when I do this and use apply a Ground and a Terminal 1 to opposite ends of the wire using the "magnetic insulation" COMSOL shows me that the field is 0 everywhere, which isn't correct. I don't understand why my 2D model produces no results since it's EXACTLY like the 3D model, except having one less dimension.

Any help with this would be greatly appreciated, I have attached the following files to help understand my problem.
1. 3D model of the wire loop
2. 2D model of the wire loop
3. Jpeg of results of 3D model, for norm of the B field squared
4. Jpeg of graph produced from taking "slice" of data across the x-axis showing the two "sharp" points

I used COMSOL 4.2a, again, thanks in advance for any help.

Regards,

FG

Ralf Tschiesche

April 16, 2012 6:03am UTC in response to Frederick Gertz

Re: Wire Loop Produces No Magnetic Field

Hi,

your image of B-field is ok; look for helmholtz_coil in the model library.

Ralf

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