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Evaluation of surface charge density, point evaluation divided by a factor of 3
Posted Oct 7, 2014, 12:51 p.m. EDT Low-Frequency Electromagnetics, Results & Visualization Version 4.4 1 Reply
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Hey,
I have problems to understand Point evaluation of the surface charge density.
I'm simulating a R= 1 m radius sphere with a potential of 0.5 V in vacuum, using 2D axial symmetry. Basically the sphere is in a box, with box edges set to ground, and sphere boundary set to 0.5 V potential.
When I plot the surface charge density along the sphere boundary in 1D plot, the results show constant charge of 4.5e-12 C/m^2, as expected from the theory. However, when I use point evaluation to evaluate the surface charge densities at the three points (0, R), (R,0) and (0,-R) (axis of symmetry z, sphere at (0,0), coordinates as (r,z)), the point at (R,0) gives the correct surface charge density, but the points at the axis of symmetry give a charge of 1.5e-12 C/m^2, that is the correct charge divided by a factor of 3.
Can anybody explain why?
Best regards,
Pekka Peljo
I have problems to understand Point evaluation of the surface charge density.
I'm simulating a R= 1 m radius sphere with a potential of 0.5 V in vacuum, using 2D axial symmetry. Basically the sphere is in a box, with box edges set to ground, and sphere boundary set to 0.5 V potential.
When I plot the surface charge density along the sphere boundary in 1D plot, the results show constant charge of 4.5e-12 C/m^2, as expected from the theory. However, when I use point evaluation to evaluate the surface charge densities at the three points (0, R), (R,0) and (0,-R) (axis of symmetry z, sphere at (0,0), coordinates as (r,z)), the point at (R,0) gives the correct surface charge density, but the points at the axis of symmetry give a charge of 1.5e-12 C/m^2, that is the correct charge divided by a factor of 3.
Can anybody explain why?
Best regards,
Pekka Peljo
1 Reply Last Post Oct 7, 2014, 5:17 p.m. EDT
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Posted:
1 decade ago
Yup, I can see an error like that also (but I get a factor of 2x less charge density at the poles, not 3x). Interestingly, the error doesn't appear on a line plot of the surface charge along the arc of the sphere. So I suspect this is a numerical issue regarding post-processing interpolation/evaluation. By the way, I've seen other errors of this sort appear occasionally in the past in Comsol Multiphysics in other circumstances, when plotting values computed at points on symmetry axes.
Perhaps the official Comsol folks would like to comment here?? (And maybe even produce a minor update to address this sort of thing?)
:-)
Perhaps the official Comsol folks would like to comment here?? (And maybe even produce a minor update to address this sort of thing?)
:-)