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Stray Capacitance to ground

Posted 30 apr 2022, 03:34 GMT-4 Electromagnetics, Low-Frequency Electromagnetics Version 6.0 3 Replies

Tony Sibo
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Hey,

I am trying to calculate stary capacitance to ground in a frquency domain study. The object being studied is a resistive voltage divider that conatins a conductor attaching 11 resistors in series. My task is to calculate the stray capacitance between earth and each portion of the conductor that is located between two resistors. I was advised to assign 1V to each portion of the conductor until I reach earth where the potential is zero. Then calculate C = Q/V where V = 1 and Q is obtained by surface integration of the surface charge density. The density is not given in the task and by searching under "Derived values" I only found Space charge density [C/m^3]. I tried both ES and EC Physics but still no good outcome.

I attached the comsol file and any idea/advice on how to calculate the stray capcitance is appreciated.

3 Replies Last Post 1 mag 2022, 17:33 GMT-4
Edgar J. Kaiser Certified Consultant
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Posted: 3 years ago 30 apr 2022, 10:22 GMT-4

In 2D axial you must use line integrals on the boundaries, because this is where the charge sits. You can check the surface integral checkbox to get integrated charge around the circumference of the respective edge.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
In 2D axial you must use line integrals on the boundaries, because this is where the charge sits. You can check the surface integral checkbox to get integrated charge around the circumference of the respective edge.
Tony Sibo
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Posted: 3 years ago 1 mag 2022, 07:55 GMT-4

Thank you for your reply! Yeah it is actually working only for ES Physics but the answer is very small (order 1e-20 F), I'm a bit skeptical of this result. When applying it to EC Physics, it is giving complex values for the line integration of the surface charge density, which is also unreasonable.

I am thinking that the capacitance in this case is a fixed value C = Q/U. i.e. for each potential applied to all parts of the conductor equally, the charges sitting on each surface will change in a way that keeps the ratio (Q/U) constant. are there any other methods for calculating the stray capacitance in comsol ?

Thank you for your reply! Yeah it is actually working only for ES Physics but the answer is very small (order 1e-20 F), I'm a bit skeptical of this result. When applying it to EC Physics, it is giving complex values for the line integration of the surface charge density, which is also unreasonable. I am thinking that the capacitance in this case is a fixed value C = Q/U. i.e. for each potential applied to all parts of the conductor equally, the charges sitting on each surface will change in a way that keeps the ratio (Q/U) constant. are there any other methods for calculating the stray capacitance in comsol ?
Edgar J. Kaiser Certified Consultant
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Posted: 3 years ago 1 mag 2022, 17:33 GMT-4

In order to verify your methods you may consider to use a simple model with an analytical solution. In 2D axial symmetry this could be a coaxial capacitor.

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
In order to verify your methods you may consider to use a simple model with an analytical solution. In 2D axial symmetry this could be a coaxial capacitor.

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