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calculate the capacitance

Posted 03.07.2011, 16:15 GMT-4 Low-Frequency Electromagnetics Version 4.2 4 Replies

Xinyu Liu
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Hi,
I start to learn Comsol and use Version 4.2. I build a coaxial cylinder and can get the electric potential and electric field, but I can not figure out how to calculate the capacitance . Can any one help me with this? Thanks!


4 Replies Last Post 04.07.2011, 15:30 GMT-4
Jakub Zlamal
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Posted: 1 decade ago 04.07.2011, 03:11 GMT-4
Use definition:

Q=C*U (U is potential on electrode, Q is charge on electrode, C - capacitance).

Charge can be calculate using Gauss law if is not provided by COMSOL.
Use definition: Q=C*U (U is potential on electrode, Q is charge on electrode, C - capacitance). Charge can be calculate using Gauss law if is not provided by COMSOL.
Xinyu Liu
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Posted: 1 decade ago 04.07.2011, 09:22 GMT-4
Hi Jakub,
I found in the help that if I set the suitable boundary conditions, I can get the capacitance by evaluating the global variable (capacitance), but I could not find the terminal node in the electrostatics menu. I do not know how to set a terminal boundary condition. Could you help me?

Thanks,
Xinyu
Hi Jakub, I found in the help that if I set the suitable boundary conditions, I can get the capacitance by evaluating the global variable (capacitance), but I could not find the terminal node in the electrostatics menu. I do not know how to set a terminal boundary condition. Could you help me? Thanks, Xinyu
Jakub Zlamal
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Posted: 1 decade ago 04.07.2011, 14:01 GMT-4
I prepared simple 2D planar condensator


Its capacity should be C=epsilon_0*S/d

side length is 0.2m so C=8.85e-12*0.2/0.2= 8.85e-12 [F/m]

When You calculate field You can evaluate capacity from E=1/2*C*U0^2 so capacity

C=2*E/U0^2 where E is total energy (integral of energy density) see Results-Derived values-Capacity. It gives the same value.

Then capacitance is X=1/(C*omega)

I never calculated capacitance using COMSOL internal variable but this procedure seems to work well.
I prepared simple 2D planar condensator Its capacity should be C=epsilon_0*S/d side length is 0.2m so C=8.85e-12*0.2/0.2= 8.85e-12 [F/m] When You calculate field You can evaluate capacity from E=1/2*C*U0^2 so capacity C=2*E/U0^2 where E is total energy (integral of energy density) see Results-Derived values-Capacity. It gives the same value. Then capacitance is X=1/(C*omega) I never calculated capacitance using COMSOL internal variable but this procedure seems to work well.

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Xinyu Liu
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Posted: 1 decade ago 04.07.2011, 15:30 GMT-4
Ok, thanks
Ok, thanks

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