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Capacitance

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Hi, I want to measure the capacitance of a simple parallel capacitor inside Comsol. I defined a global variable using “Options/ Integration Coupling Variable/boundary variables”. I named it C and used the expression as follow: “4*epsilon0/(5[um]+w2)”.

After solution, when I want to plot the variable in “post processing/Global variable plot”, I face the Error message No. 8032 which says “ C is not a valid global expression”.

Please help me what is wrong in defining the variables?

6 Replies Last Post 22.06.2010, 20:17 GMT-4

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Posted: 2 decades ago 14.08.2009, 01:31 GMT-4
Hi there, I think your variable is only defined on the boundary and is not a global variable. In any case if you want to plot your variable try going to Post processing->domain plot parameters select the "surface" tab then select the boundary on which you defined the variable C then type C in the expressions field then hit apply.
Hope that helps
Ryan
Hi there, I think your variable is only defined on the boundary and is not a global variable. In any case if you want to plot your variable try going to Post processing->domain plot parameters select the "surface" tab then select the boundary on which you defined the variable C then type C in the expressions field then hit apply. Hope that helps Ryan

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Posted: 2 decades ago 02.09.2009, 10:15 GMT-4
I am also trying to model a simple parallel plate capacitor but I do not really know where to start. I used the AC/DC electrostatics 3D model. Where can I find more information about modeling the electric field between the plates as well as measuring the capacitance?
I am also trying to model a simple parallel plate capacitor but I do not really know where to start. I used the AC/DC electrostatics 3D model. Where can I find more information about modeling the electric field between the plates as well as measuring the capacitance?

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Posted: 2 decades ago 02.09.2009, 11:57 GMT-4
To Ryan: Thank you very much. I tried the other way and I successfully get the result.
To Christo: I recommend you to read the examples in model Library of MEMS Module and try to do them yourself. It's really helpful.
To Ryan: Thank you very much. I tried the other way and I successfully get the result. To Christo: I recommend you to read the examples in model Library of MEMS Module and try to do them yourself. It's really helpful.

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Posted: 2 decades ago 07.09.2009, 10:32 GMT-4
Thanks! It works.
Thanks! It works.

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Posted: 1 decade ago 22.06.2010, 19:52 GMT-4

Ryan: I think your variable is only defined on the boundary and is not a global variable.


I get the same error message. I defined my variables in global expressions yet I face this problem. I can plot using surface/point in post processing mode, but this doesn't let me plot multiple global expressions in a single plot. If plotting from global expressions work fine, making multiple plots in a single graph would be really easy.

Any suggestions?

Regards,
Rajesh.
[QUOTE] Ryan: I think your variable is only defined on the boundary and is not a global variable. [/QUOTE] I get the same error message. I defined my variables in global expressions yet I face this problem. I can plot using surface/point in post processing mode, but this doesn't let me plot multiple global expressions in a single plot. If plotting from global expressions work fine, making multiple plots in a single graph would be really easy. Any suggestions? Regards, Rajesh.

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Posted: 1 decade ago 22.06.2010, 20:17 GMT-4
Hi Rajesh, that is very odd, you should be able to plot your variables in "Global variables plot.." I am able to do so in my models, perhaps you could email me your model and I could have a quick look at it. Did you update your model after defining the variables??
Regards
Ryan
Hi Rajesh, that is very odd, you should be able to plot your variables in "Global variables plot.." I am able to do so in my models, perhaps you could email me your model and I could have a quick look at it. Did you update your model after defining the variables?? Regards Ryan

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