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Hello,

I've been trying to model a parallel plate capacitor using 2D Axisymmetryic geometry using ComSol 4.1.

I've parametrized the whole model dimensions as well as the voltage to be applied on one of the electrodes. I've built the model geometry as follows:

1. A rectangle to represent the air
2. Vertically spaced electrodes to represent the electrodes. These electrodes were removed from the air box using the difference command to simplify the model
3. A Ground boundary condition was applied on the lower electrode
4. A terminal boundary condition was applied on the top electrode. The terminal type was "Voltage" with its value set as a previously defined parameter "V" whose value is 1 [V]

I'm facing the following problems:
1. When using the terminal voltage set as the parameter "V", the solution shows the voltage to be zero across all the solution region! Even at the terminal.
2. When setting the terminal voltage to an explicitly state value, say 1 V, the solution seems to be correct showing the expected solution of the problem. Yet, the value of the capacitance extracted from the model is far from the expected value by at least a scaling factor of 10^-9. I've double checked the geometry and made sure every thing is scaled correctly buy still in vain. I tried to change the model dimensions and I even sometimes got negative capacitance!

All your help is highly appreciated. I tried to download the model available on the model Exchange but it wouldn't open on Comsol 4.1

Regards,
Mohammed Omar