Discussion Forum

Hello friends,

I have an interesting simple problem that is giving inconsistent results and I would like to know exactly why this is going on. I believe it can help many out there as well. In one configuration COMSOL gives the correct capacitance value but in the other configuration it does not - even though the problem has not changed. The first configuration matches measurements using an Impedance analyzer.

I am using COMSOL 4.1 build 4.1.0.154

Electrostatics Module
- Parallel plate capacitor (8.5in x 6in) with separation of 7.5inches
- Dielectric is Air

1st Configuration
- I make a simple air box and put a Terminal BC with 1V on one side of the cap
-The other terminal is GND
-In Results-> Derived Values -> Global Evaluation ->es.C11 gives me 1.19pF.

This is approximately equal to a calculated value of C=epsilon_0*A/d = 1.5pF

2nd Configuration
- I put a bigger air box around the capacitor Air box - Dimensions are 2.75m x 5m x 2m
- The BC are the same for the little capacitor box (1st configuration) and for the larger box the BC states that there is Zero Charge on the outer walls
-In Results-> Derived Values -> Global Evaluation ->es.C11 gives me 8.1427pF.
-If I do a energy density volume integration of the little capacitor box in Results->Derived Values -> Surface Integration -> ec.We -> 5.96e-13 J
-This energy corresponds to a capacitance of 1.1912pF - which is correct (0.5*C*V^2=5.96e-13 and V=1)

So why is it that it calculates the initial capacitance wrong?

I first thought it had something to do with the mesh and the size of the airbox - changing either changes the capacitance value.

But here is my problem with that - it does not represent the reality of measuring the capacitance using an impedance analyzer in air. If changing the air box closes the field lines more increasing the capacitance thats understandable but in my example my air box is rather large

It should be the same in both cases. I have played with the Tunable MEMS Capacitor example by changing its mesh but the capacitance does not vary - which is good.

Any light on this will be greatly appreciated and would help the COMSOL community trying to resolve simple issues like this.

The file is attached.

Best
Shahriar