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Alejandro
Spain
14 Posts |
 Posted - May 27 2014 : 17:45:42
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Hello,
i have one question about FasterCap calculations:
As it is said in the help, the capacitance matrix for two conductors is  [C11+C12 -C12; -C21 C22+C21]; but when i calculate the capacitance for the second conductor without including the first one in the FastCap file i get that its capacitance is equal to C22+C21 when 2 conductors were used, instead of C22. I don't know why and this has happened me for to different scenarios.
Thanks.
Regards.
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Enrico
550 Posts |
Posted - Jun 04 2014 : 22:45:06
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I'm afraid you are confusing a bit the meaning of the terms and their e.m. meaning.
Let's take as reference example the cubes.lst file you can find in the Samples directory for FasterCap. Here you have two cubic electrodes.
The Maxwell capacitance matrix has the form [C11+C12 -C12; -C21 C22+C21]. Now if you remove one cube, you get another result, let's call it C'11, which is not, and must not be, the same as C11 of the previous matrix.
This is because the presence of the second conductor HAS an effect; from the physical point of view you can visually imagine that some lines of the electrical field generated by the charges on the first cube are not closed at infinity but on the second cube. How many and how large this effect is depends on the dimensions of the cubes and their relative position.
You will see anyway that C'11=C11+(C12*C22)/(C12+C22), i.e. the series of C12 and C22 in parallel to C11. Representing the capacitors between the conductors as done in Fig.1 of the paper "The Maxwell Capacitance Matrix" should help understanding this relation.
Best Regards,
Enrico
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Alejandro
Spain
14 Posts |
Posted - Jun 08 2014 : 19:04:02
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Many thanks Enrico. What i really don't understand is why C'11=C11+(C12*C22)/(C12+C22). If there is some paper or theory behind that, which i don't know, please comment to me.
Regards. |
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Enrico
550 Posts |
Posted - Jun 09 2014 : 16:29:32
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Nodal analysis says it. Please look at the following picture. If you need to calculate the capacitance of the left cube, you can simplify the right node. So this is the series of C12 with C22, and the resulting capacitance goes in parallel to C11.
Best Regards,
Enrico
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Alejandro
Spain
14 Posts |
Posted - Jun 10 2014 : 20:08:04
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| Thank you again. |
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Capacitance value
