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Electric charge much lower than expected in plate capacitor

Posted Aug 17, 2020, 7:55 a.m. EDT Electromagnetics, MEMS & Nanotechnology, Structural & Acoustics 4 Replies

Jonas Schnettker
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Hi there,

the charge I get from global evaluation is much lower than expected on my terminals.

My model is a bending beam, which should touch the bottom of a cavity by electrostatic attraction. I started off with electrostatics, but global evaluation of the terminal charge returns a 1E-9 smaller value.

Preliminary considerations are based on a plate capacitor ( Q = V * C ) with 30 V potential difference, 20 mm^2 plate area, 70 um plate distance and the relative permittivity is 80. This results in a charge of about 6.07 C. But global evaluation of the terminal charge returns 6.06E-9.

However, when I set the terminal constraints to charge, an electric field is not generated at all.

Any idea where I could be mistaken?

Thanks in advance,

Jonas

4 Replies Last Post Aug 17, 2020, 11:27 a.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago Aug 17, 2020, 8:11 a.m. EDT

Jonas,

the result of your hand calculation is much too high. The Comsol result is in the correct order of magnitude.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Jonas, the result of your hand calculation is much too high. The Comsol result is in the correct order of magnitude. Cheers Edgar
Jonas Schnettker
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Posted: 4 years ago Aug 17, 2020, 8:30 a.m. EDT

Thanks for the quick response!

Can you explain, why that is the case?

Jonas

Thanks for the quick response! Can you explain, why that is the case? Jonas
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago Aug 17, 2020, 8:58 a.m. EDT

Just take the textbook formula for a plate capacitor.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Just take the textbook formula for a plate capacitor.
Dave Greve Certified Consultant
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Posted: 4 years ago Aug 17, 2020, 11:27 a.m. EDT

Whenever the NUMBER is right...but the order of magnitude is wrong...check the units.

(In fact, check the units anyway.)

Whenever the NUMBER is right...but the order of magnitude is wrong...check the units. (In fact, check the units anyway.)

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