Non-ideal conductor in a capacitor

If the capacitor plates are not ideal conductors (and have relative dietetic constant > 1), how does that change the capacitance calculations? It is my understanding that capacitance is purely related to geometry and the material BETWEEN the plates and not the plates themselves, is that correct? (The specific material I have in mind is biological tissue such as bone, muscle...)

• A little perhaps. Depends on your overall geometry. Ask yourself how close of an approximation it would be to assume ideal conductor. – winny Mar 2 '18 at 19:52
• plates can get galvanic corrosion and dielectric can have a galvanic voltage offset and double layer charge effect in essence , many RC values in parallel. R is the effective series reistance (ESR) at the conductor/dielectric interface which may get contaminated. It means the C value depends on Vdc amplitude and frequency to some extent and ESR may also change with frequency... Do you have any data? – Tony Stewart EE75 Mar 2 '18 at 21:07

1 Answer

If the conductivity of the plates in a capacitor is much better than the medium between them, then the component will look reasonably 'capacitive', albeit with some resistive losses.

Resistance in the plates will look like equivalent series resistance (ESR). Resistance in the dielectric medium will look like a parallel leakage resistance. The dielectric constant of what's between the plates will control the capacitive part of the component's impedance.

As the materials you envisage are bone and muscle, it sounds like a measurement situation. As such, you may get good results by simply sending an AC current into the capacitor and measuring the voltage. You may get better results by demodulating the voltage into components in phase with and in quadrature to the applied current to separate the capacitive and resistive components, you could increase the specificity of the measurement to the actual changes you want to track. There are one chip solutions available to do this sort of network analysis.

• Hi Neil, Thanks for the information. If you could direct me to any references that discuss the ESR concept, I'd very much appreciate it. – Parisa Mar 3 '18 at 21:04
• @Parisa wikipedia, though it sounds like you are writing the book on your particular capacitor type. This is a one-chip network analayser IC. – Neil_UK Mar 4 '18 at 6:38