Can you assign a Capacitance to a non conducting structure?

[kthackst]

My working definition of capacitance is C = Q/V.

Where V is the potential difference between the bodies you wish to compute the capacitance between. This works cleanly because conductors are equipotential surfaces generally, and the potential difference is the same everywhere.

But what if the body is not a conductor? Consider an electret with permanent polarization. This has a permanent bound charge on its surfaces, and a potential between those bound charges. Is there a more general equation I could use to assign a capacitance to such a structure? Or is this not a meaningful thing to quantify?

 

[Vraj]

Your way of thinking is so different !
capacitor must has TWO CONDUCTIVE PLATES as the charge can go through it and store in the dielectric medium.because,if there is no conductive plates, then that nonconductive plates will work as dielectric medium. and i know that in circuit, you will connect it by wires. which has low surface area. and that will be consider as the surface area of capacitor. which is so little and , by this equation, your capacitance will be,C=e A/d will so so so little, you also know that there is already so less capacitance we are having. in practical it is in uc!

in eqaution, e is permittivity,A is LARGE plate area, d is SMALL distance between plates.

 

[kthackst]

I agree we EEs often think about capacitors as two plates. But this isn't necessarily true. Cavity resonators are modeled as RLC resonators, transmission lines have a capacitance to the. Dielectric resonators, which have no conducting parts, are often modeled as RLC circuits as well! In bioelectricity, nerves are modeled as transmission lines with capacitance, but are really quite poor conductors.

So why not extend this definition of capacitance to a polarized object? It seems reasonable enough to me. There is charge separated by an electric potential. The ratio between is still just a function of geometry and material properties, just like "proper" capacitance.

 

[BradtheRad]

Don't know if these examples touch on what you're looking for...

* A classmate once showed me his proximity alarm, attached to a doorknob. If a hand touched the doorknob, the circuit buzzed. Or if it got within a fraction of an inch. I failed to see how it could work since there was no path for current. I'd been taught electricity needs a complete loop. Perhaps that was when he told me it worked on the principle of capacitance, or else I read about it later.

* When Zircon first marketed an electronic stud sensor, an article explained that it detects changes as it measures the dielectric constant behind walls. I looked up 'dielectric' and found that it is associated with capacitance. So I thought about how I'd tape aluminum foil to a wall, then attach an audio oscillator. The frequency should change depending on what was behind the gypsum board. I tried to duplicate the Zircon sensor with my homebrew method. Of course I got nowhere.

* Stormclouds develop electric charges. An opposite charge develops on an area of ground below. Capacitance is operating, and if current can find a path through the dielectric, lightning occurs.

* Static charge is all over the place. On us, on physical objects. A jfet can be made into a detector. I walk across the carpet in winter and acquire a charge. Doesn't that make me one plate of a capacitor?

 

[kthackst]

Thanks Brad, this gives me more ideas on what to look up in terms of methods for computing such atypical capacitances.

 

[c_mitra]

capacitor must has TWO CONDUCTIVE PLATES as the charge can go through it and store in the dielectric medium...

Wrong, in simple words.

Consider one metallic sphere and you add some charge. The potential of the sphere is increased because of the addition of charge. Then C=dQ/dV.

In terms of the two plate concept, the other plate is at infinity (because the way potential is measured). The dielectric here will be air or vacuum- yes, they are good dielectric media.

The energy is stored in the electric field.

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* A classmate once showed me his proximity alarm, attached to a doorknob. If a hand touched the doorknob, the circuit buzzed. Or if it got within a fraction of an inch. I failed to see how it could work since there was no path for current. I'd been taught electricity needs a complete loop. ...

There IS a path for the current- Maxwell called that the displacement current. Displacement current is produced when a charge moves.

When your hand is close to the doorknob, the capacitance of the doorknob changes and a current flows. You can consider it in other way- the charge on the doorknob (it must not be at ground potential but can be a virtual ground) induces opposite charge on your body and that causes a small change in the potential of the doorknob. Small changes but measurable. The circuit is completed via the ground (potential).