capacitors

[chicopee]

Back in the early 60's I had course in the fundamentals of electricity. I remembered my instructor told the class that there was no current flow thru capacitors. I am bringing this subject now because the question recently surfaced and thru my infinite winsdom I restated what my instructor said about 35 years ago. However reading recent material I think that I may have stuck my foot in my mouth. I would appreciate some comments from the experts.

 

[jbartos]

Suggestion: There is no direct current flow through capacitors. They serve as blocking capacitors. When it comes to alternating current, the capacitors are passing this current as other passive components.
Namely, current I in amperes is:
I=jx2xPIxfxCxV
where:
PI is 3.14
f is frequency, e.g. 60Hz
C is capacitance in Farads
V is voltage in volts

 

[mpparent]

I believe in electromagnetic field theory, the current between a capacitor's plates is called displacement current, defined in Maxwell's Equations.

Mike

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 100 "Dictionary"
Displacement current is the time rate of change of the electric flux density.
Visit

http://rd11.web.cern.ch/RD11/rkb/PH14pp/node108.html

for Maxwell's equations

 

[SidiropoulosM]

The impedance of the capacitor is given by 1/jwc, where w is the angular frequency. With w=0 we have direct current and the impedance is infinite. Therefore, direct current cannot flow through. As the frequency increases the impedance gets smaller and current flows through.

Michael Sidiropoulos

 

[IRstuff]

A capacitor is, by definition, two plates separated by a dielectric.

The intent is to make the dielectric non-conductive, so there is, by design, no DC current path. There are leaky capacitors, whose dielectrics are no longer working properly, and hence somewhat conductive

TTFN

 

[peebee]

A mechanical analog for a capacitor would be a tank with ports at the left and right and a vertical rubber membrane dividing the tank in two:

--(|)--

You can't get any one-directional flow through such a device -- at least, not indefinitely, although a small "charging current" would flow for a short time while the rubber stretched out, and it would tend to help maintain a constant pressure (like a water tower, which is also providing "capacitance" to the system).

But an oscillating water pressure would be transmitted through this "water capactor", nearly unaffected.

Electrical capacitors work almost identically to the analogy outlined above.

A better model would perhaps incorporate a small hole in the membrane, which would represent a leakage resistance.

 

[chicopee]

OK, FROM THE INFORMATION PROVIDED ABOVE, I SEEM TO HAVE HEARD ALL THE ARGUMENTS WHICH REINFORCES WHAT I KNOW.
I AGREE THERE IS NO ACTUAL FLOW OF CURRENT WITH DC ELECTRICAL SYSTEM. W/ AN AC SYSTEM THERE IS AN AC FLOW WHICH IN A SENSE IS INTERRUPTED BY THE PLATES OF THE CAPACITOR ,HOWEVER THAT INTERRUPTION IS STORED IN BOTH PLATES VIA A MAGNETIC FIELD BETWEEN THEM. THAT AC MAGNETIC FIELD POLARITY REVERSES, MAKING ALTERNATING CURRENT APPEAR TO FLOW THRU THE CAPACITOR BUT IN ACTUALLITY IS NOT. DO IO HAVE RIGHT? NEED MORE INPUT FROM PARTICIPANTS. THKS!!

 

[jghrist]

One correction. The energy is stored in an electric field, not a magnetic field.

 

[jbartos]

Suggestion: The magnetic energy is stored in an inductor and electric energy is stored in a capacitor. If the inductor and capacitor in a circuit are in the resonance, then one energy transforms into the other and back during oscillations/resonance.