air gap capacitor

[hoola]

I have always heard that the energy stored in a capacitor is within the dielectric, and not stored on the plates. What about an air gap capacitor? Is the air between the plates storing charge? Seems that normal convection currents within the area of an old school tuning cap would make the oscillator unsteady and stations would drift, so it seems it doesn't hold true. What if the plates were in a vacuum? Then where would the charge stay? Seems there is some charge storage capability in the plates, but a minor amount, Large values can only be stored in a dielectric material, or so it would seem.....edd

[EdEarl]

Capacitors store charge as electrons on a plate, which would be in or near the boundary between the dielectric and plate or in the dielectric, whatever the dielectric material may be.

[Pulvinar]

hoola, I think I see what's confusing you here: the energy in a capacitor is in the electric field created by the charge, not the charge itself. A dielectric is an insulator so no charge can flow in or out of it to build up. That includes a vacuum as a dielectric, which has the baseline permittivity (ε0, 8.85 pF/m). Material dielectrics are compared to ε0 for relative permittivity. In those, the electrons are slightly displaced by the field, thereby partially counteracting it and thus increasing the capacitance.

[hoola]

I thought I remembered reading that in a dielectric, the orbits of the electrons of the atoms in the dielectic material would be elongated by the field and the charge would be stored in the amount of elongation in those orbits.....I didn't think much of it at the time, but that explanation didn't seem correct at least in some cases such as air gap. The energy stored in the electric field sounds more likely...but how is that field sustained without an accordant number of electrons sitting on the negative plate? So, isn't the field more a resultant of the stored energy, than the actual storage mechanism?...edd

Edited December 23, 2013 by hoola

[Pulvinar]

I thought I remembered reading that in a dielectric, the orbits of the electrons of the atoms in the dielectic material would be elongated by the field and the charge would be stored in the amount of elongation in those orbits.....I didn't think much of it at the time, but that explanation didn't seem correct at least in some cases such as air gap. The energy stored in the electric field sounds more likely...but how is that field sustained without an accordant number of electrons sitting on the negative plate? So, isn't the field more a resultant of the stored energy, than the actual storage mechanism?...edd

 

Electric charge is an excess or a deficit of electrons relative to the number of protons in a material, not a slight displacement of them. Electrons need to be able to flow into or out of the material for it to have a charge, and that normally only happens with conductors, such as the plates. The slight displacement of electrons in a material dielectric requires a force (the electric field) and is a form of potential energy, but it's not a charge.

 

The electric field is sustained by the charged plates (assuming no external circuit), and will be reduced by a material dielectric's slightly-displaced electrons.

[hoola]

"Electric charge is an excess or a deficit of electrons relative to the number of protons in a material" .....seems to me that the charge relates to the number of electrons in one plate as compared to the number of electrons in the other plate. The protons being safely sequestered within the nucleus, and out of capacitor functioning dynamics. "Electrons need to flow in or out of the material for it to have a charge".....well, a fully charged 40 mfd 450V electrolytic capacitor will remain charged, disconnected and sitting on the bench, much to the surprise to someone coming along to pick it up....electrons need to flow in or out in order to create, dissipate or increase a charge, not for it to have a charge. " The slight displacement of electrons in a material dielectric requires a force (the electric field) and is a form of potential energy, but is not a charge." a form of potential energy is the charge in the case of the capacitor......whether or not the charge is stored in the dielectric, a charge field, or on the plates, or all 3 is a good question, but seems to me that if there were some stored potential energy within the dielectric, it would add to overall energy output as field collapse occurs as the capacitor gets discharged.....where else would it go? Heat perhaps?....edd

Edited December 23, 2013 by hoola

[Enthalpy]

Electric energy in vacuum: yes. When a distant star radiates light, it loses energy immediately, while the telescope gets (a part of) the energy years later. Meanwhile, the energy was in vacuum, needing no matter for it.

 

Atom filled with dielectric: some situations are modelled that way. For instance a phosphorus atom in a silicon crystal. Phosphorus brings one electron more, one proton more, and only four electrons participate in the crystal bond. If cold enough like 4K, the extra electron stays attracted by the extra proton, but at a "big" distance due to the "permittivity" of silicon. "Big" means here a few atomic radii, so using the macroscopic permittivity is a bit cheeky, but the computed energy to separate the electron isn't bad.

 

An air capacitor has some 1+10^-4 times the capacitance of a vacuum one, so the air brings only worries here.

 

Vacuum capacitors do exist commercially. Because discharges do exist in vacuum as well - the processes aren't well understood - the energy density isn't significantly better than ceramic ones, but losses are smaller. Such capacitors are used at the final stages of RF transmitters (SW, maybe MW and LW) of highest power and antenna matching, to ease cooling.

[hoola]

very interesting enthalpy as to the vacuum caps !....perhaps the dielectric in these units are the virtual particles, or dark energy as it is sometimes referred to....as I think they are direct equivalents.....if true, by knowing the permittivity of the vacuum capacitor, you are really having a new understanding of one property of the dark energy phenomena.....edd

Edited December 23, 2013 by hoola

[Pulvinar]

seems to me that the charge relates to the number of electrons in one plate as compared to the number of electrons in the other plate. The protons being safely sequestered within the nucleus, and out of capacitor functioning dynamics.

Except that the plates could be made of different metals with different electron densities and be different thicknesses, so we can't just compare electron counts. But yes, the difference between the charges of the two plates determines the field strength.

 

well, a fully charged 40 mfd 450V electrolytic capacitor will remain charged, disconnected and sitting on the bench, much to the surprise to someone coming along to pick it up....electrons need to flow in or out in order to create, dissipate or increase a charge, not for it to have a charge.

 

I don't really see the distinction there since the plates won't have a charge until it's created, but you've got the idea.

 

a form of potential energy is the charge in the case of the capacitor......whether or not the charge is stored in the dielectric, a charge field, or on the plates, or all 3 is a good question, but seems to me that if there were some stored potential energy within the dielectric, it would add to overall energy output as field collapse occurs as the capacitor gets discharged.....where else would it go? Heat perhaps?....edd

 

Yes, that potential energy in the dielectric gets added -- it's part of the whole, just not a necessary part.

[decraig]

The energy, W can be expressed in two parts.

 

From the electric field alone, the energy density [math]w_f = (1/2) \epsilon E^2[/math]. This is from the electromagnetic stress-energy tensor. This has to be integrated, or added-up over the volume.

 

[math] \epsilon[/math] is the permittivity of the dielectric.

 

From the Lorentz force for the electric field, [math]W_p= \int^{d} F dx = dEq[/math]. q is the charge on one plate. d is the plate separation. For other than a vacuum, we would have to include the charge displacement within the dielectric. This is via use of the microscopic Maxwell equations were there is no displacement current term, D.

 

[Sensei]

very interesting enthalpy as to the vacuum caps !....perhaps the dielectric in these units are the virtual particles, or dark energy as it is sometimes referred to....as I think they are direct equivalents.....if true, by knowing the permittivity of the vacuum capacitor, you are really having a new understanding of one property of the dark energy phenomena.....edd

 

Nonsense.

 

Imagine two metal plates inside bulb with evacuated air.

Simply saying the simplest and the earliest version of vacuum tube - Crookes tube

http://en.wikipedia.org/wiki/Crookes_tube

 

You can apply voltage to these plates only to some point, after exceeding breakdown voltage with high enough voltage, free electrons are starting flowing between plates (vacuum arc appears) and we can see them because they emit photons like you can see on this picture.

 

 

 

In vacuum capacitor distance between plates are minimal (in Crookes tube from picture it's good 20 cm), and voltage is smaller in comparison to Crookes tube which need thousands volts to start emitting electrons between plates.

 

Traditional capacitors like electrolytic after using too high voltage, are permanently damaged.

Perhaps vacuum capacitor can survive it, because it's just emitting electrons to other plate.

Edited January 9, 2014 by Sensei