The vacuum has a finite impedance of ~375ohms so it is not a perfect insulator. This comes from the fundamental magnetic permeability constant mu naught divided by the speed of light. Also interesting is the fact that the vacuum impedance is an exact number because it’s solely based on fundamental constants.
Given a small enough distance, arcing is possible between 2 surfaces in vacuum as charges are ejected from the surface (that’s how vacuum tubes and these transistors work).
It is essential to distinguish between characteristic impedances (of free space or other) and resistances. Although both are real values measured in ohms, characteristic impedances are non-dissipative.
Just because EM waves can propagate in vacuum does not mean you can induce a current density.
Right. I see now that I was unclear in that second part - I should have written something like "just because EM waves can propagate in vacuum does not mean you can induce a current density in vacuum." Of course EM waves can induce current density in antennas and other structures.
> Obviously you can't induce a current in vacuum ...
Yes. I wrote that second part because 'mmmBacon seemed to be arguing that one could induce a current in vacuum; that because vacuum has a characteristic impedance, it isn't an insulator (i.e. can support current because it has characteristic impedance).
@twtw was not arguing that one can induce a current in the vacuum. However, I did say that a surface can arc in vacuum. This is because a surface can emit charges either thermally or in presence of a large electric field. This is correct and didstinct from current induction in media. Please note this phenomenon should not be confused with arc discharge in gas.
IIRC diamonds are good isolators because their crystal structure is both very close to perfect (few defects) and because their crystal lattice has all electrons tied up.
Vacuum tubes (thermionic valves) work by heating a tungsten filament-- the cathode-- until the filament is hot enough to emit electrons, which are then attracted to the positively-charged plate (anode), traveling through the vacuum (and thus flowing current). Current only flows in one direction in this setup, which has two electrodes, leading to the moniker "diode." Adding a third electrode in between the cathode and anode-- called the "grid" based on its physical shape-- allowed a vacuum tube (now a 'triode') to use a small signal to control a large one. An amplifier!
The key to the function is the heating of the cathode filament. Vacuum tube designs that use a separate heating current (which is, today, most of them) do not conduct if the heating current is not applied. "Cold cathode" type tubes are not vacuum tubes-- they are typically filled with a low-pressure gas.
An insulator by definition is matter that doesn't have freely flowing electrons so vacuums aren't an insulator. However, all insulators have a "breakdown voltage" at which point the electrons start "ripping" through the material so a vacuum has all of the properties of an insulator except the mass: the breakdown voltage is the the amount of energy required to eject electrons from the materials surrounding the vacuum and until you reach that point, no current will flow through the vacuum.
You are correct. Except, this quality of the vacuum is very fragile. "What's perfect is imperfect, it is the imperfect that is perfect." (Example: a perfectly rigid system will collapse under stress while an imperfect one might withstand it.)
Vacuum is a special case of an insulator. Others will conduct beyond some threshold voltage; vacuum will not. On the other hand, vacuum will conduct at any voltage as soon as free charges are injected into it.
A capacitor shouldn't allow electron flow between its (dielectric separated) plates. The charges should build up on the plates, then exit through the terminals when discharging. If charges cross the dielectric, then you have leakage.
Given a small enough distance, arcing is possible between 2 surfaces in vacuum as charges are ejected from the surface (that’s how vacuum tubes and these transistors work).