[kenfox]

A perfect vacuum is a perfect insulator. There are capacitors that use vacuum.

[mmmBacon]

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).

[twtw]

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.

[baybal2]

Vacuum dielectric strength is 20 megavolts per metre.

Diamond is almost perfect though, at 20 gigavolts per metre, and that only given if the diamond has at least some microscopic imperfections.

[analognoise]

I think those might be reversed...

[RandomOpinion]

> A perfect vacuum is a perfect insulator.

If that were true, vacuum tubes (used for radio and early computers) wouldn't work.

[i_am_proteus]

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.

[Koshkin]

It is better to call vacuum "the perfect semiconductor," then - it only conducts when free charges are injected into it.

[akiselev]

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.

[blattimwind]

Valves work because the heating filament fills the vacuum with free electrons.

Vacuum: isolator.

Vacuum filled with electrons: not so good isolator.

[Koshkin]

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.)

[0db532a0]

Electrons can still pass through perfect vacuums.

[Koshkin]

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.

[arkades]

I think that’s implied by his pointing out that capacitors use vacuum. A cap that doesn’t allow electron flow wouldn’t be a cap.