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by mmmBacon 2756 days ago
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).
1 comments
twtw 2756 days ago
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.

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analognoise 2756 days ago
First part is correct. Second part: ...that's the basis of all RF systems, and exactly how antennas work, isn't it?

Obviously you can't induce a current in vacuum, but you can definitely convert EM waves to current flux in a receiving structure.

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twtw 2756 days ago
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).

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mmmBacon 2756 days ago
@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.

See: https://en.m.wikipedia.org/wiki/Vacuum_arc

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twtw 2756 days ago
Right, but that effect is pretty much unrelated to the characteristic impedance of free space and doesn't mean that vacuum is not an insulator.
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mmmBacon 2756 days ago
I’m sorry but I fundamentally disagree. GP said vacuum is a perfect insulator. Comment was in response to that demonstrating that it is not.
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analognoise 2756 days ago
Ah, gotcha, appreciate the clarification.
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