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People say yes, and give a wonderful example of vacuum tubes, CRTs. But can we really say that vacuum (..as in space) is a good conductor of electricity in a very basic sense?

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The conductivity of the vacuum is not a very trivial issue. In fact, depending on how you look at it, it behaves in two different ways.

Firstly, there is no retarding force on any charged particle with constant velocity in vacuum. To this extent, no extra work is required in maintaining a constant current through any surface in vacuum.

In stark contrast however, is the presence of free charges in conductors. Normally, when an electric field $\mathbf{E}$ is applied across a conductor, we get a current density due to the 'internal' charge flow, given by: $$\mathbf{J} = \sigma\mathbf{E}$$ where $\sigma$ is the conductivity. Clearly, $\sigma = 0$ in a vacuum - electric fields do not spontaneously cause currents to flow. Thus, in this sense, the vacuum is not a conductor at all. Even everyday insulators have low but non-zero values of $\sigma$.

Thus, the resistance of the vacuum is in fact, infinite, as long as we define resistance in terms of the response of the charge carriers of a material. In this sense, we might say that it is an insulator - there are no charge carriers.

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No, in the very basic sense it is not a good conductor, because very high voltages are required to shoot them through. But yes it still is a conductor, because it allows the flow of current.

Compare this to a diode, which similarly only allows current (in the same very basic sense) to flow if a certain voltage is applied.

Such non linear behaviour exceeds anything one would describe as basic, but if the basic sense of a conductor is that it allows current to flow, then it is a conductor indeed.

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But can we really say that vacuum (..as in space) is a good conductor of electricity in a very basic sense?

No, because vacuum is not a material object. The word conductor was meant for material bodies. It is not usually used to describe vacuum, because vacuum is not merely a different body from metal or dielectric, but it is a different concept - a lack of matter.

(Language note: it is possible to let charges pass through it with no resistance, but I would not call it conductor just because of that. Conducting is associated with influence of the conductor on the motion of the conductee - directing the motion - which vacuum does not have.)

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Electricity is a flow of electrons. Electrons can flow across a vacuum. The problem with doing this over a long range is that you need a force to get the electrons to travel across the vacuum.

In a CRT the cathode is heated, which gives the electrons the energy they need to escape the cathode. A large electric field then accelerates the free electrons across the vacuum and onto a target (screen). In this case, other fields are also used to steer the beam to get an optimal picture.

If you have a different system - imagine an anode and a cathode in a vacuum seperated by a small distance - with no deliberate heating taking place - then the potential difference (ie potential energy or voltage) between the two electrodes must be large eneough that the electrons can "leap" between them. They need to leap because the vacuum is a perfect insulator and so there is no medium in which they can flow (like through a metal conductor) so they must aquire all of the energy necessary to cover the distance before they can escape the cathode. Larger gap to be traversed implies larger potential difference required to get the electrons to make the leap.

Hope that helps.

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I think the problematic part of the question is the word "electricity", which is not a useful modern description of phenomena surrounding electric charge and electromagnetic fields. Charge is a phenomenon that is invariably coupled to matter. Electromagnetic fields are a phenomenon of the vacuum. Both are connected in a very deep way trough quantum mechanics. Ultimately both matter and electromagnetic radiation are different expressions of the same quantum field that permeates all of the vacuum, but there is virtually no way to express that connection properly on the level of macroscopic "electricity". What the vacuum does is to allow matter to pass trough it. Matter can carry charge, moving charge is "electricity", but it's ultimately not the charge that transports energy, but it's the electromagnetic field that is linked to it, and that field can transport energy without the need of charges, at all, but the latter is usually not called "electricity", which makes the word "electricity" of limited use to describe proper electrodynamics.

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    $\begingroup$ Electricity is not problematic word. It can be measured and it can be theoretically described. Ultimate quantum field theoretical views do not seem to be what Swami is asking. $\endgroup$ – Ján Lalinský Apr 28 '15 at 16:23
  • $\begingroup$ Electricity is what electricians deal with. Physicists are dealing with charges and electromagnetic fields, which are ultimately connected by quantum field theory. That is the problem. Swami is asking an electrician's question and physicists can only give a physicist's answer. $\endgroup$ – CuriousOne Apr 28 '15 at 16:28
  • $\begingroup$ Electrician is, according to a dictionary, a person whose occupation is the installation, maintenance, repair, or operation of electric equipment and circuitry. electricity - The physical phenomena arising from the behavior of electrons and protons that is caused by the attraction of particles with opposite charges and the repulsion of particles with the same charge. - The physical science of such phenomena. - Electric current used or regarded as a source of power. $\endgroup$ – Ján Lalinský Apr 28 '15 at 16:58
  • $\begingroup$ It seems clear to me that the original question is about what physicists deal with, not about what electricians deal with. $\endgroup$ – Ján Lalinský Apr 28 '15 at 17:00
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    $\begingroup$ Also I think is important to say that when something cannot be stated or understood in the language of QFT, it does not mean it is not physics. Physics is a rich subject. $\endgroup$ – Ján Lalinský Apr 28 '15 at 17:03
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To make things even more confusing, there is a sense in which we could assign the vacuum a "resistance" of 377 ohms: https://en.wikipedia.org/wiki/Impedance_of_free_space

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Summary: (?) A vacuum is the absence of atmosphere and is a neutral force--offering neither resistance nor conduciveness to proton/electron flow. The state of "vacuum" does not compare with the state of "space". Any/all space is a physical measure of distance and can be overcome by the optimal difference of potential.

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protected by ACuriousMind May 14 '17 at 16:56

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