From Nessman on Flickr

From Nessman on Flickr

Here was an interesting discussion on a science teacher’s listserv, which came down to the question — can a vacuum become a conductor?  What is it that we really need in order for charge to flow between two points?  What is the physics of electron flow?  The physics teacher in question wrote:

The Paul Hewitt book has a test question that reads:  In order for charge to flow from one place to another, there must be a
A. Potential difference between the two places.
B. Conductor, such as a wire, connecting the two places.
C. both A and B.
D. none of these.

The book’s answer is C (both A and B).  I’m wondering why A isn’t the answer. In the case of the van de Graff or lightening you create a potential difference between two locations (say me and the van de Graff) and the charge will eventually flow … I guess the air acts as the conductor from van de Graff to me? But is there a situation where there is enough of a potential difference between to places and charge doesn’t flow? Would the van de Graff not work in a vacuum?

A veteran physics teacher (Al Sefl, who always knows more physics than moi) responded:

The key to the Hewlett question is that it asks about charge flow.  Current cannot flow through an insulator until the point of breakdown is reached.  If you have a sphere X with a positive charge and a sphere Y with a negative charge there will be no flow between them until a conductor connects them.  Before that conductor is there, there will be lines of electrostatic force but no flow.  If the potential were great enough the air would break down to become a conductor and you would have flow.  So, C is the correct answer, you must have a potential difference AND a conductor to conduct the charges.

Yes, a Van de Graaff generator will work in a vacuum.  High voltage capacitors used in broadcast transmitters and radar units are vacuum capacitors where the charge is stored between two plates surrounded by an evacuated space.  The electrostatic lines of force do not need air to exist.

That’s all fine and dandy, but most of the people on the listserv didn’t understand that second paragraph (and neither did I), not knowing enough about broadcast transmitters and such.  A teacher asked, do you mean that the Van de Graaff will store charge in a vacuum, but not throw off sparks?  After all, what would the charge be flowing through if the Van de Graaff is in a vacuum?  There’s no air to ionize (or “break down”).

Al responded with a clarification:

A vacuum may also be a conductor.  The old cathode ray tube of years past sends a beam of electrons from a gun through a high vacuum to impact the phosphor screen.  So, when the potential becomes high enough current will flow through a vacuum.  In a CRT it does get an assist from thermionic emission in the gun.

The miniature lightning bolts we see from a Van de Graaff are really the paths of broken down insulator air that has become conductive and ionized.  You would not see that in a vacuum.  If you put a sharp point on the negative terminal then the charge concentration will push off electrons that will travel to the positive terminal.  The vacuum will become a conductor.

So, C is still the correct answer.  If charges FLOW they must do so through a conductor.  ANYTHING will become a conductor if the electrostatic charge exceeds its dielectric.  If electrons are flowing through something it *is* a conductor.

Perhaps where the Hewlett presentation becomes unclear is the definition of what a conductor is.  Most of us immediately think of a piece of copper wire *but* it can be anything if the potential is high enough.

So, a vacuum can become a conductor, even though there’s nothing to ionize (and thus you won’t see the glow from the electrons as they travel through a vacuum, as you do in the air).  But by definition, if charge is flowing, it’s flowing through a conductor!  Paul Doherty explained that when there is an electric field that is large enough (it has to be very very large), then it will produce electron/positron pairs in the vacuum.  Those electrons and positrons are what flow to conduct electric current.

On a side note — the charged particles given off by the Sun aren’t visible as they pass through the vacuum of space… but they are visible when they hit our magnetosphere as the aurora borealis.

And another teacher offered a clarifying comment:

I was taught to get over the idea of being protected by an insulator. We were told that an insulator is a bad conductor. My trade teacher felt that insulator was a weak word and preferred to talk about everything being a conductor, just good conductors (copper) or bad conductors (glass).

So, the discussion got interestingly esoteric here.  The original questioner then posited:

If any space can be considered a conductor given a high enough potential difference, then I think the answer to Hewett’s question should be we just need a potential difference to get a flow of charge.  After all, he didn’t explicitly state that we need to have charged particles, which I think would be necessary to have a flow of charge.  So why state that an omnipresent conductor is necessary?

Also, if a vacuum has charged particles moving through it, is it still a vacuum?

Paul Doherty emphasized that the correct answer to the question is still “C.”  You can have a potential difference and no flow of charge, because the voltage may not be low enough to create its own conductor out of the insulator between the two places.  With enough potential difference an insulator is turned into a conductor, but you STILL need both a potential difference and a conductor for charge to flow.

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