current flow in vacuum

[TimFox]

Whats interesting to think about that kinda makes it different then a regular circuit is what happens with the electrons if you were to some how hide the electrode the electrons are flying towards really fast. They leave the circuit

In the electron gun of a typical CRO CRT, the electrons are accelerated to an anode, but pass through a hole in the middle of the first anode to continue to further electrodes at higher positive voltage (with respect to the cathode) and the phosphor screen.
Some of the electron emission current will hit the first anode, the rest should pass to the phosphor screen in a good device.
Typical electrostatic lenses used to focus the beam use coaxial cylinders or discs (with holes) at different potentials:  there is a large literature about "electron optics".

[coppercone2]

I assume the CRT is leaky or something so it does not build up too much of a positive charge, but it must build up some charge thats why stuff sticks to it so good. IIRC the old big screen CRT tv would get balloons stuck to it and stuff

[ejeffrey]

most of which was fixing vacuum leaks.
(Perhaps things have changed in 50 years, but I doubt it.)

That's not entirely accurate, sometimes you have to fix water leaks too.

[coppercone2]

does a non grounded plasma cutter shoot out electrons that go outside the nozzle ? I mean if you just point it at a dielectric and leave the ground clip disconnected. It makes a huge difference in  cutting performance if you disconnect the ground. I thought maybe some electrons escape through the hole

[TimFox]

most of which was fixing vacuum leaks.
(Perhaps things have changed in 50 years, but I doubt it.)

That's not entirely accurate, sometimes you have to fix water leaks too.

I didn't have much trouble with water leaks (and they were easy to locate).
Vacuum leaks, leakage currents, and high-voltage breakdown (in air and power supplies) were the worst problems.

[TimFox]

I assume the CRT is leaky or something so it does not build up too much of a positive charge, but it must build up some charge thats why stuff sticks to it so good. IIRC the old big screen CRT tv would get balloons stuck to it and stuff

Typically, the phosphor screen (usually on a metallized surface) was grounded through a wire from the metallization, but the phosphor itself was an insulator that could hold a charge.
Often, the interior of a CRO CRT envelope was coated with Aquadag^TM, a colloidal suspension of graphite.

[TimFox]

Another form of electronc emission is thermionic emission.  Heat up a cathode in vacuum and electrons will "boil" off.  This is what was used in most vacuum tubes.  Even a small potential (say on the grid electrode of a triode) is enough to push them around.  Once they fly past the grid, they now see the large anode potential with accelerates them, adding energy.  In this case, the maximum current is limited by the rate thermionic emission as well as the repulsion from other electrons pushing them back towards the cathode.  Small changes in the grid potential can generate large changes in current flow at first, but eventually saturate.

The vacuum itself doesn't have resistive losses, although the vacuum is different from metalic conduction.  In metals, the charges of electrons are screened from each other and they propagate relatively freely.  In vacuum, the electrostatic repulsion can be significant.

In a simple thermionic vacuum diode, if you heat the cathode it will emit electrons that just hang around as a "space charge" cloud (with no voltage applied to the anode).
When you add a positive voltage to the anode, there are two important limiting conditions for the anode current as a function of cathode temperature and anode-cathode voltage:
1.  "Saturated emission":  with a very high anode-cathode voltage, the electrons are drawn away from the cathode fast enough that no space-charge cloud persists.  The current is almost independent of voltage (if high enough), but a very strong function of cathode temperature.
2.  "Space-charge limited":  normal operation for a vacuum tube.  The Child-Langmuir law gives a current proportional to V^3/2, roughly independent of cathode temperature (if high enough).  For typical amplifiers, this operation is preferred since it is not so sensitive to cathode temperature and depends mainly on the voltages applied to the device.

Adding the grid to control the current, the E-field at the cathode is a function of both the plate-cathode and grid-cathode voltages.  Since the grid is much closer to the cathode than is the anode, it has a stronger control of the cathode field (the relative factor is called "mu") than the anode voltage.  The field at the cathode surface
E_k is proportional to (V_ak/(mu) + V_gk).
In this ideal case, if V_gk = - V_ak/(mu), then the field at the cathode goes to zero and the tube "cuts off".

[IanB]

does a non grounded plasma cutter shoot out electrons that go outside the nozzle ? I mean if you just point it at a dielectric and leave the ground clip disconnected. It makes a huge difference in  cutting performance if you disconnect the ground. I thought maybe some electrons escape through the hole

A plasma cutter is like an arc welder. There is an electric circuit between the torch tip and the work and back again. If you don't ground the work to complete the circuit, the cutter won't work very well.

The difference between a plasma cutter and a regular arc welder is the addition of a high velocity gas jet inside the arc, which blows aside the molten metal created by the arc and leaves a hole.

[Stray Electron]

you guys are making it sound like its easier to pass current through air than a vacuum

  It is easier to pass current through a partial vacuum than it is to pass it through a gas at STP (Standard Temperature and Pressure) but it's not a purely linear function. (Which is why I recommended that you look at some of the technical data sheets and/or the CRC Handbook.) And it is much harder to pass current through an absolute vacuum.  But it's very nearly impossible to create anything close to an absolute vacuum so a lot of the arguments here aren't really relevant even though they are theoretically correct.

[IanB]

But it's very nearly impossible to create anything close to an absolute vacuum so a lot of the arguments here aren't really relevant even though they are theoretically correct.

They are close enough correct that you won't readily get any current to flow through a thermionic diode unless you turn on the heater.

[coppercone2]

does a non grounded plasma cutter shoot out electrons that go outside the nozzle ? I mean if you just point it at a dielectric and leave the ground clip disconnected. It makes a huge difference in  cutting performance if you disconnect the ground. I thought maybe some electrons escape through the hole

A plasma cutter is like an arc welder. There is an electric circuit between the torch tip and the work and back again. If you don't ground the work to complete the circuit, the cutter won't work very well.

The difference between a plasma cutter and a regular arc welder is the addition of a high velocity gas jet inside the arc, which blows aside the molten metal created by the arc and leaves a hole.

but does it shoot some electrons. it looks like maybe some might miss and go through the opening

[IanB]

but does it shoot some electrons. it looks like maybe some might miss and go through the opening

Why electrons? A plasma arc consists of ionized gas molecules. You can't really have free electrons in air, there are too many gas molecules in the way.

[TimFox]

But it's very nearly impossible to create anything close to an absolute vacuum so a lot of the arguments here aren't really relevant even though they are theoretically correct.

They are close enough correct that you won't readily get any current to flow through a thermionic diode unless you turn on the heater.

It is literally impossible to create an absolute vacuum.
However, compare the average density of molecules/cm³ and the corresponding average distance between molecules to the important dimensions of your contraption, such as electrode spacing.
In 1950s science fiction, interstellar space was usually assumed to be 1 molecule/cm³.
At a really, really good laboratory vacuum of 10^-10 Torr, the molecular density at room temperature is about 3.3 x 10⁶ molecules/cm³:  feel free to check my math.
This is maybe 0.07 mm average spacing between molecules, so there are lots of molecules in a normal lab system.

[IanB]

I have not done the experiment. Given a typical thermionic diode, can you strike an arc and get a stable plasma discharge between the electrodes supported by the residual gas pressure inside the envelope?

[TimFox]

I haven't done that exact experiment on purpose, but I have seen (visible) gas discharges inside glass vacuum tubes operated at high voltage.
When electrons hit the internal electrodes, they can knock off adsorbed gas molecules:  typically, grid current due to gas ions drops during minutes of operation as gas is absorbed in the getter.
Note that vacuum-tube residual pressure values can be much higher than "ultra-high vacuum" laboratory systems (including large-scale particle colliders).

[CatalinaWOW]

The 1B3 experiment is going to be interesting.  The newest of these available are probably going on 40 years old.  Some are undoubtedly gassy, and I would bet on quite a bit of variability in the "vacuum" in the remainder.  I would also bet that OP if he runs the experiment won't understand or implement your instructions for carefully cleaning the outside of the tube.

[radiolistener]

The formula of breakdown voltage in a partial vacuum can be found here: https://en.wikipedia.org/wiki/Breakdown_voltage#Gases_and_vacuum

Regarding ideal vacuum, I think it's hard to talk about ideal vacuum if there is some conductors at temperature > 0 K and electric field between them, because such vacuum will consists a lot of electrons detached from a conductor due to its speed + applied electric field.

And if you increase electric field (applied voltage between conductor plates in the vacuum) it leads to more electrons emission and at some point energy of flying electron will be high enough to get sputtering, so the vacuum will be contaminated not only with electrons, but also with ions.

So when you put two conductor plates into a vacuum and apply some voltage or just heating these plates, the vacuum transforms into some gas or plasma and it will not be a vacuum anymore. 

[MathWizard]

I want to do a pile of electromagnetism this winter. I have some good textbooks on it. I need to find some simulator program for some of that, so you can check your equations, and hopefully have some graphs or something to visualize.

[Xena E]

Out of sequence now in this otherwise interesting and informative thread.

There will be many that appreciate the thread content but a side issue is learning to deal with narcissistic personalities.

...if you think im annoying, its becuase i dont have gullible as my profile name, nor idiot, nor airhead. seems you are mistaking annoying with a proper functioning brain. kids also seem annoying when they seek things they desire that the people they ask dont want to give them it.

What is annoying is when someone comes along and tries to teach the teachers that they are wrong. Because obviously the intuition of someone who has nothing to go on other than their imagination is going to be more likely than the work of careful experimenters over the past 200 years. What do you think scientists were doing for 200 years? Scratching their heads and being confused until someone was able to tell them where they were going wrong?

[Xena E]

I haven't done that exact experiment on purpose, but I have seen (visible) gas discharges inside glass vacuum tubes operated at high voltage.
When electrons hit the internal electrodes, they can knock off adsorbed gas molecules:  typically, grid current due to gas ions drops during minutes of operation as gas is absorbed in the getter.
Note that vacuum-tube residual pressure values can be much higher than "ultra-high vacuum" laboratory systems (including large-scale particle colliders).

Yes.

The vacuum tube only had to be fit for purpose, and it was quickly learned that in use any attempt at perfection was lost on first use.

Therefore the inclusion of the getter was a pragmatic solution to the problem of outgassing of the internal surfaces and structures within the envelope.

I don't have any citation of it, but was once told that very high powered transmitter tubes were in some circumstances continuously pumped down during use.

Regards,
X.

[Zero999]

im probably just going to find out from experimentation. i find it difficult to believe it requires a million volts per mm to get current to flow across a vacuum gap. the supposed electrons in the wires connected to the power supply already have a force upon them like water in a hose with a nozzle at the end. squeezing the nozzle handle opens the dielectric barrier to allow current to flow. since the vacuum gap has no dielectric breakdown voltage, the current should just flow from the power supply, through the wire, along the field through the vacuum gap, through the "anode" back to the power supply, because there is nothing blocking the flow to overcome. if a vacuum gap actually behaves like a dielectric, its because something is hindering the flow of electrons, which would mean a vacuum isnt actually empty. i know you dont need a continuous strip of conducting material in order to make charge carriers to flow through an electric field. electric field experiments which show a charged object flowing from one charged plate to the other through the electric field shows this. there is no plasma or material with a continuous flow of charges used as a conductor like electrons bound to a atom of copper wire which already has sufficient force upon it to remove it from its atom to flow down the wire, so, the force applied during induction should be sufficient to make electrons flow across a vacuum gap as the electrons already are given the force needed to remove them from their atoms and should be able to continue to flow along the electric field across the vacuum gap, if they cant its not from having insufficient "work function" it would be because properties of the vacuum are preventing it. if you think im annoying, its becuase i dont have gullible as my profile name, nor idiot, nor airhead. seems you are mistaking annoying with a proper functioning brain. kids also seem annoying when they seek things they desire that the people they ask dont want to give them it.

Do you want mayonnaise with that?

What is annoying is when someone comes along and tries to teach the teachers that they are wrong. Because obviously the intuition of someone who has nothing to go on but their imagination is going to be more likely than the work of careful experimenters over the past 200 years. What do you think scientists were doing for 200 years? Scratching their heads and being confused until someone was able to tell them where they were going wrong?

I'm surprised so many people have the patience to read his word salads.

I just politely suggest they improve their posting style, then walk away, if they continue on the same line.

Yes, it seems counterintuitive that it's more difficult for current to flow across a small gap containing a vacuum than a much larger air gap, but one has to have patience to learn and take the time to understand why. I don't have much else to say, other than those who've responded have done a great job of explaining it.

[TimFox]

I haven't done that exact experiment on purpose, but I have seen (visible) gas discharges inside glass vacuum tubes operated at high voltage.
When electrons hit the internal electrodes, they can knock off adsorbed gas molecules:  typically, grid current due to gas ions drops during minutes of operation as gas is absorbed in the getter.
Note that vacuum-tube residual pressure values can be much higher than "ultra-high vacuum" laboratory systems (including large-scale particle colliders).

Yes.

The vacuum tube only had to be fit for purpose, and it was quickly learned that in use any attempt at perfection was lost on first use.

Therefore the inclusion of the getter was a pragmatic solution to the problem of outgassing of the internal surfaces and structures within the envelope.

I don't have any citation of it, but was once told that very high powered transmitter tubes were in some circumstances continuously pumped down during use.

Regards,
X.

Some high-power klystron tubes came with pumps (sputter-ion or Vacion^TM) attached to maintain the vacuum.