Question about DC voltage arcs

[FrankenPC]

To simplify, I'm specifying DC.
Assumption: charge is define as motion of electrons through solid metal conductor. 

When an HV charge arcs from it's conductor across an air gap, what is actually happening?  I can't visualize what's happening to the charge and EMF field.  What's it doing? 

I imagine it's converting to ions when a plasma is formed.  Is that all there is to it?  It converts to a plasma, continues the matter path to the other conductor and goes on it's way having lost EM field strength to heat and light?  Then it converts back to normal electron charge motion as it proceeds down the other conductor to the ground return path??

[Rick Law]

To simplify, I'm specifying DC.
Assumption: charge is define as motion of electrons through solid metal conductor. 

When an HV charge arcs from it's conductor across an air gap, what is actually happening?  I can't visualize what's happening to the charge and EMF field.  What's it doing? 

I imagine it's converting to ions when a plasma is formed.  Is that all there is to it?  It converts to a plasma, continues the matter path to the other conductor and goes on it's way having lost EM field strength to heat and light?  Then it converts back to normal electron charge motion as it proceeds down the other conductor to the ground return path??

Hi FrankenPC,

I think you must first change your assumption.  Charge is not electrons moving, but the energy to move electrons.  So to have a charge is to have an amount of energy stored that can be use to move electrons.

But, you are spot on with ionization (plasma).

As the energy builds, its ability to move electron increase.  To have 10eV energy (electron volt: typical unit used in particle physics) means I have the energy to move 10 electrons across 1 volt potential, or to move 1 electron across 10volt potential.

Air is not an insulator in an elementary particle sense.  Say an electron sits somewhere between the earth and the moon (no air), putting some air in between is not going to stop the electron moving across.

First, a side track explanation for the next paragraph: How electrons moves in metal: It moves in metal by pushing an electron in, the one you push in is not the one that came out - like pushing a ping-pong ball into a tube full of ping-pong balls.  A ball came out the other end, but likely not the one you push in.  Unlike a copper wire, air just doesn't have any "space" for you to push an electron in.  Electrons in air are not free to move around like electrons in metal.

Now back to how it moves:

Lets use a Van De Graaff generator as an example (typical in movies).  As the charge build on the metal ball, the repulsive force between electron builds.  It no longer needs an existing electron-track like a metal wire.  As the electron jumps the surface (moving farther out than normal while "orbiting"), it has enough energy to ionize the air molecules next to it.  Ionize means separating (air) molecules into + and - part.  It gets close enough to disrupt the + attracting to the - thereby disrupting the bond in that molecule.  Now the electron has a + near by that increases its ability to go farther out.  The +ion is donated by the molecule it ionized.  As it moves and ionize the air around it, the ionized air (ions of air molecules)  becomes by definition plasma.  Plasma is the forth state of matter where +ion and -ion are dissociated (no longer bonded to each other) but the + and - are still in balance.  So, in effect, it created a track for itself and a conducting track for other electrons to follow.  It enlarges (in a arc) as the following electrons ionize more air widening the track.

Above is somewhat simplified but convey the idea adequately.  But in fact, an electron does not even need that.  Copper oxide is not a conductor, yet everyday, we flip a switch and light comes on flowing across the oxidized copper switch.  Quantum tunneling effect allows the electron to jump across an energy barrier.  The lower the jump, the higher the probably that an electron will "tunnel" by "borrowing" energy from nature.

If it does not even need to borrow energy since the charged ball has plenty, it just need a jolt to turn some air into plasma and the (air isolator) barrier now acts like a broken dam.  A huge rush of electrons follow.  The plasma track created by the "pioneering" electrons IS the conductor.

Rick

[FrankenPC]

If it does not even need to borrow energy since the charged ball has plenty, it just need a jolt to turn some air into plasma and the (air isolator) barrier now acts like a broken dam.  A huge rush of electrons follow.  The plasma track created by the "pioneering" electrons IS the conductor.

Rick

AHHH!  OK.  So, the "magic" is in the buildup to the plasma state and that involves weirdness like quantum tunneling and excited energy states.   But once the plasma state is established, it's really nothing more than an exotic conductor at that point.

Does that sound right?

[Rick Law]

If it does not even need to borrow energy since the charged ball has plenty, it just need a jolt to turn some air into plasma and the (air isolator) barrier now acts like a broken dam.  A huge rush of electrons follow.  The plasma track created by the "pioneering" electrons IS the conductor.

Rick

AHHH!  OK.  So, the "magic" is in the buildup to the plasma state and that involves weirdness like quantum tunneling and excited energy states.   But once the plasma state is established, it's really nothing more than an exotic conductor at that point.

Does that sound right?

Yup.  Plasma is a great conductor since all the ions are already dissociated.  So, the electrons can just "move in" and bond with / blend with other ions in the "soup".  That electron will repel other electrons so they move away.  Those that moved away in turn push others away - until another "ping-pong ball came out the other side" thus the current flow.

Plasma is also present in flames (fire).  If you put two conductors very close by and use a torch blasting a flame in between (with the flame touching both conductors), you should see them conducting.  How conductive will depends on if your torch flame forms a good plasma bridge.

Electrons are so small any movement they make must be looked at using quantum mechanics.  But quantum tunneling is not so exotic.  It is the start of so many things.  That is why I brought up regular light switch.  Without quantum tunneling, you will have to keep the copper on the switch absolutely fresh (no oxides) as copper oxide is a very poor conductor.  We just utilize its effects without knowing.

[ConKbot]

A less physics based explanation(maybe to help grasp the basic behavior before expanding with more indepth physics?)  is that, once the electric field gradient exceeds the dielectric strength of the air, it causes dielectric breakdown, and ionization of the air, which provides a conductive path for electrons to flow in.  Air's dielectric breakdown is 3kV/mm, however if you have a 1mm gap, you dont need 3kV to make it flash over.  If you have 2 large spheres with a smooth surface, the electric field change is smooth between them, and it will take 3kv to jump that 1mm.  However if you have 2 needle points, at the tip of the needle the field changes rapidly, and its quite easy to make it exceed the 3kv/mm with a lot less than 3kv. 

see how close the field lines are on the needle point and spread out on the larger sphere?

Also, not only can flame be used as conductor...



It can be used to make a triode, no heater filament required!

[SandunDhammika]

If it does not even need to borrow energy since the charged ball has plenty, it just need a jolt to turn some air into plasma and the (air isolator) barrier now acts like a broken dam.  A huge rush of electrons follow.  The plasma track created by the "pioneering" electrons IS the conductor.

Rick

AHHH!  OK.  So, the "magic" is in the buildup to the plasma state and that involves weirdness like quantum tunneling and excited energy states.   But once the plasma state is established, it's really nothing more than an exotic conductor at that point.

Does that sound right?

ionization of air have two stages. Arc could be just a corona discharge or a complete electrical breakdown. In most cases there are no enough energy to have electrical breakdown air, instead of lighting like situation. In such situation air acts completely like a conductor.

In theory of insulators electrical breakdown does have a definition.
http://en.wikipedia.org/wiki/Electrical_breakdown
after that point any good insulator turned into a good conductor.

[T3sl4co1l]

More than you ever wanted to know:
http://mysite.du.edu/~jcalvert/phys/dischg.htm
Great website.

Tim

[FrankenPC]

More than you ever wanted to know:
http://mysite.du.edu/~jcalvert/phys/dischg.htm
Great website.

Tim

Thank Tim!  This is exactly what I was looking for even though I didn't know it.

"For a discharge to occur, there must usually be a source of electrons at the cathode, and the nature of this source controls the form of the discharge. Cosmic rays and natural radioactivity continually produce a small number of electrons and ions in all gases at the surface of the earth, and this gives air a small conductivity. The electrons will migrate to the anode, the ions to the cathode, and a small current will flow. ..."

and this...

"More copious sources of electrons are necessary for a good discharge. One source is the photoelectric effect, when light of sufficiently short wavelength falls on a metal or semiconductor and liberates a photoelectron. Photons can also be absorbed by a molecule, which gives up an electron and becomes a positive ion. "

and this...

"Electrons already in the discharge, such as the random electrons produced by cosmic rays and radioactivity, can add to their number by ionizing gas molecules by collision. Each ionizing collision produces a new electron, and a positive ion that moves the other way, an ion pair."

I was looking for an explanation as to how the cascade is initiated.   Good read!