“The vacuum is one of the places where our knowledge fizzles out and we’re left with all sorts of crazy-sounding ideas,” says John Baez, a mathematical physicist at the University of California at Riverside.
Electricity means the flow of electrons and conductivity is a material’s ability to allow this flow. Normally, conductors include metals (more precisely, materials with free electrons). But when exposed to extreme conditions, even the ones excluded from the list can be compelled to allow electric flow.
To get the best understanding, we’ll look into the best insulator, a Vacuum.
Questions we’re gonna think about:-
- Is not a vacuum completely devoid of any particles (mass)?
- And if you have no matter, you have no atoms, which means no electrons?
- So, is there going to be any electricity within a vacuum?
Short answer: NO. But we are pretty good at approximating stuff in Physics when we can’t really get to its true essence, just the way we handle uncertainties.
Electricity needs electrons. Period.
Even with the latest technology, we cannot throw all the particles out of space. The LHC at CERN, for instance, requires very high speeds of particles to make them collide hard enough to get the most reliable results. But unfortunately, they still are always left with extremely tiny fractions of particles even after weeks of starting the evacuating process for the collider. But we still carry out experiments in vacuums, don’t we? Well, realistically, it is nothing but very very low proximity of particles (usually extremely expanded gases) because that is the closest we can get to a true vacuum, if not exactly there.
If a vacuum in the general sense is a gas, we know that gases are insulators. So does it conduct electricity? Yes, it does. In fact, every insulator conducts after a certain point.
How do insulators conduct electricity?
What they never teach you at school:
- Electricity cannot exist without extra electrons being supplied.
- Although insulators do not have mobile electrons, they still develop temporary polarization by shifting their electrons away from the invading external electrons (because like charges repel).
There are two scenarios in an insulator:
- If we apply an external electric field to an insulator, the field polarizes it, thereby charges it (remember, it’s not yet conducting).
- If the supply voltage is strong enough, some external electrons might also enter the lattice of the insulator, eventually increasing the charge (still not conducting).
In both cases, the static charge density increases. The insulating material restrains this static charge from flowing up to its dielectric strength.
The potential difference in an insulator should overcome the dielectric strength because the charges are immobile. The potential difference at which the strength breaks is what we call the Breakdown Voltage. This is the point when conduction starts.
The two cases of a Vacuum
The vacuum has its own dielectric strength just like other insulators: 20-40 MV/m (still varies depending upon the type of vacuum you consider). A dielectric is an insulating medium. The sole fact that a vacuum has a dielectric strength means that it isn’t a vacuum.
I. Vacuum with a conducting path
Many times the vacuums we work with have enough particles which can provide a conducting path. In that case, the only thing you need to do is break the dielectric strength of the vacuum and maintain the breakdown voltage so that it stays a conductor and allows its otherwise insulating lattice to pass on the current using polarization and thus releasing free electrons. (See: Avalanche breakdown)
II. Vacuum without a conducting path
Ultra-high vacuums have no considerable particles to allow effective charge shifting or increased charge density and line up as a conducting path. Then you don’t just care about the breakdown voltage (to make it a conductor) or breaking its dielectric strength. Rather, you also have to allow external electrons to flow through it and pass on the current themselves.
Dielectric Strength Explained
In order to introduce electricity, some source of free electrons is needed on both sides of a path, so that it can fetch electrons from the rich side and supply it to the needy one. This is usually done by using electrodes.
Now think about your life’s dream (you did not see that coming). You want to make it come true at any cost, and you love working for it, but you don’t always have the motivation. However, you still believe in yourself and one day you introspect on what’s stopping you. As soon as you know the problem, you try to find the solution. The same happens with an insulator.
The lack of motivation here is a metaphor for the lack of energy supplied.
The concept of free electrons in metals
In the case of metals, the valence electrons are comparatively farther from the nucleus and due to better shielding, much loosely bonded (the reason why we call them free electrons). Although these electrons are free to move when nudged a bit, they can’t travel much far as the nuclear influence is still not that weak. When we introduce a potential difference, the incoming electrons from the anode cause the atom they interact with to lose an electron of its own while they join it (to maintain stability). The lost electron then quickly rejoins the next atom, while that atom loses an electron as if it had a shock and the process continues.
So, electrons are not traveling very far in conductors because they are quickly grabbed by adjoining atoms, but wouldn’t do so if they had no atoms catching them in the way. If this was possible, then batteries would drain out as soon as they were made.
Conduction in an Insulator
In an insulator, the electron’s dream is to fulfill the potential between the electrodes. It is not as privileged as the electrons in a conductor who can just pass on their dream to the next atom’s electrons. The insulator’s electrons overcome this by reaching the suitable Drift velocity. (Also see: Electron Mobility, Speed of Electricity)
The extraction of the electrons from the electrodes can be done through Thermionic emission, Field emission, Particle exchange mechanism and sometimes Photoelectric effect (if you use high enough frequencies).
Once the electrons have entered the vacuum, depending on the type of vacuum:
- either the voltage has to stay large in order to pull the ejected electrons toward the other electrode
- or the drift velocity gained from emission processes should be high enough to make the current flow through the vacuum.
Speed of an electron in a practical vacuum= one million (106) m/s or about ten million (107) miles per hour
Sparks and Arcs: what’s the difference?
Continuity. In short, arcs are continuous electron discharges while sparks just end in a blink. When a material breaks down, it does so under very high voltages or maybe due to very high static electricity. This causes the abruptly released electrons to travel with a very high kinetic energy which is mostly enough to ionize the atoms present in their path.
That’s one case when you see visible arcs (due to ionization). Remember that electrons are not emitting any photons themselves. They hit other atoms in the way, making the atoms ionize. In a vacuum without a conducting path, the arc is invisible. Electrons are still there, traveling at enormously high speeds. The only difference is that they have nothing to ionize, hence no light to emit.
How easy is it to get an electric arc?
It usually depends upon the:
- the density of particles and
- the fluency of the breakdown voltage.
A breakdown is the most ideal case to get an arc because it has enough high voltages to ionize the atoms as well. So, denser the insulating medium, better will be the ionization and noticeably continuous will be the arc. That’s why arcing is easier in the air than in a vacuum.
The arc will stop abruptly as a spark instead if you lower the voltage below the breakdown point even a single volt. Lightning, for example, occurs due to the accumulation of very high static electricity (increasing the potential between the height). This finally leads to a radical discharge of charges which soon ends as the potential reaches back to normal.
Violation of the Ohm’s Law
Insulators depict one such case where the Ohm’s Law is infringed as they do not keep up the linear relationship between the voltage and the current before breakdown due to extremely high resistance, but the curve drastically increases during the breakdown.
So far so good.
The Real Vacuum
I can go on with this forever. But for the sake of my pending assignments I’d walk y’all through a glimpse:
This is the real vacuum with no mass at all. It is not practically possible (thanks to our technology). Physicists have predicted that the perfect void will be called a void in the sense that all forces and energy within it are apparently canceling each other out. Wait up! For that, we must know how matter exists in space-time.