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Self assembling "wires".. sooooo cool
#EE #electronics #ElectricalEngineering #HV #HighVoltage #Science @Science
@freemo Something seems off in his narration. The ring at the edge of the Petri dish is explicitly set to a negative voltage, while the wire hanging over its centre is described as having a "large voltage" and notated positive. But he then goes on to say "electrons get sprayed down to the ball bearings in the dish." If anything, they should be getting sucked up into the wire, right?
But I don't even think that's the whole story. There's a couple points in the first demo where electrons actually do start flowing, and the arc is easily visible. In normal operation, I think he electrically polarises the air without an appreciable quantity of electrons travelling in either direction.
@freemo Huh. I thought air was an extremely good insulator up until breakdown voltage. In fluid dynamics we treat it as a near-perfect insulator unless we're at very high temperatures (e.g. rocket exhaust, reentry heating) where it starts to get weird ionic species in its composition, or actively breaking it down with high voltage.
@khird It is a very good insulator up until breakdown voltage, and breakdown voltage is the point you see sparks. However while it is a good insulator it is not a perfect insulator., electrons will still flow. In fact you can **see** them flowing before breakdown or a spark gap forms.
Check out the image I attached. I took it myself. That is at about maybe 10kV or so, but what your seeing int hat picture is NOT a spark gap. witht he lights on you'd barely notice any glow except maybe right at the tips of the electrodes. Bot no spark is formed yet. That blue glow is electrons flowing and forming a corona. It isnt out of focus either, a spark would look like a sharp blue hair like path between the electrons. Prior to spark formation the corona is diffuse though still flows between the electrodes.
Because no spark gap is formed and as you pointed out the air is a good insulator, much less current is flowing for the same voltage. Once a spark gap forms current will increase tremendously, usually to the point the voltage will drop and not be able to maintain the spark.
@khird I think the part you are missing here is that air is a non-linear resistor (in technical terms we would say it has "Negative Differential Resistance"... in other words, its resistance changes as a function of the voltage/current across and through it.
Youa re corrent that normal dry air is a very close to perfect insulator. However its conductivity is directly proportional to the ions present in the air. So when you provide a high voltage across air very few electrons flow at first, but they cause ionization, the resistance drops, more ions flow, and it drops some more and it becomes increasingly conductive. At a certain point you get a runaway reaction and a spark gap forms.
Attached you will see an example of the I-V relationship in air across a very small gap. Notice the curve of is not a straight line as would be expected with a normal linear resistor. Notice how the current escapes exponentially as the voltage increases.
For this reason at low voltages the air is close enough to a perfect insulator. But at 20kV as is used int he experiment it is quite a bit more conductive than that.
@freemo I'm a little confused by the second chart. Is the knee in the curve, off to the right-hand edge, the breakdown voltage? If so, I think that's kind of showing what I was getting at - up until breakdown it's less than 50nA. That's *nanoamps*! In pretty much any context we'd treat that as an open circuit, no current - but if he thinks that counts as electrons spraying out I guess it's a matter of interpretation.
On the other hand there are some other features of the chart which make me wonder if I'm not getting something. It shows -100nA at 0V and 0A at 50V, roughly, depending on the conditions. I would expect it to pass through the origin instead, and also to be approximately antisymmetric about the y-axis. So it could just be that I don't know what I'm looking at.
@khird No the voltages are too low to show breakdown in those diagrams. breakdown is ~10kV per cm
@khird the fact the second image has a current at 0v is strange... here is the source:
@khird so seems the second graph doesnt imply. i read the study and it is testing plasma made from air, not air. the first one is more accurate.
@khird well you are certainly right that he spoke incorrectly when talking about the direction of flow of the electrons.. but otherwise he is on point. with or without an arc electrons are definately flowing. At 20kV I can tell you from expiernce electrons are flying into the air. To be clear electrons are flowing both when an arc is visible and when it isnt however far more are flowing when the arc is visible. Arcs form as cascade reactions where air resistance is lowered by ionization and more and more current flows resulting in increased ionization until the air transitions suddenly from resistor to conductor.
Also keep in mind this experiment should behave identical regardless of what side is polarized as + or -.