Help settle an old dispute

[Seraphiel]

That's good news. Can anyone draw up two simple circuits, that'll demostrate both situations in real life?

 

[BrownTown]

Yeah, just watch for example this tesla coil. You'll see that tesla coil forms arcs at "random" between the high voltage of the top of the tesla coil and the grounded surroundings. Like I said before you can say nothing in life is truly "random" and to my understanding these sorts of arcs have alot to do with things like cosmic radiation coming in and ionizing a path which starts the arc going and stuff like that, but for all practice purposes it is random, you would not be able to generate a function which would tell you the current going through any given path at any given time (not even close). For example if you have a 5 battery connected to two resistors in parallel one of which is 10 ohms and the other is 20 you can know to a high degree of accuracy that .5 amps will be going through the 10 ohm resistor and .25 amps will be going through the 20 ohm resistor. For the tesla coil it will be 0 amps 99.9999% of the time and then maybe BOOM 1000 amps across some random path for a tiny fraction of a second.

EDIT: anyways, the "difference" between what people are telling you here is based on exactly what range you are considering. For most applications a "resistor" is assumed to have a constant resistance, however any REAL material will not have a constant resistance nor will it have a linear I-V transfer curve. If you are looking at a metal film resistor at 0V to 100V then maybe you are fine and the resistance is constant, but get up to 100,000V and your gonna arc across the terminal and suddenly have a much different resistance. The same thing is true for every other material. It is even true in low voltage applications with the right types of materials (IE: semiconductors), you can make junctions where the resistance is FAR from linear even in the range of a few volts (a diode might have a forward biad voltage of .7V and reverse bias voltage of 5V). In a diode such as that the resistance changes orders of magnitude based on the applied voltage. You can even have a NEGATIVE resistance in some materials at different conditions.

 

[silverpig]

Actually, I think it is possible. You have to cheat a little and use electromigration, but I think it would work. I'll explain more later when I have time.

 

[KIAman]

Originally posted by: BrownTown
That is a nice model that they try and teach you in freshmen "Circuits I" class, however it doesn't take into account a whole lot of the properties of electric fields, and in this case electric fields in non-linear materials. For example lets say we had the two paths here, one with a diode with threshold voltage 0.7V and another with threshold voltage 1V and both with internal resistance of 100 ohms. Now if we go and apply 0.8V the vast majority of the current will be going through the path with the 0.7V diode and not with the 1V diode even though they both have the same nominal resistance (in reality they don't have the same resistance because their resistance is a function of applied voltage and not a constant which is the whole point here). Now the instance people here are discussing of an air gap is exactly the same thing, as you increase the electric field the resistance stays the same (incredibly large value) until you reach the breakdown threshold and an arc forms across only ONE of the two paths. At this point the resistance in that path decreases 10+ orders of magnitude as the air is ionized while the other path is still very high in resistance. At that point ALL (99.9999999999%) of the electricity is going through the arc. The important thing is that this is more or less random as to which way it goes, but the point is that initially both paths have the same resistance and the same electric field applied across them and yet in the end 99.999999999999% of the current is flowing in one path and not the other.

Air and wire are very different materials. Air is an insulator and requires the potential voltage to be high enough for the arc to be created which will drop resistance in the arc due to plasma. It is the drop in resitance that allows most of the current to flow on 1 path.

Wires are conductors and does not require a high potential voltage. The resistance goes up as temperatures go up due to current flow. Given 2 equal paths, the current will flow through both paths cause by the electromagnetic force applied to both paths. If for some reason one path has a little more current than the other, the resistence goes up and the current flow is balanced with the other wire.

 

[BrownTown]

Originally posted by: KIAman
Air and wire are very different materials. Air is an insulator and requires the potential voltage to be high enough for the arc to be created which will drop resistance in the arc due to plasma. It is the drop in resitance that allows most of the current to flow on 1 path.

Wires are conductors and does not require a high potential voltage. The resistance goes up as temperatures go up due to current flow. Given 2 equal paths, the current will flow through both paths cause by the electromagnetic force applied to both paths. If for some reason one path has a little more current than the other, the resistence goes up and the current flow is balanced with the other wire.

OK, so I don't get the point of your post? Are you agreeing with me or not? I mean you quote my post and then talk about the difference between an insulator and a conductor. Since the OP never specified which material was to be considered I used the example of air to explain a way in which his circuit could be achieved.

 

[Seraphiel]

Thank you, BrownTown and everyone else. I actually think, I understand it now:

In the particular example given in my first post, with say all-cobber wires, then the flow of current would be almost the same. The only random element would be the difference in current that actually flows at any given time, but the difference can be ignored in that particular example.

However, if setup and materials were set to this particular configuration:

http://img267.imageshack.us/my.php?image=quexb9eg8.jpg

... and where the light-grey area is air, then there would be an unpredictable (not random by default) majorrity of current flowing from C to A, or from C to B.

This true?

Hope it isn't a problem, but I really want to understand this, and not just know the correct result. I know, this is probably outside my realm of understanding, but I am going to try anyways.