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Voltage without charge = current?

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johnpjust
#1
May6-11, 08:53 AM
P: 17
This would be better if drawn out, but hopefully everyone can use their imagination. Consider a DC voltage source between two wires on one end, and the other ends being open (unconnected leads). Both wires are the same length, and one lead can be considered "ground" or zero reference, and the other we'll call "high". There would be a certain amount of charge "stored" in the system based on the distance between the wires, and the length of the wires, etc.

NOW -- consider that directly between the charged leads (and in the same plane) but without touching them we place two wires of the same length at different distances from the wire we designated as reference voltage (as measured perpendicularly from the reference wire). Each of these wires would be at different voltages (higher than reference voltage, but lower than the "high" voltage wire). Now, if on each end of these unconnected wires, we put resistors that connect the wires together, would there be current flow due to the voltage differences of the two wires, even though neither of the wires is "charged"?

Obviously an EMF (inductance) can create a current flow without having a charge on the wire, but I'm trying to figure out if there is some sort of complimentary situation with voltages induced by electric fields instead of magnetic ones.....
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vanhees71
#2
May6-11, 09:58 AM
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What you describe here, is a system of capacitors. If you have a DC voltage source like a battery, in changing the setup there will only be quite short "transient currents" which lead to the rearrangement of charges (electrons) inside the wires to adapt to the corresponding static condition due to that arrangement.
johnpjust
#3
May6-11, 10:32 AM
P: 17
Yes I see your point. So if we were to turn this situation into a wire loop (no resistors) that is in the same plane as the reference and high conductors, what do you suppose the voltage of the loop would be, given that each different little "dL" length of the wire loop would be a different distance from the reference wire, yet a conductor is an equipotential surface?

Thanks.

Evil Bunny
#4
May6-11, 11:01 AM
P: 237
Voltage without charge = current?

The current you see in either scenario (wire or resistors) would be so small that it would be almost impossible to measure and it would only happen for a very small fraction of a second.

I think I see what you're getting at, but the reason you cannot get current to flow is simply because there would be no potential difference anywhere once you closed the loop. The loop would equalize almost instantaneously and then be at rest.
johnpjust
#5
May6-11, 01:04 PM
P: 17
I apologize because I kind of changed the question in my second post, but my second question was really more concerned about how to determine the voltage of the loop, not the current. Hopefully that makes sense - and maybe the symmetry is such that it's not possible to determine voltage without measuring it physically.

I do understand what you're saying about current (the current would be what power engineers call a "charging current" in the case of open transmission lines - except here since it's DC, it would only happen upon initial energization).

Thanks.
Evil Bunny
#6
May6-11, 01:09 PM
P: 237
Voltage is a measurement between two points.

If you pick any two points in your loop, your voltage will be zero. The closed loop has equalized
johnpjust
#7
May6-11, 02:53 PM
P: 17
Right - when I say voltage, I mean with regards to reference voltage as defined above. As I stated above, the loop would be an equipotential surface.

Thanks.
Drakkith
#8
May6-11, 05:57 PM
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Quote Quote by johnpjust View Post
Right - when I say voltage, I mean with regards to reference voltage as defined above. As I stated above, the loop would be an equipotential surface.

Thanks.
That would depend on the amount of charge difference on each wire and how close they are to the loop.

Also, you cannot have voltage without charge. If you applied a voltage between two wires, but had 0 charge buildup, the wires would be neutral. However, this does not happen when you apply a voltage in real life. A charge would build up on the wires depending on how much voltage was applied.


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