# Two circuits connected by a single wire, will there be current in wire?

by jaumzaum
Tags: circuits, connected, current, single, wire
 P: 248 If 2 circuits are connected by a single wire, will there be current in the wire? If there is current, it can only goes throgh one way (ex: right or left) . In a 2 circuits like the example, Where c and d are generators, e/f/g resistors. The wire AB is inicially unplugged and then plugged. In which case of voltage of the generators/resistance of the capacitors There would be current i n AB and in which case there wouldn't be? What would happen if c or d were capacitors?
P: 146
 Quote by jaumzaum If 2 circuits are connected by a single wire, will there be current in the wire? If there is current, it can only goes throgh one way (ex: right or left) . In a 2 circuits like the example, Where c and d are generators, e/f/g resistors. The wire AB is inicially unplugged and then plugged. In which case of voltage of the generators/resistance of the capacitors There would be current i n AB and in which case there wouldn't be? What would happen if c or d were capacitors?
You just need to calculate which side of the wire has higher potential. If they're equal, no current will flow. If one is higher, current will flow from there. But I don't think we can tell without values for those components which way it will flow.
 P: 248 If c = 80V, d = 20V e = 5 ohm f = 5 ohm g = 2 ohm What would happen?
 P: 882 Two circuits connected by a single wire, will there be current in wire? @Vortex But, if you have all components values? OK. Take: c: 4.5V, d: 9V, f: 100ohm, e: 300 ohm, g: 1000 ohm. Could you tell us now what will be a currend A-B and which direction?
 P: 882 @jaumzaum: nothing happens. Maybe small current flows for a very short while just after you connect A and B - discharging their initial difference in potentials (that would be equivalent to an initially charged capacitor connecting those halves). Current always flows along some loop - A-B do not form one.
 P: 248 Thanks xts And why the current is small (or almost none) in AB? How can we calculate it? Will there be ac urrent until the 2 circuits get the same potencial difference?
 P: 248 Ok, So nobody of the whole site knows how to solve this question?
 Mentor P: 17,509 There is no current. You can analyze with node voltage or loop current, my preference is node voltage.
 P: 248 Thanks Dalespam I already know theres no current (its a question I've already s een before) I want to know why
 Mentor P: 17,509 Just use standard analysis techniques like node voltage: http://people.ee.duke.edu/~gary/ECE27/nvm.pdf
 P: 5,462 I think this is a trick question. What happens if you redraw it thus? Attached Thumbnails
 P: 248 In G: R3.i1 + R1.i1 -V1 = 0 -> i1 = V1/(R1 + R3) In H: -R2.i2 + V2 = 0 -> i2 = V2/R2 i1 = i2 + i3 -> i3 = V1/(R1 + R3) - V2/R2 But what that thing has to do with the other?
Mentor
P: 17,509
 Quote by jaumzaum But what that thing has to do with the other?
It is the same circuit. Just write the current through AB in terms of the loop currents G and H. Everything will drop out and you will get 0.
 P: 248 I didn't understand? I'm still getting i3 = V1/(R1 + R3) - V2/R2 Couldn't you solve it? My test is in 2 days, and I have a lot of things I wanted to ask too I would be thankful []s John
P: 1,403
 Quote by DaleSpam It is the same circuit. Just write the current through AB in terms of the loop currents G and H. Everything will drop out and you will get 0.
But the current through AB isn't equal to the current through the wire AB in the circuit in the origional post. (and it isn't 0 either). The wire AB was eliminated from the first circuit because the potential at A and B is obviously the same, but from the resulting circuit you can then no longer draw any conclusion about the current through this wire.

The current through the first AB wire is obviously 0 because there is no return path. If there was a current, electrons would pile up on one side, creating a potential difference, that would stop the current.
P: 280
 Quote by DaleSpam Just use standard analysis techniques like node voltage: http://people.ee.duke.edu/~gary/ECE27/nvm.pdf
Kudos for the link, great document
Mentor
P: 17,509
 Quote by jaumzaum I'm still getting i3 = V1/(R1 + R3) - V2/R2
That is correct, but i3 is not the current through AB. Let's label the current from A to B as I and express I in terms of the loop currents G and H. By applying KCL at B we see I+H-H=0 so I=0. By applying KCL at A we see -I+G-G=0 so I=0.
Mentor
P: 17,509
 Quote by willem2 But the current through AB isn't equal to the current through the wire AB in the circuit in the origional post. (and it isn't 0 either).
Yes it is, it just isn't equal to I3.

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