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

[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,

[PLAIN]http://img594.imageshack.us/img594/838/imagemgpp.png

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?

 

[VortexLattice]

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,

[PLAIN]http://img594.imageshack.us/img594/838/imagemgpp.png

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.

 

[jaumzaum]

If c = 80V, d = 20V
e = 5 ohm
f = 5 ohm
g = 2 ohm

What would happen?

 

[xts]

@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?

 

[xts]

@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.

 

[jaumzaum]

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?

 

[jaumzaum]

Ok,

So nobody of the whole site knows how to solve this question?

 

[Dale]

There is no current. You can analyze with node voltage or loop current, my preference is node voltage.

 

[jaumzaum]

Thanks Dalespam

I already know there's no current (its a question I've already s een before)

I want to know why

 

[Dale]

Just use standard analysis techniques like node voltage:
http://people.ee.duke.edu/~gary/ECE27/nvm.pdf

 

[Studiot]

I think this is a trick question.

What happens if you redraw it thus?

 

[jaumzaum]

[PLAIN]http://img809.imageshack.us/img809/6809/imagemzs.png 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?

 

[Dale]

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.

 

[jaumzaum]

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

 

[willem2]

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 original 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.

 

[meldraft]

Just use standard analysis techniques like node voltage:
http://people.ee.duke.edu/~gary/ECE27/nvm.pdf

Kudos for the link, great document

 

[Dale]

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.

 

[Dale]

But the current through AB isn't equal to the current through the wire AB in the circuit in the original post. (and it isn't 0 either).

Yes it is, it just isn't equal to I3.

 

[jaumzaum]

Why couldn't the current be like this?

[PLAIN]http://m.UploadEdit.com/b97/2212158.gif



Where the darker the higher the current

As willem2 said, if there was a current, electrons would pile up to one side, reducin g the potencial difference and the current stops (that what happenned). But I was not able to draw the current reducing because I didn't have enoug h colors for that.

Anyway, couldn't it be like this?

 

[Studiot]

I suggest you study this reference that analyses a similar circuit in great detail.

You can put zero values into the formula to remove the extra resistors.

http://forum.allaboutcircuits.com/showthread.php?t=30963&highlight=loop+current&page=2

 

[Dale]

Why couldn't the current be like this?
... electrons would pile up to one side

That is why. Electrons piling up on one side is explicitly prohibited by the basic assumptions of circuit theory.

I don't know what textbook you are using, but a good circuits textbook should explicitly list the assumptions of circuit theory early on in the book. My textbook is Electric Circuits by Nilsson and Riedel and in section 1.1 the second assumption listed is "the net charge on every component in the system is always zero".

 

[jaumzaum]

I'm high school (sorry)

My textbook is only the basic :' (

 

[Dale]

OK, so from Nilsson and Riedel the three assumptions of circuit theory are:

1) Electrical effects happen instantaneously throughout a system
2) The net charge on every component in the system is always zero
3) There is no magnetic coupling between the components in a system

Assumption 2 is what allows you to use KCL, and assumption 3 is what allows you to use KVL. Assumption 1 is what allows you to neglect the finite speed of light.