Author Topic: Can current flow in opposite directions simultaneously through same wire?  (Read 6321 times)

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Offline geratheg

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Attached is a schematic. The two voltages are unknown and could be different values. Both are after some circuit above them, also unknown. The circuits are separate from each other above the point of voltages.

Assume the entire circuit uses the same power source.

Up until now, I thought the connection of those two schematics is exactly the same thing, just different points.

1. But then I questioned the behavior of current, and wonder if it would sometimes be better to wire it one way over the other because of this. Is this ever a consideration? Or are those setups exactly the same and current adjusts behavior such that it would work?

2. For example, lets say currents from both of those voltages both share load 1 and load 2. This makes sense in setup 2.
In setup 1 only one load appears to be able to be shared, either load 1 or load 2, unless current is able to flow in two directions in the same wire simultaneously. For example, assume load 1 is shared, load 2 would have to get its current from the left "+" because current from the right "+" is blocked from going to the left due to opposite directions of current as shown by the arrow with a question mark in the attached picture.
Is this possible?

I hope my explanations make sense.
« Last Edit: August 19, 2014, 06:41:34 am by geratheg »
 

Offline Richard Crowley

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Assuming those black lines are practical (near zero-resistance) wires, then your "two diagrams" are identical.  That entire circuit as shown is a single "node" and settles to whatever voltage results from all the currents flowing into (and out of) the node at any given moment.  Of course, absent any details of the voltage or impedance of the connecting signals, there would be no way of predicting what that voltage may be at any given time.

The simple answer to your subject-line question is: No.
 

Offline geratheg

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Ok so current behavior can be different with those two setups.
But it will still behave in such a way that both of those setups work exactly the same. Correct?

Is it ever something I would need to worry about? Such as considering current behavior? Or does current sort itself out by itself such that I can wire it either way and it will always work the same?
« Last Edit: August 19, 2014, 06:43:16 am by geratheg »
 

Offline Simon

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The only difference between the diagrams will occur if one of the wires is too small for the current and will introduce a large voltage drop of it's own. Essentially they are the same in theory if you assume the wire has 0 resistance, if the wire is the correct size then for practical purposes you can.
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Offline Richard Crowley

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Unless you are talking about extraordinarily low impedances and high currents, there is NO DIFFERENCE between those "two" diagrams.
They are identical for all practical purposes. There are NO "two setups".  There is no "either way".
 

Offline Cside

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superposition
 

Online hamster_nz

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Yes and no

At low frequencies and low power they are identical.

At high frequency they will perform differently.

At high power they can perform differently to.
Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing.
 

Offline Simon

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oh dear lets not confuse the poor chap, when i saw the title I thought it would be about back emf
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Offline Kremmen

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In a practical circuit every wire is a low ohm resistor due to the intrinsic resistivity of the conductor material (copper, usually). If the wire cross section is appropriate to the application you can assume the resistance is zero for all practical purposes. But in the exact sense there will be some resistance and accordingly, some voltage drop. So, in the world of nit-picking precision every wire connection and splice constitutes a network node linked to other nodes with a low ohm resistor at least.
Enter Ohm and Kirchhoff.
Ohm's law states that U = RI, i.e. the voltage between any 2 nodes equals the resistance between said nodes times the current flowing between them.
Kirchhoff's law states simply that the sum of currents flowing into a node is always zero. Note that currents in this context are signed quantities with in and out flows having opposite sign.
If you have 2 sources driving opposing currents between 2 nodes, the resultant is the superposition of the 2 currents. In the end only one current (the resultant) flows between the nodes.
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Offline zapta

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Unless you are talking about extraordinarily low impedances and high currents, there is NO DIFFERENCE between those "two" diagrams.

Did you mean 'high impedance or high currents' ?

Drain the swamp.
 

Offline German_EE

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The answer to your question is yes, but currents flowing in opposite directions will cancel each other out with the wire carrying the difference between the two.

Imagine two 1,5V cells in series and two 1,5V bulbs in series all connected together, both bulbs will light. Now connect the center tap of the batteries to the center tap of the bulbs using a short wire and the bulbs will still remain lit. That short piece of wire is carrying current from the + end of cell one and the - end of cell 2 with current 'passing in both directions'. In practice there is zero current along the wire as the two currents have cancelled each other out and your wire is a 0V center tap  :)

Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline geratheg

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Kremmen, I thought the voltage loop and current joint rules apply to two or more power sources. I've never used it with one power source. In my question power is all coming from a  single power source.

Thanks for all the responses! I think I got my answer  :-+
 

Offline IanB

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In setup 1 there will be little current flowing in the connection between the two circular junction points. In setup 2 there will be a large current flowing in the connection between the two junction points. This may affect the required wire gauge or trace thickness of that connection if large load currents are involved.
I'm not an EE--what am I doing here?
 

Offline Richard Crowley

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I can't see that geratheg provided ANY information ANYWHERE in this thread that even hints at what kind of voltages, currents, and impedance we are talking about here.
Given that complete lack of detail, I am unable to understand how IanB can make such a definitive statement. Perhaps he can explain his reasoning here.

geratheg is showing a SCHEMATIC DIAGRAM. By definition, a schematic diagram makes NO REFERENCE  to the PHYSICAL configuration of any particular circuit node.
The wires could be cheek-by-jowl soldered to the same terminal, or they could be miles apart.  There is no physical information in a schematic diagram.

If geratheg wants to give us some Real-World reference to what he is talking about here, we can give him a more definitive response.
Else, this is a very poor example of a generic question, and we can offer only broad generalities in response.
 

Offline WarSim

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Please people read the authors question. 
No current dose not flow both ways down a wire at the same time.  Differential current does. 
If the voltage sources are different they will not share the load.  Two voltage sources of different potential connected together will attempt to resolve their difference by dropping their difference across the others output stage and or any intrinsic resistance between them.  If the potential difference is large enough you will end up with one power source and three loads.  If both sources survive. 


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Offline IanB

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If geratheg wants to give us some Real-World reference to what he is talking about here, we can give him a more definitive response.

Geratheg did:

But then I questioned the behavior of current, and wonder if it would sometimes be better to wire it one way over the other because of this.

Wiring is physical, and current affects the required size of wires.

I think I answered the question. When you consider physical wire size you should consider the current flowing through each wire. If the question is phrased in terms of "power sources" and "loads", then the implication is that the currents involved may be non-trivial. If that is the case, then you must consider the current flowing through each wire (or circuit board trace) and make sure it is sized appropriately.
I'm not an EE--what am I doing here?
 

Offline algorithm

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If geratheg wants to give us some Real-World reference to what he is talking about here, we can give him a more definitive response.

Geratheg did:

But then I questioned the behavior of current, and wonder if it would sometimes be better to wire it one way over the other because of this.

Wiring is physical, and current affects the required size of wires.

I think I answered the question. When you consider physical wire size you should consider the current flowing through each wire. If the question is phrased in terms of "power sources" and "loads", then the implication is that the currents involved may be non-trivial. If that is the case, then you must consider the current flowing through each wire (or circuit board trace) and make sure it is sized appropriately.

How would 1 go about determining what a particular traces current is? Would it be determined by components on the trace?
 

Offline Richard Crowley

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How would 1 go about determining what a particular traces current is? Would it be determined by components on the trace?
Ohm's law.  But you need to see the entire circuit. Seeing only an isolated circuit "node" with no visibility of what else is connected makes it completely impossible to predict anything here.  In terms of an electronic schematic circuit diagram, both of geratheg's diagrams amount to a single "connection dot". It would be like guessing the phrase on "Wheel of Fortune" with nothing visible but a single vowel.
 


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