Is there a better way to assign current direction in circuit analysis?

  • #1
Is there like a rule of thumb, or a simple hack for assigning current direction in circuit analysis? Even when I m consistent with the direction I choose, I still get the wrong answer most of the time.
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1. Do current source or voltage source affect when choosing current direction?
2. Especially in nodal analysis, would I follow a single current flow throughout, or I assign current direction by loop?
Thanks
 

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  • #2
phinds
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Is there like a rule of thumb, or a simple hack for assigning current direction in circuit analysis?
It is utterly irrelevant. Just assign a direction and do the math.
 
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I think what confuses people in this matter is the opposition between what is conventionally called the direction of current flow, and the actual direction of electron flow, along with the arbitrariness and imperspicuity of which charge is called negative and which positive. In ordinary English, a surplus goes to fill a deficit, and in real life, electrons move toward protons much more than protons move toward electrons, but for historical reasons, the convention for direction of current flow is the reverse of that.

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  • #4
lewando
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in real life, electrons move toward protons much more than protons move toward electrons...
This statement is true due to the difference in mass of the objects, but this is not what is concerning the OP.

The convention is what it is. @phinds nails it.
 
  • #5
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This statement is true due to the difference in mass of the objects, but this is not what is concerning the OP.
It's confusing to say that electrons flow in one direction and 'current' flows in the opposite direction. We don't do that because it's perspicuous, and we don't embrace the imperspicuity because it's independently preferable; we tolerate the misleadingness of the convention because the convention is so strongly established that the cost of correcting it is something we're unwilling pay in exchange for the comparatively small benefit that would accrue from doing so. I think it's still beneficial to point out the flaw as as a source of confusion, even if it's not precisely the source of the empuzzlement expressed by @Electgineer.
The convention is what it is.
I think that the convention isn't what it ideally should be. Yes, the convention is what it is, for historical reasons, instead of being something more directly, as distinguished from more inversely, corresponding to what is really going on. It's obvious that re-naming something doesn't change its physical properties. Calling 'dephlogisticated air' oxygen didn't change any chemical reactions, but it did contribute to making chemistry more readily understandable.
@phinds nails it.
Nothing I said disagreed with what he said, but I would caution that, when dealing with electricity, it's often important to distinguish correctly between source and sink.
 
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  • #6
Joshy
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My personal preference is to make them all face the same direction ie. all inward or all outward. I don't have to worry about any bookkeeping and I'll let the mathematics dictate the direction.

Something like this:

242225


Here's an example with a classic Voltage Divider. I know it's simple, but the point is to show that the direction didn't matter.


242226
 
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  • #7
NascentOxygen
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Is there like a rule of thumb, or a simple hack for assigning current direction in circuit analysis? Even when I m consistent with the direction I choose, I still get the wrong answer most of the time.
Like
1. Do current source or voltage source affect when choosing current direction?
Where there is a voltage source, the current direction through the external circuit due to that source is from its + terminal....through the external circuit....then back to the source's negative terminal. So mark the current arrow to show that direction. If the external circuit is a resistor, R, then you can mark the value of that current as V/R.

But suppose that you (or someone else) has already drawn the current arrow in a direction opposite to what I described, then you can work with that, but the value of the current that you mark on that arrow becomes –V/R.
 

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