Electronics > Beginners

Mesh Current Method, Sign Convention

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rstofer:
Back in the early '70s, I was taught that:  When I went through a battery from - to + that was a gain and was considered positive.  If I went through a resistor I assumed I was going from a higher voltage to a lower voltage and that was considered negative, a loss in voltage.

So, for a simple battery in parallel with a resistor I would have +Vbat - I1*R1 = 0  or Vbat = I1 * R1 which seems pretty good, voltage isn't piling up anywhere.

Now I see many tutorials where the sign convention is reversed.  Going up through the battery is considered negative and going down through the resistor is considered positive.  I realize it doesn't matter because it will all work out in the end.  If the assumed current direction is wrong, it will have a minus sign.  No big deal!

The problem is, I didn't get the memo!  Is everybody on board with the newer sign convention?  Is that really the way it is being taught today?  What is the justification for considering the battery voltage as negative when clearly there was a gain?

sibeen:
I had to think about this and grinned to myself when reading this as I suspect that I change which way I define the direction depending upon which day of the week it is. This may be a result of my electrical education. I started out with a four year apprenticeship as an electronics tech in the late 70s. The interesting tidbit is that military techs, at least in the English speaking portion of the world, get taught electron flow. I've never been able to track down the why of that and it's always interested me as to why that occurred. A few years later I decided to go down the university route and was taught the correct conventional current flow.

Still to this day when I'm looking at a circuit I'll revert to electron flow and this occasionally causes consternation with people I'm showing the workings to as "I'm doing it wrong" only to be a tad bemused when my answer turns out to be the correct one.

As to the OP, I didn't realise that any change had been made, but then I haven't looked.

RoGeorge:
No idea what exactly you describe.  Do you have a link?

There is an international convention that current, I (A), flows from + to -.  In electric and electronic this is the only convention in use.

Sometimes, in physics there is a distinction about "electrons current", but that's something else than what we usually call "current" (or more pedantic "conventional current", I).

rstofer:
I don't have a workable link.  My question is really "How would you write the loop equations?" for a battery in parallel with a resistor.

I would start in the lower left corner and draw the loop in a clockwise rotation.  I would go up through the battery and consider that a gain of Vbat.  I would continue over to the resistor and see the voltage at the top of the resistor as higher than at the bottom, justifying my clockwise rotation.  I would compute the voltage drop as the current I times the resistor R and assign it a negative sign because I am entering at the + terminal.

When I was done I would have +Vbat - (I * R) = 0.

Apparently, the new math is the opposite:  Consider the battery as negative and, since the current is entering the resistor at the + terminal, consider that voltage as positive.  The equation would be -Vbat + (I * R) = 0

Either way, the equation simplifies to Vbat = I*R -> the voltage in is equal to the current flowing through the resistor times the value of the resistor.  Solving for I = Vbat / R.

Like I said, the same answer but the convention seems backwards from what I was taught.

The only reason I bring it up, knowing that the proper application of either approach will yield the same answer, is that my grandson is taking a circuits course next semester and I need to get it right,

https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/circuit-elements/a/ee-sign-convention?modal=1

RoGeorge:
Now I see what you mean, you were talking about the voltage arrow, right?

I didn't know there are two conventions, American and European   ::)
https://tex.stackexchange.com/questions/37182/voltage-sign-convention-european-vs-american-in-circuitikz
https://tex.stackexchange.com/questions/298538/european-convention-of-voltage-in-circuitikz

About the signs in a loop, it shouldn't matter.  We can pick arbitrary directions for any arrows in the circuit.  After solving the circuit and finding the numerical values, wherever is a -x, it means we picked the wrong direction for our arrow, wherever is a +x, it means we were lucky and picked the correct direction for the arrow.  This is how I remember it.

Like the electron current vs. conventional current wouldn't have been already enough confusion, the Khan Academy link you posted have the voltage arrow reversed from how I would draw it.   ;D

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