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# Labeling voltages

Labeling voltages on a schematic is not a matter of "right" and "wrong". It simply establishes how the voltage appears in the analysis equations. Created by Willy McAllister.

## Want to join the conversation?

• Why was the current labeled as flowing out of the positive side of the battery? Doesn't positive current always flow into the positive side?
• Just think like charges repel. Positive charge is always going to flow away from the positive terminal and into the negative one. Remember the conventional way of labeling current is the path a positive charge would take. Hope that's helpful.
• At , If I is going out of the positive terminal why the result is not negative?
• It is a positive current of positive charge that goes from the positive terminal through the circuit. David, you confuse electron current with conventional current and what you state above is very true for electron current. This is even explained in an earlier video.
• @ or so you could have been more clear and specific that the plus side of the V1 is still considered to be 1V less then the minus side of V1. Although this is for many counter intuitive it is still correct yes. I needed to replay this part 3 times to exactly listen what you said and where you were pointing too. Just wanted to add for more clarity.
• Welcome to the world of many-teachers vs many-style-of-explanation thing. It is not easy sharing a topic to a 30 people class, let alone to a 8 billion khan class. huhu..

Anyway, I kind of like your question/post. It was challenging to be a good teacher, the kind that all-people-can-understand. I wish we can all be one of them. /(^_^)
It was also very challenging to be a good student, the kind that are so fast to reach the gist(point) of a lecture/class. I am a very slow learner, and not as hardworking, let alone doing revision. I envy them very-very much.

Nevertheless, let's learn something (if not everything) with all our hearts. And may we become wiser, and our kids (and other people kids) become smarter.
• Where does the V1 and V2 come in?
• From nowhere.

For this video/section, the purpose of the video is to share how to label our required variable/parameters when it was not given/labeled in the problem/real_circuit. It is to share that we CAN work out any circuit and get every parameters value ( V, I, R, power, equipment_rating) but we need to be systematic in approach and consistent in naming/labeling parameters. This is VERY useful when you are troubleshooting an electronics/power system that you don't have much info to start with. :)
• Generally, what is the most used assumption in circuits: the electron flow [which is the current comes from the negative terminal] or the conventional flow [in which the current comes from the positive terminal same as the voltage (for simplicity)]?
• Hi Voncarlo,

Conventional flow!

The test equipment, right hand rule, and textbooks all assume a current flows from positive to negative.

If you read the fine print the charge carriers (primarily electrons) are traveling from negative to positive.

This is as good an explanation as any: https://xkcd.com/567/

Enjoy!

APD
• lets say i had "assumed" a starting direction for the voltage and when calculating the current, get the current to be negative, does this mean that the direction is opposite to my originally assumed direction?
also when i substitute my current solution to get V1 or V2, do i just use V=IR with no signs or do i follow the signs i had indicated previously, like if my assumed current flow at the begining got a negative sign for a certain voltage, will i have V=-IR when proving or just V=IR?
• IMHO :
does this mean that the direction is opposite to my originally assumed direction? > Yup.
will i have V=-IR when proving or just V=IR? > Just V = IR.

Since this is a question of current direction, then we technically discussing a vector, in which, I don't have a clear way of typing it. :p I would rather draw. :| Sorry.

So, I'll use an analogy. let say, we are an alien.. seeing the blue dot (earth, as human call it). we (alien) wanted to study, how fast does this dot move around the star (human call it the sun).
The answer have two part, one scalar, one vector. One is the amount of the blue dot displacement change, and one is the direction of the rotation. We need both.

The displacement change amount can be calculated, independent of where we are flying from. But for the rotation around the star, we may get 'clockwise' or 'anticlockwise'. The direction calculated depends on which way are we viewing this dot and star. (human : it's anticlockwise if we view from the earths' north and clockwise if we view from the earths' south)

Both are true answer(for the alien), but we(alien) can only calculate/observe one at any one point of reference.

Similarly, (back to human) putting V1 and V2 means we assume the direction of the current flow. If we calculated a positive value, means it flow in our assumed direction. if we get a negative value, means it flow in the opposite of our assumed direction.

hope that helps. :)
• why is the second diagram negative one volt? resistors don't have polarity
• For future readers with the same question: I believe it is clearer if you imagine someone else handed him the schematic with the voltages already marked—which label the voltages entering and leaving the resistor, not polarities of the the resistor itself. (Nothing changes the fact that the resistor is causing a voltage drop.) And now he is using the KVL to analyze a loop.

The signs around the resistor do not alter the effect of the resistor (which is revealed by how many ohms it is). The signs simply affect whether the voltage is a rise or drop in calculations, but note that a negative number for a rise is the same as a drop, and a negative number for a drop is the same as a rise. So when you do the calculation, if you see a situation where you want to reverse the signs around the resistor, you can instead just flip the sign of the value of number of ohms. It doesn't change the role of the resistor in the calculation, it just affects the math we do.
• At you use Kirkoff Law, meaning the sum of voltages=0. Is this a proprety we can apply in every and any circuit? or is there some kind of requirement?
• I'm looking at this serie on circuit analysis to remember what I learnt before, but you got to wonder, since now all I saw was some really basic notations, and all of a sudden he uses K Law without even an introduction ? Imo spend less than 10 videos about notations and at least one to present this important law ...
• That it works out mathematically makes sense since the arithmetic and algebra are very simple but WHY would you ever depict current flowing into a negative terminal and thereby have to reverse the sign of the current ? It seems like this is just complicating things unnecessarily by deliberately introducing something that is wrong and deciding to mathematically compensate for it.
(1 vote)
• Hello Galba,

In the near future you will encounter systems where the direction of current flow cannot be determined by inspection. To solve using "nodal" and "mesh" analysis you will need to guess. Sometimes we get it correct, sometimes not. It is only after the simultaneous equation are solved that the direction of current is known.

Know that current can enter a voltage source from the positive terminal. Think of this as charging a battery.

Regards,

APD
• I still struggle to understand why the sign convention puts differences of potential in the opposite direction of the current for passive components. In my head I think of the current i as a kind of vector, so V would have to be a vector too. Because V=iR, and R>0, V would have to point into the same direction. As an additional problem, this way of thinking still works with Kirchhoff's voltage law (ΣV=0). So why would V have to point into the opposite direction?
(1 vote)
• I feel your pain. This is one of those humps you have to get over at the beginning. Fear not, it is a small hump. Your reasoning about vector current and voltage is valid, but it happens to not be the one we use. Instead, be sure you've seen this video on the sign convention: https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/modal/v/ee-passive-sign-convention.

With circuits we don't bother with a vector notation for current, because there are only two possible directions, this way and that way. This distinction can be taken care of with just the sign of the current, so we don't need vectors.

The definition of current we use is "positive current is the direction positive charge moves (or would move if it was present)." That's the first decision. We use this even though we know negative electrons are moving in wires.

Next, we want Ohm's Law to give the right answer for the voltage polarity when there is a current flowing in a resistor. Suppose you have a battery connected to a resistor. We define current to flow OUT of the positive battery terminal on its way to the resistor. If you measure the voltage with a voltmeter, the more positive voltage is on the end of the resistor closest to the + battery terminal. So we label the voltage that way, with positive voltage sign next to where the current is coming INTO the resistor.

This is the sign convention we use at KA and in almost every EE text I've ever seen. It is totally arbitrary that we do this, but we are super consistent about it.

It is possible to use the opposite convention, which means we define current to flow in the direction electrons move. That's what you've described in your question. This is also valid, but it is not commonly used. The only example I've ever seen is in the training material used by the US Navy and other branches of the US military. (search for "NEETS").