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Course: Electrical engineering > Unit 2
Lesson 1: Circuit elementsSign convention for passive components and sources
A standard practice for labeling current and voltage on resistors, capacitors, and inductors. Labeling voltage and current sources. Written by Willy McAllister.
Sign convention for passive components
We need a simple, widely understood way to refer to voltages and currents in a circuit. The purpose of the sign convention developed here is to define what we mean by positive and negative voltages and currents.
Why do we need a sign convention? Passive components (resistors, capacitors, inductors) have a defining equation (Ohm's Law and others). These equations establish a relationship between voltage and current. We can't just assign voltage polarity and current direction any which way. Voltage polarity and current direction have to be consistent with each other. The universal convention for voltage polarity and current direction for two-terminal components is shown below:
This is called the sign convention for passive components.
Voltage polarity: The illustration above shows voltage polarity with two notations in orange:
Current direction: The blue arrow shows the direction assigned to positive current flow. Current arrows should be drawn so current flows into the
All three of the current arrows in the next image mean the same thing.
The reason for this convention is so the signs of current and voltage come out right when we apply the defining equations for each component, like Ohm's Law for a resistor.
Example 1
This
Something (not shown, a voltage source or surrounding circuit) has caused
What is
To find the current, apply Ohm's Law:
The voltage arrow tells us the top of the resistor is
Example 1X - the wrong way
What would happen if we labeled the resistor with the wrong sign convention? The diagram below shows the same resistor with the same voltage polarity, but the current arrow points out of the positive terminal, so the sign convention for passives is not being used.
Apply Ohm's Law, exactly the same as Example 1,
Ohm's Law is telling us the current is
Example 2
This
What is
We use Ohm's Law to solve for the unknown voltage. Since we've been careful to use the sign convention, all we have to do is plug in the actual values shown on the schematic. (Avoid the temptation to flip signs around in your head as you write these equations. This often leads to errors.)
The answer came out with a negative sign, meaning the resistor terminal with the
This labeling convention for passives is not just a good idea, it is the only way to get the right answer when analyzing a circuit.
Sign convention for ideal sources
Voltage sources
The voltage across an ideal voltage source is independent of the current flowing through it. An ideal voltage source can be defined by an equation like this:
If you need to label the current through a voltage source, it can be done a few ways. In general, the options are:
- No current label. Usually you don't need to label current through a voltage source. The surrounding circuit context determines the direction of the current, (illustration 1).
- If you are doing power calculations,
- If it is important (or comforting) for the sign of current in a voltage source to have a positive sign, then use a convention where the current arrow points out of the positive voltage terminal, (illustration 3).
In most cases, the current flows out of the positive terminal of a voltage source. If you apply the passives sign convention to the voltage source, in most cases the current ends up with a negative sign. This current arrow direction may feel "wrong," or you may find it annoying, but it is not technically an error. It just means the current has a
My preference for labeling voltage sources is the first option: no convention. Different textbooks teach all versions of this sign convention. Be tolerant of those who learned from a different book. Everyone gets the right answer in the end.
The label does not have to match the actual voltage
The label on a voltage source is usually oriented with the polarity arrow going in the same direction as the actual voltage generated by the source (1a.), but there is no law that says it has to. The black
A voltage source with two alternative labels,
For a battery symbol, the longer black line indicates the positive terminal of the battery. A battery with two alternative labels,
When might you want to point the voltage label "backwards"? When we learn about Kirchoff's Voltage Law sometimes it is helpful to point all the voltage arrows in the same direction going around a loop, (to make it easier to get the signs in the equation right). If one of the elements in the loop is a battery or voltage source, the voltage arrow may point opposite the actual voltage polarity.
Remember, the voltage labels are just labels; they are there to establish a reference direction for voltage in the context of the overall circuit. The labels don't determine the internal properties of the voltage source or battery; that's the job of the black symbol.
In some ways, a voltage label is similar to a force vector in mechanics, if you assign a vector going up and then run through the math and find your answer is negative, it means it's actually going down. The direction is so you have a clear idea of which way things are actually moving when all is said and done.
Current sources
The current through an ideal current source is independent of the voltage across it. The equation describing a current source is:
Current sources are usually labeled with a current arrow matching the direction of the symbol arrow, and no voltage indication. The actual voltage across the current source will emerge from the analysis of the surrounding circuit. If you need to label the voltage for some reason, it is usually done as shown in option 2, similar to the sign convention for passive components.
Want to join the conversation?
Is there a topic i can study here regarding the math on ohms law?
Better yet, can someone walk me through the part where it said:
v = −20μA⋅10kΩ
to
v = (-20 x 10^-6) x (10 x 10 ^+3)(12 votes)
If you are having trouble with the transformation from letter prefixes like 20 uA changing to 20x10^(-6), check the article on Numbers in Engineering: https://www.khanacademy.org/science/electrical-engineering/introduction-to-ee/intro-to-ee/a/ee-numbers-in-electrical-engineering.
There are other articles in the Introduction to Electrical Engineering that cover the symbols for current, voltage, and resistance.(17 votes)
I don't fully understand the current direction and voltage polarity for two-terminal components. First, don't electrons move from negative to positive, so why does the current enter from the positive terminal? Also, why does the voltage polarity face the opposite way from the current?(9 votes)- Check out this video on how we define the direction of current: https://www.khanacademy.org/science/electrical-engineering/introduction-to-ee/intro-to-ee/v/current-direction
The definition of current direction used by all electrical engineers is to point the current arrow in the direction positive charge would move. Since electrons have a negative charge, they move in the opposite direction the current arrow is pointing. This quirky definition of current direction was created long before anyone knew that electrons existed, and that they carry the current in wires. There are no plans to change this definition, and, once you get used to it, it starts to feel normal.(12 votes)
Maybe a stupid question, but why would one need a "voltage arrow"? Voltage is the energy per charge... so how can it have a direction? I'm so confused. Feels like these tutorials assumes that I know stuff that I don't know...(7 votes)
Hello David,
Instead of direction let's call it polarity. You could also call it a rise or fall in potential. From a practical perspective it tells you if the voltmeter will read a positive or a negative voltage.
Yes, I understand your confusion. At first glance this passive sign convention seems completely useless. However, as you continue on it will start to be more and more important. You will appreciate the concept when you learn about Kirchhoff's Voltage Law (KVL).
Ref: https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws
Regards,
APD(6 votes)
What is the difference between current source and voltage source? I think they are the same because both of them produce electricity.(6 votes)- Both sources create voltage and current. A voltage source creates a constant voltage, and the current adjusts to whatever is required by the attached circuit. A current source outputs a constant current, and the voltage adjusts depending on what is connected.(6 votes)
How did you get from "i=+2V/250Ω" to "i=+8mA"
I got "i=1V/125Ω" = 1A/125(3 votes)
And if you divide 1 by 125, you will get 0.008 Ampere, which is 8 milli-Ampere (milli = one thousandth)(9 votes)
- Why would the label not match the actual?(3 votes)
- In general the label is like a vector in mechanics, if you assign it going up and then run through the math but find your answer is negative it means it's actually going down. The direction is so you have a clear idea of which way things are actually moving when all is said and done.(7 votes)
- Example 1: "What is i?"
i = +8mA
Example 1X: "What would happen if we labeled the resistor with the wrong sign convention?"
i = +8mA
"We got the wrong answer."
Did we though? Because those two answers look awfully similar to me, despite one using the wrong convention. If one of those examples could be i=-8mA due to being misled somehow, then that ambiguity should be pointed out and explained. However, even if the math does work out to be the same for both examples somehow, comparing the two examples is not useful for justifying the use of the correct convention. Also, we're talking about a resistor in these examples, and AFAIK it doesn't actually matter which way they're oriented, mathematically or practically, so again maybe not the best illustration to justify convention? If what you're trying to say is that +8mA IS the wrong answer for a diagram labeled with current flowing the other way, then that is not clearly stated and it also implies that the actual direction of current should be changed instead of fixing the label, which is where the real problem is here. Either way, something needs to be clarified or corrected.(4 votes)- The answers look similar, but one of them gives you the wrong current direction.
In both Example 1 and 1X there is +2 volts on the resistor, with the + voltage applied to the resistor's upper terminal. In both cases the resulting current will be 8 mA flowing from top to bottom in the resistor.
The sign convention tells us to draw a current arrow pointing into the positive voltage terminal of the resistor. With this convention, Ohm's Law gives you i = +8mA, which is the correct answer. If you accidentally draw the current arrow pointing the other way (coming out of the positive voltage terminal, or equivalently, pointing into the negative voltage terminal) when you apply Ohm's Law you again compute i = +8mA. BUT, since the arrow is pointing up, that says the current is flowing up, which it is not (it is flowing down).
Part of the challenge here is that current direction is indicated by mingling two signs. First you have the arrow symbol, which can point in two directions. Then you have the sign of i, as in i = +8mA or i = -8mA. The arrow direction and the arithmetic sign combine to give you the overall direction of current.(3 votes)
- If the convention is to have current flow into the positive terminal of a battery, as this article states, then why are most circuit schematics modeled with the current flowing out of the positive terminal? To me this seems explicitly contradictory.(3 votes)
- Hello Sean,
Yes is is contradictory. Shrug - It's just the convention we use - positive to negative. It is regrettable that this is the first lesson electronics. It's not just here but in just about every electronics and physics textbook.
I think Randall Munroe said it best in his comic https://xkcd.com/567/
Regards,
APD(5 votes)
- I am confused about the labels of voltages (in applying the Kirchoff's voltage law (KVL) and the passive sign conventions for current. Why do we even need the passive sign convention in the first place? Don't we take into account the signs of voltage supply and absorbed when we apply the KVL for the circuit.(3 votes)
- I am confused with the labeling of voltage sources. I thought the convention is to have the conventional current flow from the positive terminal to the negative terminal. But the 3rd illustration is contradictory to the illustration of the wrong way of labeling the current.(2 votes)
- We use the word "convention" a lot. So first, let's calm that down a little. Whenever we say "current", all we have to say is "current", since we no longer need to be long-winded with "conventional current".
The first part of the article is about how to assign current and voltage directions to passive components (R,, L, C). The reason we have this convention is so when you have a positive current you know the direction of the corresponding positive voltage.
The next part we talk about putting signs on an active component (voltage source). Active sources are a bit quirky when you apply signs, because they generate power (that's why they are so different from passive components.) I offer three choices for doing this, along with some reasons to help you make your own choice. You may be thinking the third choice is wrong, but it is ok. In fact it's my second favorite. There is no "wrongness" here because it is not an attempt to distinguish between conventional current and electron current. The choice is just a matter of controlling and making sense of where you get positive current and positive voltage.(5 votes)