Confused about voltage potentials & polarity in a circuit - Page 1
Forum section: electronics / beginners

author: Ronan
author_rank: Newbie
timestamp: April 16, 2019, 11:08:18 pm
content: Thanks for your comments on trigonometry. I watched several YT videos and since several of youREALLYemphasized it, I'll be signing up for a trig & calc refresher course soon.But I'm still lost in the wind. Maybe I'm making this harder than it should be, but I'm totally confused on at least three things. The first thing is the voltage drops or "voltage potentials add when going from point to point" with respect to polarity of items in a circuit, as see in the attached image.Going from b to c, we're going from "+" to "-" so we drop 6 voltsGoing from b to a, we're going from "-" to "+ so we go up 4 volts"What I don't get is if these are resistors, how is there a polarity since resistors aren't placed in a specific direction like LEDs are?Why would one be a + - and the next one be a - +?Secondly, I'm confused about direction of current. It seems arbitrary for one tutorial to use a "conventional flow theory" where the current is coming out of the positive end of a DC circuit, and then in another video tutorial the speaker will use the "electron flow theory" where the current is coming out of the negative terminal. I understand that there are positive and negative charges where the flow of electrons is the negative current, but is the conventional current the flow of protons? I hear it's a negative current, so then I get all confused on how to envision the current and apply it to figuring out voltage drop across an item.Thirdly, what does it mean if power is consumed or generated in these two examples?current is going from "+" to "-" which has a positive voltage and positive current so power is consumed.current is going from "-" to "+" which has a negative voltage so we get negative power so power is generated.I don't even know if I'm explaining myself clearly with respect to positive and negative current, and voltage and power generated or consumed. The instructor used this terminology but didn't explain the definition. I get the water reservoir analogy for voltage, but negative voltage confuses me. I'm just friggin' lost on these specifics.Thanks in advance.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 12:16:11 am
content: 1. Voltages.The mountain explanation where voltages are equivalent to height is a very good one. Let's pay a bit more attention to it....If you have a circuit that is joined - such as the example given - then if you traverse from one point to the next, then the next, etc. until you get back to your starting point, then you have a net change in voltage of exactly zero. This is the exact same thing as traversing the mountain. You first pick a reference (eg up is positive and down is negative) and you have to keep using this same reference throughout your travels. Now whether the changes are positive or negative will depend on which way you decide to go. For example, if point 'x' is higher than point 'y' then if your journey goes from point 'x' to point 'y', then (using the previous reference) you will be travellingdown- but if your journey takes you from point 'y' to point 'x', then you will be travelling up.When applying this to a circuit, imagine yourself walking around it. Defining your reference direction between two points will have you connect the red lead of your DMM to one point and the black lead to the other. Let's say that as you walk between these two points, you will have the red lead in front of you and the black lead behind you - then you mustalwayshave the red lead in front of you and the black lead behind you. (The maths works exactly the same if they were the other way around, but once you pick a direction, you have to stick with it.)If we look at your circuit, using the "red lead in front, black behind" reference, let's look at what happens when travelling from point 'a' to point 'b'. Firstly, the black lead will be on point 'a' and the red lead on point 'b'. There is a difference in potential of 4V with point 'a' being higher than point 'b' and with our "red lead in front" reference, the DMM will show -4V.When you start walking from point 'b' to point 'c', you have to move both leads so that the red lead stays in front - which means the red lead will be on point 'c' and the black on point 'b'. The difference in potential here is 6V with point 'c' higher than point 'b' and with our "red lead in front" reference, the DMM will show +6V.Using this same process of "red lead in front and black behind", when you go from 'c' to 'd' you will get a reading of +3V and from 'd' back to 'a' you will get -5V.To prove you got this right, just add up all four measurements you took:-4V+6V+3V-5V----0VYour answer should always be zero - since you are back where you started. This is Kirchhoff's Voltage Law.Extension: Your circuit could have many other connections and components weaving in and around, but as long as everything was in steady state (Electrically this means DC) then you could do this exercise around ANY closed path and you will always get a total of zero.

author: Ronan
author_rank: Newbie
timestamp: April 17, 2019, 12:25:00 am
content: Thanks Brumby,I get that each loop has to add to zero, but I'm still confused about the polarity of a resister which causes an addition or subtraction. How can there be a polarity for a resister, and how can that polarity change? As seen in this attachment.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 17, 2019, 12:27:38 am
content: Sometimes I find it helps me to redraw the circuit so it's clearer to me. This circuit with 4 resistors can be visualized as two resistors in series across a 9 volt battery and a second two resistor in series across the same 9 volts.for the first pair, the top resistor on the left in my drawing drops 3 volts, and the lower resistor drops 6 volts. If you measure the voltage across the top resistor with your negative (black) meter lead on the top lead of the top left resistor (point d) to the bottom lead on the top resistor (point c), you will measure -3 volts because point c is 3 volts more negative than point d.The resistors do not have polarity like a diode and can only have a voltage drop across them but you have to consider where your point of reference is for measuring the voltage drop.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 12:32:20 am
content: 1a PolarityPolarity is a term that is used in several ways. In basic terms:For a circuit element that provides voltage, it is used to identify which connection has the higher potential and which has the lower. Traditionally, a higher potential is called positive and the lower is called the negative, but this is not an absolute truth - it is arelativetruth. What makes things "positive", "negative" or zero depends on where you put your meter probes!For a circuit element that "uses" electricity, there can be two situations. The first is where that circuit element will work properly no matter which way it is connected. Resistors are the classic example, but so are low value capacitors and neon bulbs. The second is where that circuit element will only work properly when it is connected a particular way around. These - especially those with only two connections - are described as having a "polarity". Diodes and electrolytic capacitors are classic examples.Same word - but two very different meanings. The fact they are interrelated in electronics makes for some confusion.

author: Ronan
author_rank: Newbie
timestamp: April 17, 2019, 12:38:14 am
content: Quote from: Brumby on April 17, 2019, 12:32:20 amQuote from: ArthurDent on April 17, 2019, 12:27:38 amThanks. I think both of your guys' explanations helped clarify things a bit.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 12:54:28 am
content: 2. Direction of Current.This is an old topic of discussion and there has been much said - but suffice it to say this:When electricity was first discovered, they had no idea as the the actual mechanism that was producing this wonderfully versatile form of energy - but they soon realised it was directional and that there was something flowing. They then picked one level of potential and arbitrarily called that positive and they just as arbitrarily decided the direction of movement of the energy carrying stuff, which required that stuff to be positive as well. (This has become known around the world today as "conventional current flow".)After many years of using this conventional current flow model - very successfully, I might add - someone actually worked out that it was electrons that were the energy carriers. So, since we had already defined what "positive" meant, then the electrons had to be negative.In short, the arbitrary decision many years before got the current flow direction wrong. (Hey, they had a 50-50 shot.)HOWEVER, there had been an entire electronics industry built on "conventional" current flow -because it works. There are some areas where understanding the design process needs understanding of electron flow - such as thermionic devices (like valves and CRTs) and when you get down to the physics of semiconductors - but for the most part you can undertake a high level career in electronics by simply following conventional current. All circuit diagrams are drawn up using conventional current flow.The practical advice is this: By all means take the time to understand electron flow - but then forget about it.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 01:16:00 am
content: Quote from: Ronan on April 16, 2019, 11:08:18 pmIn the four point circuit provided - there is no information to tell us what is happening with regards topower- as used in the normal context of electronics. For a statement on power, we need to know that there is a non-zero current flowing. This four point example circuit might have zero current flowing - or there may be several amps flowing. All we know is that, as connected, there is a voltage equilibrium.In regards to "negative" or "positive" power - there are some clear statements that can be made:1. "Negative" power is not a thing. Using terms like "negative" or "positive" power means you need to gain better understanding - which you are trying to do2. Energy can be supplied or consumed. When it is supplied, referring to the amount available in "watts" does make some sense - but it is more accurate to refer to it as V.A (volts times amps). When energy is consumed, the measure is in watts.3. For regular electronics, power does not have a sign. It is always a positive value. Signs only come into things like voltages and currents.Hope this helps, but there might be someone who can say it better or cover something I missed/messed up.As always, never be afraid to ask questions!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 17, 2019, 01:17:51 am
content: Quote from: Brumby on April 17, 2019, 12:54:28 amThis is similar to the reply I wrote before I inadvertently deleted it by my thumb touching the screen.(I hate iPads!)I can't go along with "forgetting about Electron Flow", though.None of the people who learnt both back in the day had any problem with using either where appropriate.People who use Newtonian Physics in their everyday work don't forget that Einsteinian Physics exist, after all!

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 01:29:45 am
content: Quote from: vk6zgo on April 17, 2019, 01:17:51 amUnderstandable - but my point was that you shouldn't get your knickers in a twist trying to make sense of both at every turn. A focus on conventional current understanding should be one's principle approach to electronics.QuotePeople working on Newtonian Physics don'tneedthat knowledge of Einsteinian Physics in their everyday, so they can - for all intents and purposes - "forget about it" as far as their designs and calculations are concerned.However, having a basic understanding of Einsteinian Physics will allow them to cope with those rare occasions where Newtonian Physics doesn't quite work. In those cases, their archived knowledge will ring a bell - and they can make use of it. This is why I say "by all means learn about it".

author: rstofer
author_rank: Super Contributor
timestamp: April 17, 2019, 01:50:00 am
content: When you write the loop equations, you can arbitrarily choose the direction of current flow. It doesn't matter! Why not? Because, when all is calculated out, current flows that are opposite your assumption will be negative. Simple as that!Some people advocate for drawing all loops clockwise - that as good as any and completely arbitrary. We almost always go with the flow through batteries so if there is a battery, draw that loop as flowing negative to positive.You will find that the assumed current flow is 'down' through a resistor from a loop to the left of the resistor but 'up' through the same resistor for a loop to the right. As long as you keep the respective signs correct (there are two current through the resistor and both loop equations need to deal with both of them), everything will work out fine.Watch the first few lectures at Digilent Real Analog:https://learn.digilentinc.com/classroom/realanalog/

author: pwlps
author_rank: Frequent Contributor
timestamp: April 17, 2019, 09:32:25 am
content: Quote from: Brumby on April 17, 2019, 12:54:28 amIt's not necessarily"wrong" after all, both positive and negative charges can carry current. In an electrolyte there are both positive ions and electrons moving, both contributing to the total current. In a semiconductor there are negative electrons and positive holes both contributing to the current. For example if you consider a PNP transistor the majority of the current is carried by positive carriers (holes). (NB. Of course some might object that holes are only a concept and only the "true" particles, electrons, are moving - but then the discussion would shift to some subtle points of the quantum mechanics and multi-electron wavefunctions).

author: IanB
author_rank: Super Contributor
timestamp: April 17, 2019, 09:45:18 am
content: Quote from: Ronan on April 17, 2019, 12:25:00 amIn your picture at the top of the thread the yellow things are NOT resistors. They are just unknown "things". Some of them could be (indeed must be) batteries or other voltage sources.Resistors don't have polarity, but current has direction. In the direction that current is flowing through a resistor there must be a voltage drop in this direction.As far as conventional current or electron flow is concerned, you should ignore electron flow and do not think about electrons or protons. You are not trying to be an atomic physicist, so these things are not relevant at the early stages of learning. They will just cause confusion. All electronics texts will use conventional current and conventional voltage so just follow standard conventions.

author: Brumby
author_rank: Unknown
timestamp: April 17, 2019, 09:51:23 am
content: @pwlpsIons in solution are not the same thing. Also, "holes" are not a positive entity - they are a lack of negative charge.Please- let's not add alternative points of view, edge cases and armchair warrior causes. The OP is a beginner and has posted a question in the Beginner's section which indicates a fundamental confusion.It would be best to have them become familiar with the most common view - and help them learn from that basis. Throwing all these other thoughts around is just adding to the confusion. It is not helping them at this time - andmay never beof any practical use.

author: apis
author_rank: Super Contributor
timestamp: April 17, 2019, 03:03:13 pm
content: For the maths to work you need to specify what direction you consider as positive voltage drop (it depends on which direction the current flows). When you analyse the circuit you can get both a positive and negative voltage drop across the resistor. Together with the polarity you specified, the sign tells you in which direction the current flow and which side is the high/low voltage side. It has no physical meaning for the component, and how you pick the polarity is completely arbitrary, but it's important for how you interpret the result you get from the circuit analysis.Say someone tells you that resistor A has a voltage drop of -2 V. In order to know in which direction the current flows and which side is high/low voltage of the resistor you also have to know how the polarity of the resistor was defined.

author: Shock
author_rank: Super Contributor
timestamp: April 17, 2019, 05:14:40 pm
content: Firstly the bit about the mountains is confusing because there is no d. So of course between c to a takes you directly to the top of the mountain. Secondly in the schematic you must travel through d no matter if you go clockwise or anticlockwise. Thirdly the schematic states that c is 6V more positive than b and a is 4V more positive than b.So in other words don't climb that mountain after looking at the schematic.Quote from: Ronan on April 16, 2019, 11:08:18 pmIt is indicating by show voltage drops (the differences in voltage potential) that between the junctions the voltage goes more positive (higher) or more negative (lower).QuoteTypically you would choose conventional flow or whatever the text tells you. But in calculations it's not important as long as you stick to same direction calculating current in a circuit. But do use electron flow for the physics interpretation of what is occurring.QuoteThe labelled junctions are showing the difference in relative voltage potentials between the resistors. Starting out for things to make a little more sense and fill in the gaps you need to learn Ohms law, Kirchhoff's current law and Kirchhoff's voltage law with the associated formulas.I've attached a schematic as an example of how you could view the circuit. Note that even though its backwards it's essentially the same thing. The positive and negatives are in the actual direction of the voltage drops.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 18, 2019, 04:02:43 am
content: Quote from: IanB on April 17, 2019, 09:45:18 amFor Pete's sake!Sixteen year old kids in their first semester of study of Electrical Theory learnt how to apply the two different "current flow" concepts without getting confused, back in 1959, have we dumbed down that much?

author: Brumby
author_rank: Unknown
timestamp: April 18, 2019, 04:32:12 am
content: Quote from: vk6zgo on April 18, 2019, 04:02:43 amIs the OP undertaking a course of study that will have a planned teaching stream - or are they making their own way through the subject matter?If they are following a course of study, then their teacher/lecturer can make the point clear - and they can then move on. In such a case, your point will have some validity.HOWEVER, if they are muddling their way through on their own - then what we are trying to do is get them to follow the standard conventions that have been successfully used all around the world for decades so they can find their feet. Once they have a handle on that, they can progress.Right now - it's enough that they know there is a difference - and that conventional current flow understanding will get them where they need to go.