Electronics > Beginners
Confused about voltage potentials & polarity in a circuit
rstofer:
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/
pwlps:
--- Quote from: Brumby on April 17, 2019, 12:54:28 am ---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.
--- End quote ---
It'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).
IanB:
--- Quote from: Ronan on April 17, 2019, 12:25:00 am ---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.
--- End quote ---
In 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.
Brumby:
@pwlps
Ions 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 - and may never be of any practical use.
apis:
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.
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