Resistor Polarity Convention--Vol I, Ch. 7 (Education)

Thread Starter

doubleoinfo07

Joined Jan 7, 2016
2
Good Afternoon Everyone,

This forum and website has been very helpful as I dust up on my reviews of the basics! In Vol. I, Ch. 7 Building Series-Parallel Resistor Circuits: Series Parallel Combination Circuits, the last two diagrams show resistors with their polarities labeled. Can someone explain to me 1) a simple way to label the polarity of a resistor in a circuit and 2) how it applies to the circuit shown in that chapter? I think understanding the polarities will help me better understand the wiring configuration used in the terminal strip shown in that chapter. Many thanks in advance!
 

wayneh

Joined Sep 9, 2010
16,128
You need to know the voltages on either end. Just as water flows downhill, current flows from high to lower voltage.
 

kubeek

Joined Sep 20, 2005
5,650
Actually in the AAC textbook we use electron current convention, so current flows from negative to positive. But that doesn´t matter as long as you choose one direction and consistently stay with it.
 

wayneh

Joined Sep 9, 2010
16,128
Oops! Not a can of worms I care to reopen. But anyway, my comment stands about the relationship of current to voltage. It's Ohm's law, after all. Sometimes you know voltage and calculate current, and sometimes it's vice versa.
 

GopherT

Joined Nov 23, 2012
8,012
You need to know the voltages on either end. Just as water flows downhill, current flows from high to lower voltage.
Actually in the AAC textbook we use electron current convention, so current flows from negative to positive. But that doesn´t matter as long as you choose one direction and consistently stay with it.
I love the decision taken by AAC for the eBook! Since the water analogy is used so often in electronics, it is so much more intuitive to talk to a new EE student and correlate the natural flow of water to that of electronics flowing up-hill instead of the awkward version where current flows down-hill.
 

kubeek

Joined Sep 20, 2005
5,650
OT: Exactly GopherT, why don´t we just declare electrons positively charged and call it the non-conventional voltage convention? It would make things even less awkward.
 

GopherT

Joined Nov 23, 2012
8,012
OT: Exactly GopherT, why don´t we just declare electrons positively charged and call it the non-conventional voltage convention? It would make things even less awkward.
He is looking this information up in an eBook so he is trying to learn. What differentiates his attempt to learn from the students' attempts to learn who ask in the homework section? Members seem much more willing to help here. Is it different because he is not paying an institution to help him? I was trying to make a sarcastic comment about the 'help' strategy used in the AAC homework section but I cant even think of additional differentiators to make a joke - now I think the strategy is just sad instead of worthy of a joke.
 

WBahn

Joined Mar 31, 2012
24,974
OT: Exactly GopherT, why don´t we just declare electrons positively charged and call it the non-conventional voltage convention? It would make things even less awkward.
You certainly could, just be sure to reverse the polarity of all voltages, too, including things like batteries.
 

WBahn

Joined Mar 31, 2012
24,974
Good Afternoon Everyone,

This forum and website has been very helpful as I dust up on my reviews of the basics! In Vol. I, Ch. 7 Building Series-Parallel Resistor Circuits: Series Parallel Combination Circuits, the last two diagrams show resistors with their polarities labeled. Can someone explain to me 1) a simple way to label the polarity of a resistor in a circuit and 2) how it applies to the circuit shown in that chapter? I think understanding the polarities will help me better understand the wiring configuration used in the terminal strip shown in that chapter. Many thanks in advance!
You can randomly pick a direction for current to flow in each branch and label it as such. When you calculate the actual current in the branch that current will either be positive or negative (let's leave zero as a special, trivial case). If it is positive that means it is flowing in the direction of the arrow. If it is negative that means that it is actually flowing in the direction opposite to the arrow. With practice you will be able to made educated guesses so that the currents in most/all branches will be positive. Any that are negative are worth a bit of consideration to convince yourself that the current really is flowing in the other direction and then a bit more consideration to see if you can figure out if it would have been possible for you to see that when you assigned your current labels.
 

AnalogKid

Joined Aug 1, 2013
8,227
A hysterical historical question - Since all language is subjective, how did the charge of an electron come to be called "negative"?

ak
 

WBahn

Joined Mar 31, 2012
24,974
A hysterical historical question - Since all language is subjective, how did the charge of an electron come to be called "negative"?

ak
Thank Ben Franklin for that. Prior to him, there were thought to be two types of electricity, vitreous and resinous, that could cancel each other out when in balance. Franklin posited (and he wasn't alone) that there was only one kind of electrical fluid and that an excess of it resulted in a positive charge and a deficit of it resulted in a negative charge. He essentially declared that when you rubbed a glass rod with silk that fluid was transferred from the silk to the rod leaving it positively charged. I don't think anything has ever been found that might indicate he had any particular reason for making that declaration and, in fact, there was no reason why he should have given a reason -- he could have simply flipped a coin. In actuality, electrons are transferred from the rod to the silk. So the "fluid" consists of negatively charged particles in order to be consistent with the declared polarities of objects when rubbed against each other. By the time this was figured out it was way too late to try to go back and reverse the polarity of everything in all the publications and all the batteries and everything else, so we are stuck with it, very possibly for the rest of human existence.

But it really doesn't matter because electrical current is not the transport of physical things, it's the transport of charge -- something the electron-flow crowd doesn't seem to be able to comprehend and treat consistently, resulting in the need for magical mystery minus signs in order to make things work out.
 
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