What Exactly Is Voltage in Electronics?

[Ratch]

sophiecentaur,

My quote from earlier on:

Voltage has nothing to do with how many charges there happen to be - unless you are specifically referring to a 'charged' object or a distribution of charge - but that is different (in different Units) from your "Energy Density" idea. You can produce a very high voltage by electromagnetic induction with very small currents (moving charges) or produce massive numbers of moving charges with a very small voltage.

I think you were in self defence mode at the time and ignored this.

I thought it was irrelevant and ignored it.

You seem to be confusing cause and effect here: The induced emf is what causes the charges to move and not due to the charges. Charges don't move without an emf to make them move.

Isn't that what I said in post #27?

The induced Voltage is there whatever resistance the conductor has - the field is there (Volts per metre), even in empty space.

No, the magnetic field intensity units are amps/meter. It is not a electrostatic field.

Ratch

 

[Ratch]

milesyoung,

Lets say there's no conductor present, only a time-varying magnetic field.

OK, you set up the conditions, now what is the question?

Ratch

 

[jim hardy]

I think OP's last question was this:

Thanks for the help gentlemen, I can understand voltage alone now, however, I'm having a bit of trouble relating it to resistance at this point.

From what I am reading, a resistor decreases the amount of energy per charge (by converting it into heat or motion through friction), which seems that by definition, voltage would decrease after a resistor, but according to V=IR, and I being constant, voltage goes up?

Could someone clear this confusion up? Thanks again.

I think maybe you are confusing potential with voltage. It is important to be precise in terminology else we miss these little distinctions.

Let's parse your question:

From what I am reading, a resistor decreases the amount of energy per charge (by converting it into heat or motion through friction),

exactly right. It converts potential energy into heat.

which seems that by definition, voltage would decrease after a resistor,

Recall voltage is potential difference .
You didn't say in above phrase where your voltmeter is connected.
Your charge exits the resistor with less energy than when it entered .
Whether your voltmeter shows an increase or decrease depends on where in the circuit its two leads are connected.

but according to V=IR, and I being constant, voltage goes up?

Those words do not paint a picture in my alleged brain. Voltage from where to where goes up?
What V=IR is telling you is this:
>>>It takes more volts to push an amp through some ohms of resistance than through zero ohms of perfect conductor.<<
More precisely, it takes one volt to push one amp through each and every ohm.
Again, Whether your voltmeter shows an increase or decrease depends on where in the circuit its two leads are connected.

It is helpful when first starting out to imagine yourself inside the circuit drifting along with the charges, feeling the "push" of voltage just as the charges do. It'll help you keep polarities straight.
Schematics used to be drawn with positive on top and negative on bottom. It helped with that imagination business, for rising potential moves you "up" (the page), just as with gravity.

Resistance is as you said like friction against current.
It's measured in "ohms", an ohm being the amount of friction which, with one volt across it, allows one amp to pass.

We electricals have a secret - by choosing our units and including in them necessary conversion factors, they come out already metric.
Get these definitions down pat and you'll be a long way ahead:
Coulomb, Joule, Volt, Ampere, Ohm, Watt.
Then you'll be ready for Kirchoff's Laws and circuit analysis.

old jim

 

[sophiecentaur]

Jim
Uphill and downhill is definitely the way to go on (simple) circuit diagrams, whenever you can. "We electricals" were lucky with our units because nothing electrical ever had anything to do with a King's thumb or how much hay a man could carry. We had a clean sheet in the realtively modern age 'we' started in business.

My view on matters electrical is that you can't expect to 'understand' them. The best you can do is to follow the rules and, in the end, get so familiar with them that they are as much second nature as Newton's laws of motion. And, we have to remember that we were not born with Newton's laws hard-wired into our brains; we had to learn about them too.
Grumpy old devil as he was, Feynman had it right when he took that journalist to task to ask the "why' question about magnetism. Anyone who wants to come to terms with what I'm saying should watch that video. Then they must settle down and learn their basics - by rote at first, if you like. You emerge the other side a lot wiser, more capable and less likely to make up and talk nonsense.

 

[sophiecentaur]

I thought it was irrelevant and ignored it.

...

No, the magnetic field intensity units are amps/meter. It is not a electrostatic field.

Ratch

What has that to do with the induced Electric Field? Not all electric fields are "Electrostatic". Are you still saying that comment was 'irrelevant? You have been asked the question by someone else now - with no answer. You response to the question was just to sidestep the issue instead of answering the implied question.

But you could clear this up with just a smidgin of Maths (if it's as simple as you say.). You are still avoiding it and just hand waving. You could, for example, start with Maxwell's Equations, (believed and respected by both of us, I hope), which would take you most of the way there. Just slip in your modification in a valid way, where appropriate and we'd be cooking on gas. Even I would believe you, then.

 

[sophiecentaur]

Thanks for the help gentlemen, I can understand voltage alone now, however, I'm having a bit of trouble relating it to resistance at this point.

From what I am reading, a resistor decreases the amount of energy per charge (by converting it into heat or motion through friction), which seems that by definition, voltage would decrease after a resistor, but according to V=IR, and I being constant, voltage goes up?

Could someone clear this confusion up? Thanks again.

I re-read this and it seems you are confusing cause and effect. You need to split the problem into independent and dependent variables. If we're talking about PD (independent), then this is what we start with. Inserting different resistors (independent) across your supply will result in different values of Current (the dependent variable). When you say that increasing the resistance will increase the Voltage, that will only apply if we keep I constant (a different experiment - constant current supply, not constant PD). The way you state it would imply that the height of a (constant head) dam would increase as you take less power from the generators, in the analogous situation.
It is true that, for two resistors in series, the volts across one resistor will increase as its value increases - but only because the current through the circuit decreases and the added resistor gets a bigger share of the supply volts. 'The Potential Divider' is explained all over the place. I don't think anyone who comes across the potential divider will instantly get a feel for it. It becomes very clear, once you have done the simple sums associated with it though.

 

[jim hardy]

...they must settle down and learn their basics - by rote at first, if you like. You emerge the other side a lot wiser, more capable and less likely to make up and talk nonsense.

why i keep harping on preciseness in terminology:

"We think only through the medium of words. --Languages are true analytical methods. --Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method. --The art of reasoning is nothing more than a language well arranged." -----Lavoisier

that's why he should spend time getting his units defined unambiguously in his mind.

My view on matters electrical is that you can't expect to 'understand' them. The best you can do is to follow the rules and, in the end, get so familiar with them that they are as much second nature as Newton's laws of motion.

The "why" becomes apparent as they become 'second nature' because one tosses them about in thought experiments, resolving apparent contradictions in his mind.
At least that's how i learned everything from algebra to ethics.
"What" before "Why".
That's why "New Math" was such a failure, it tried to reverse that sequence. I went through junior high school just a year before that stuff thank goodness.

On electrical units - i usually lead into a joke with that line... "I pity Mechanicals - their stuff is non-linear, "g" is not an integer, and they have to put up with entropy."

old jim

 

[sophiecentaur]

The "why" becomes apparent as they become 'second nature' because one tosses them about in thought experiments, resolving apparent contradictions in his mind.
At least that's how i learned everything from algebra to ethics.
"What" before "Why".

I pretty much agree but I have to add that feeling that one has understood the 'why' thing is an illusion. When we feel we know 'why', it is because we have related enough of the new thing to our past experiences to accept it and have gone 'deep enough' to satisfy our conscious mind (your "second nature"). Every individual has their own level of a sufficient answer to the 'why' questions.
It disappoints me that so many of the questions on PF seem to assume that there exists some fundamental level of truth about the world and that they have a cat's chance in hell of getting there.
Very early on, I came to terms with my own limitations and I accept that even the sum of human understanding is never going to be complete knowledge. This is absolutely no problem at all, for me. Is that just the underlying uncertainty of the 60's hanging about in the back of my mind, perhaps? Probably the best legacy from that time, for me. (That and Mr Worthington's Maths classes).

 

[Ratch]

sophiecentaur,

What has that to do with the induced Electric Field?

Nothing, that is my point.

Not all electric fields are "Electrostatic".

Yes, they are. If you are talking about a field caused by putting a voltage across a dielectric (capacitor), that is known as a "electric field" or electrostatic field. If you are talking about a field caused by running a current through a loop of wire, that is a magnetic field or "electromagnetic field". A magnetic field does not need electricity to exist, as in a permanent magnet. So when you talked about "electromagnetic induction" and "induced emf" in post #29, I assumed you meant an electromagnetic field.

You have been asked the question by someone else now - with no answer. You response to the question was just to sidestep the issue instead of answering the implied question.

Who, where?

But you could clear this up with just a smidgin of Maths (if it's as simple as you say.). You are still avoiding it and just hand waving. You could, for example, start with Maxwell's Equations, (believed and respected by both of us, I hope), which would take you most of the way there. Just slip in your modification in a valid way, where appropriate and we'd be cooking on gas. Even I would believe you, then.

Start with Maxwell's equations? Along with the vector analysis concepts of divergence, gradient, and curl plus integral and differential calculus? Plus the difficulty of writing math in an ASCII format. Can't you instead imagine integrating Coulombs equation of force between two charges with respect to the distance to get the energy to takes to put two charges close together. That has been done a multitude of time in textbooks.

Perhaps you should ask your question again carefully and specifically so we can start over.

Ratch

 

[jim hardy]

Very early on, I came to terms with my own limitations and I accept that even the sum of human understanding is never going to be complete knowledge. This is absolutely no problem at all, for me. Is that just the underlying uncertainty of the 60's hanging about in the back of my mind, perhaps? Probably the best legacy from that time, for me.

education teaches us how litle we know. perhaps wisdom is the acceptance, not just awareness, of one's abysmal ignorance.
i exist at a level appropriate to maintenance of machinery. That was my vocation. It's been a good and interesting life.

 

[sophiecentaur]

@ratch
An "Electrostatic" field is, as the name suggests, "Static".
Maxwell considers non-static Electric fields (sloppy use of terms, I'm afraid). Where are the charges in space?

If it is important for you to be believed and understood then you need to present us with more that assurances that we could prove your ideas for ourselves. Plenty of people on PF present equations when they are relevant. I think you really need to 'put of or shut up' on this one. I don't have to "imagine" anything that you're not prepared to present explicitly. A string of buzzwords proves nothing. Where's the meat? (as LBJ once said, I believe).
The question has been asked 'explicitly' - it is just "How do you justify what you are saying and support it with evidence?" or "Give a definition of the Volt that involves the dimension of length?" How much more explicit can that be? You could even quote the particular passage in a textbook that has your definition explicity printed as a definition and not associated stuff which the definition leads to. I presume a scan and a pdf attachment is possible?

 

[Ratch]

sophiecentaur,

An "Electrostatic" field is, as the name suggests, "Static".
Maxwell considers non-static Electric fields (sloppy use of terms, I'm afraid).

Yes, I agree with the above.

Where are the charges in space?

Anywhere charge carriers like electrons or protons exist in free space.

If it is important for you to be believed and understood then you need to present us with more that assurances that we could prove your ideas for ourselves. Plenty of people on PF present equations when they are relevant.

It is not important to me whether people believe me or not. I presented a view along with the reasoning to support that view. Everyone can evaluate it for themselves. In the case of my view, equations are not relevant. If I were solving a textbook problem, then I probably would present some equations.

I think you really need to 'put of or shut up' on this one.

I believe that I have "put up" as you say.

I don't have to "imagine" anything that you're not prepared to present explicitly

I have presented my view and reasoning explicitly. You don't have to even read my view.

A string of buzzwords proves nothing. Where's the meat? (as LBJ once said, I believe).

Words like "charge", "energy", "density" are not buzzwords. They are used all the time in scientific discussion. LJB was a politician, not a person of science.

The question has been asked 'explicitly'

The question was vague and sloppy.

it is just "How do you justify what you are saying and support it with evidence?"

That was not the question you asked. You asked something about electric or magnetic fields. However, to answer your last question as I did before. Since voltage is measured in units of energy/charge, that represents an energy density with respect to charge. That is as simple as I can make it, and it is basically the same answer I gave before. So now it is up to you to pick it apart by reason and logic, not by meaningless unsubstatiated declaration.

"Give a definition of the Volt that involves the dimension of length?" How much more explicit can that be? You could even quote the particular passage in a textbook that has your definition explicity printed as a definition and not associated stuff which the definition leads to. I presume a scan and a pdf attachment is possible?

I never said that a definition of voltage involves length. I challenge you to find where I said that.

Ratch

 

[sophiecentaur]

We're back to my original problem. You introduced the term "density" and you have not justified it as fitting into the definition of the Volt. When I say 'justified' I mean with a reference or some Maths (which is what PF would normally require, I think, for something as revolutionary as that.
I should not need to explain why "density" involves the dimension of length.

You are being deliberately obtuse here, I can tell. You have done it before in threads where you didn't want to climb down. (Don't ask for a reference, they are dead an buried)

I never said that a definition of voltage involves length. I challenge you to find where I said that

All in the thread:

Sure, voltage is the electrical energy density of an amount of charge

So voltage is a measure of the energy concentration per unit of charge (joules/coulomb)

So if there is energy and charge together in a volume, that defines voltage.

Yes, a higher charge carrier or electron density will define a higher voltage

Do you know what the word "define" means in the context of Science?

 

[Ratch]

sophiecentaur,

...I should not need to explain why "density" involves the dimension of length...

Now I think we are getting somewhere. You seem to think that density only has meaning for spatial dimensions. That certainly the most common meaning, and the one everyone thinks of unless stated otherwise. Now, what if I said I was mixing 1 kg of salt with 100 kg of sugar. Would not that dry mixture have a particular density with respect to the weight of the salt? Could it not also be described as having a particular concentration with respect to the weight of the salt. Sure, there is volume involved, but that is not what defines density in this case. What if I mixed 1 kg of salt with 100 kg of flour? It would still have the same density and concentration with respect to the weight of the salt, but the volume would be greater. Therefore, volume is not part of the density definition in this case. If you know some other quantative word to call that mixture (percentage?), let me know and I will use that instead.

You can guess what comes next. We extrapolate using energy and charge. No need to go further in that direction, is there?

Do you know what the word "define" means in the context of Science?

Not just science, but elsewhere also.

Ratch

 

[sophiecentaur]

OH boy, we're re-defining the meaning of density now.
It's not a poetry circle. It's a Physics Forum.

 

[berkeman]

Thread closed for a bit until I can sort things out...

 

[berkeman]

Unfortunately, Ratch has been banned for multiple misinformation violations. This thread has run its course, and will remain closed. Sorry that I didn't see the problems earlier in the thread -- please be sure to use the Report button when there are problematic posts in a thread, so the Mentors can head off the problems early. Thanks.