Dave,

For your examples: the linked example post #1 is a question asked by someone who didn't understand something (what one would expect from an OP in a thread).

OK, perhaps I did not search enough for a good enough example.

The answers provided discussing voltage were valid to that question - unless tere is some distinct limitation of the advice offered that you wish to share?

The other things were not related to defining voltage, but I found terms like "current flow" and the statement that electric fields appear when there is separation between charges. While that is true, separation is not necessary for electric fields to be present. Only an accumulation of charge.

The other example doesn't count as it came after you started this thread!

Why not? I have seen it before and I will see it again in the future.

I think your OP approach this thread from the wrong angle, and I have yet to see a suitable debunking of other voltage models to suggest that any other model (including yours) is a superior way to visualise voltage from both a conceptual and practical perspective.

Only those who are beginning to learn can answer that question. Us old hard bitten types have too much baggage to judge a method in a fair way.

Not when we are prattling around in the world of "you say pot-ay-to and I say pot-ah-to". Ok lets move on with the thread and see where we end up...

Sounds like a good idea. Ratch

 

So when you suspend an object inside a pressure vessel, purge the vessel, and refill it with hydrogen gas, and then pressurize it additionally to 100 atmospheres, what is the pressure on any point of that object suspended inside the pressure vessel?

 

Technically, the force on a submerged object due to pressure is always perpendicular to the surface at each point on the surface, not the pressure itself. This is usually written as:

\(\vec{dF} = p\ast\vec{dA}\)

\(\vec{dF}\) is the portion of force due to the pressure on the small area, \(\vec{dA}\). \(\vec{dA}\) is defined as a vector of the incremental area having a magnitude of the area of the surface in question and a direction that equals the normal of the surface. Since \(p\) is a scalar, \(\vec{dF} \) and \(\vec{dA}\) have the same direction.
If you want to find the total force on a submerged object you need to add up all of the \(\vec{dF}\) terms due to the pressure on all of the \(\vec{dA}\) portions of the submerged object. This is generalized to a surface integral:

\(\vec{F} = \oint p\ast\vec{dA}\)

However if pressure is homogenous around the object and the surface is closed, then

\(\vec{F} = p\oint\vec{dA} = 0\)

This makes sense in that a submerged body in a homogenous pressure field will not be accelerated due to the pressure. Buoyancy in fact is caused by a non-homogenous pressure field.

So the pressure is still a scalar, and the force is still a vector.

 

caveman,

Technically, the force on a submerged object due to pressure is always perpendicular to the surface at each point on the surface, not the pressure itself.

Yes, but you would not be able to make that statement unless you take into consideration the direction of the pressure, which then makes it a vector

So the pressure is still a scalar, and the force is still a vector.

You ignored the direction of the pressure so you could call it a scalar, and used the geometry of the object instead to get the direction. You can do that with pressure because of its constant normal direction with its contact surface and still get the correct answer. But conceptionally, I believe that pressure on a object is a vector. It is all a matter of perspective. Ratch

 

I rather think this thread has meandered off topic.
I also think that there has been too much argument over definitions in a thread purporting to help explain a basic but important concept.

I would agree that some physical and mathematical terminology could have been better chosen, especially with 20:20 hindsight.

But they are the internationally accepted terms and arguing with them only merits a small place in a thread about simple explanations.

So back to discussions of Voltage.

I understand what you are saying. You want to differentiate between an active element like a voltage source and the voltage across a circuit element. I just have trouble figuring out what "potential" means. Potential for/of what? And force in EMF. What force? I would use VS for voltage source and VL for voltage loss or VD for voltage drop. That is more in line with IS for current source. Ratch

No I did not say or imply that EMFs are associated with active devices. Transistors are active devices, but not a source of EMF.

I said that the distinction is not an idle one but a real and measureable one.

I outlined a simple experiment to show this difference.

If you prefer a simple mathematical derivation look at Kirchoff's Voltage Law:

"The algebraic sum of the EMFs around a closed circuit equals the sum of the ohmic voltage drops around the circuit."

 

SgtWookie,

So when you suspend an object inside a pressure vessel, purge the vessel, and refill it with hydrogen gas, and then pressurize it additionally to 100 atmospheres, what is the pressure on any point of that object suspended inside the pressure vessel?

One hundred atmospheres, plus what ever it was pressurized to before, no matter whether the object is suspended or not. Ratch

 

studiot,

No I did not say or imply that EMFs are associated with active devices. Transistors are active devices, but not a source of EMF.

I don't consider transistors active devices. They are controllable resistors. You know the old definition transistor = transfer resistor . The power supplies make them active.

I outlined a simple experiment to show this difference.

I am too dumb to understand what you are trying to define.

If you prefer a simple mathematical derivation look at Kirchoff's Voltage Law:

"The algebraic sum of the EMFs around a closed circuit equals the sum of the ohmic voltage drops around the circuit."

Still don't know the difference between and EMF and regular voltage. Ratch

 

You ignored the direction of the pressure so you could call it a scalar, and used the geometry of the object instead to get the direction. You can do that with pressure because of its constant normal direction with its contact surface and still get the correct answer. But conceptionally, I believe that pressure on a object is a vector. It is all a matter of perspective. Ratch

That is how it is described in a physics book. You are basically saying that the pressure at a point depends on if there is a body right next to that particular point or not. Do you see why you do not want to do that?

Sure it is perspective. It is just definitions. But I'm not making this up. I've actually been researching two different physics books here. You are doing exactly what you were saying that people should not do. You are trying to logic your way through it without reference to standard convention. Don't do that. Get a good reference and use it. That way we are all on the same page.

BTW, the book I am primarily referencing is the "Handbook of Physics" from AIP press. It is based on the third German edition of the Taschenbuch der Physic. It is a respected physics reference that is very consistent with the theory taught in physics classrooms.

 

Perhaps you mean amps times ohms. But even then you would be wrong, because voltage can be present between two points without any amps being present.

As resistance increases without bound, current decreases to zero. Since there is no such thing as a perfect insulator, there is no such thing as zero current. Voltage is therefore definable as Amps times Ohms.

The problem is not communication. It is acceptance and understanding. Ratch

The problem is both! Since you do not understand or accept that the problem is communication, the problem continues to be communication! If you had looked up the definition of "definition," we might be getting somewhere. Instead, we'll just keep bippity bopping back and forth and gain nothing.

No, it is proportional to the potential energy of the charge,

So voltage is force. Thought so. Thanks.

The next question is: why should this topic remain open?

An excellent question. I certainly see no reason to leave it open. There is nothing to gain here. Indeed, this thread could prove detrimental to new students.

I am thinking of all the times I have run across posts in threads here and elsewhere where I felt that voltage was not understood or explained wrong. It is too much work to review all that I read, so I will only give two examples.

http://forum.allaboutcircuits.com/showthread.php?t=10025 ,post #1

This thread, post #18, see answer #24

Your first example cites someone with no understanding of the basics who came here to learn. Your second holds up a typographical error as "proof." You have yet to adress Dave's question. Where are the wrong explanations? Where is the misunderstanding by those offering explanation instead of asking for basic help?

I am concerned with a electrical concept, not the units that describe it.

If you were concerned with the concept of distance, would you ignore meters, feet, and parsecs?


Lastly, I believe there is a confused communication here:

Another definition says it is electromotive force (EMF). That is a circular definition, because the phrase EMF means a voltage difference, and is not a force anyway.

Still don't know the difference between and EMF and regular voltage.

For clarification, do you maintain that EMF is, or is not, one valid definition of "Voltage?"

 

Caveman,

That is how it is described in a physics book. You are basically saying that the pressure at a point depends on if there is a body right next to that particular point or not. Do you see why you do not want to do that?

Yes, you are right. My physics book also says that "pressure is a scalar quantity". They get the direction of the force caused by the pressure by multiplying a delta of the surface area, which they represent by a vector perpendicular to the surface, with the pressure. Just like you did. If that is the way they want to think of pressure, so be it.

You are doing exactly what you were saying that people should not do.

What is that?

You are trying to logic your way through it without reference to standard convention. Don't do that. Get a good reference and use it. That way we are all on the same page.

What is wrong with that? Looking at things is a new way can be a good thing that gives knowledgeable insights, as Einstein showed. No one was on his page at first. If someone is not on the same page, let him explain why.

BTW, the book I am primarily referencing is the "Handbook of Physics" from AIP press. It is based on the third German edition of the Taschenbuch der Physic. It is a respected physics reference that is very consistent with the theory taught in physics classrooms.

I have no disagreement with the textbook. Ratch

 

What is wrong with that? Looking at things is a new way can be a good thing that gives knowledgeable insights, as Einstein showed. No one was on his page at first. If someone is not on the same page, let him explain why.

What is wrong is that you came in acting as if you know absolutely the right answer, and you stated very succinctly that everyone else was wrong. If you are "feeling" your way through it, be clear about that. Don't act like a teacher when you are actively being the student.

This is precisely why I didn't find that pressure was a scalar until so far in the conversation. It is a confusing notion if you've not dealt with it so much, but you were so adamant that I presumed that you were correct. Even after I stated that it was a scalar, instead of looking it up and finding the correct answer, it was argued vehemently that it was not.

Despite that, I really enjoyed this topic because by trying to prove my point, it helped me to clear up a lot of concepts in my own mind. I hope you feel the same.

BTW, to the mods: I believe that this thread should be in the physics section as it describes fundamental physical concepts. I don't believe, however, that it should be closed just because some people feel that it isn't providing any use. By arguing through these concepts, the persons that are arguing are learning. If people don't want to get involved, they can just not read it, but if they want to, they may learn something themselves. I feel it is up to the reader to follow the whole conversation if they want to avoid getting incorrect understanding.

 

Looking at things is a new way can be a good thing that gives knowledgeable insights, as Einstein showed. No one was on his page at first. If someone is not on the same page, let him explain why.

Very well. As you are in the position of Einstein, please do explain why you are not on the same page.

BTW, to the mods: I believe that this thread should be in the physics section as it describes fundamental physical concepts.

Works for me.

 

thingmaker3,

As resistance increases without bound, current decreases to zero. Since there is no such thing as a perfect insulator, there is no such thing as zero current. Voltage is therefore definable as Amps times Ohms.

Yep, that is the resistance formula. Now, to make the definition complete, you have to define resistance.

Instead, we'll just keep bippity bopping back and forth and gain nothing.

And challenge each other with new concepts and ideas.

So voltage is force. Thought so. Thanks.

When/where did I say that?

An excellent question. I certainly see no reason to leave it open. There is nothing to gain here. Indeed, this thread could prove detrimental to new students.

I disagree. I see this as a lively exchange of thoughts and ideas.

Your first example cites someone with no understanding of the basics who came here to learn. Your second holds up a typographical error as "proof." You have yet to adress Dave's question. Where are the wrong explanations? Where is the misunderstanding by those offering explanation instead of asking for basic help?

As I said before, I have come across wrong and confusing explanations from several years of reading forums like this. And not just voltage. For instance, folks talk about "charging a capacitor". If a voltage is put across a capacitor, it is not charged. That is because one plate has an excess of charges, and the other plate has a deficiency of charges. The total net charge is zero, no charge at all. They should say a capacitor is energized to X volts. That is, energy is added to the capacitor. It is both correct and descriptive of what is really happening. I said before that I come across these gems all the time. I do not have the inclination or ambition to do a search of the thousands of posts in this forum and others, but they occur all the time. Ask me about Ohm's law or "current flow" sometime.

If you were concerned with the concept of distance, would you ignore meters, feet, and parsecs?

Indeed I would, as long as only the concept was discussed.

For clarification, do you maintain that EMF is, or is not, one valid definition of "Voltage?"

Before I can answer that, I have to know what EMF means, especially the force (F) part of it. Ratch

 

The reason that a 3 phase system adds to zero voltage is that at any point in time, each phase has some voltage on it, but because of the phase relations in time, they always cancel each other out.

Thanks Mr. Caveman. Now I wonder why did I even have to ask that, perhaps i was confused drawing vectors in my mind. The addition of different magnitudes and signs did not appear to me.

As far as pressure is concerned:
I think it has been pointed out that pressure acts the same in any direction. If we just consider pressure of a gas, its caused by molecules whose motion in themselves is random. I for one can not recall where I ever needed direction for pressure. I would thus concur with pressure being a scalar.
What I think of pressure analogy is that as far as cause and effect is concerned it does wonderfully well to explain how voltage will cause a current. The only thing perhaps that is missing from that analogy is the absence of two different charges which flow in different directions(hope I do not restart the conventional Vs electron flow debate).

however, that it should be closed just because some people feel that it isn't providing any use. By arguing through these concepts, the persons that are arguing are learning. If people don't want to get involved, they can just not read it, but if they want to, they may learn something themselves. I feel it is up to the reader to follow the whole conversation if they want to avoid getting incorrect understanding.

I share the same opinion. I got to learn a lot and the discussion is rather interesting.

Edit:

For instance, folks talk about "charging a capacitor"

That sort of talk is usually between professionals. We often say "spring is charged" for circuit breakers.

My stand is still clear: There was and is nothing wrong with any of the explanation/definition each one has its own merit. I for one prefer going down to the basics( the energy density one). That IMHO gives a better understanding of different concepts.

 

thingmaker3,

Very well. As you are in the position of Einstein, please do explain why you are not on the same page.

Why I would think that would be obvious. I had an idea that was out of the mainstream of looking at things. Ratch

 

You realize, of course, that George Simon Ohm had to wait half a century before his "out of the mainstream" idea caught on.

Perhaps I will begin to agree with you in fifty years time. For now, I'm going to bed. We can have another lively exchange of thoughts and ideas on Thursday, if you like.

 

caveman,

What is wrong is that you came in acting as if you know absolutely the right answer.

I am unsure of to what you are referring to that I had the right answer.

and you stated very succinctly that everyone else was wrong.

We covered a lot of topics. Wrong with respect to ...?

If you are "feeling" your way through it, be clear about that.

I will be clear and say that I had firm opinions about what I said, whether they were right or wrong.

Don't act like a teacher when you are actively being the student.

I stopped being the student a long time ago. I learn new things everyday, as does everyone, but my basic knowledge is in place, so it would be wrong to call me a student.

This is precisely why I didn't find that pressure was a scalar until so far in the conversation. It is a confusing notion if you've not dealt with it so much, but you were so adamant that I presumed that you were correct. Even after I stated that it was a scalar, instead of looking it up and finding the correct answer, it was argued vehemently that it was not.

I still think I have a good argument to make, and it could be looked at from either perspective. But both perspectives are valid, and no gain in this case can be obtained by going against a equally valid concept that is widely accepted.

Despite that, I really enjoyed this topic because by trying to prove my point, it helped me to clear up a lot of concepts in my own mind. I hope you feel the same.

I certainly do. I never realized that pressure was not thought of as a vector quantity by the physics texts. I also agree that this thread should not be closed down. Ratch

 

thingmaker3,

You realize, of course, that George Simon Ohm had to wait half a century before his "out of the mainstream" idea caught on.

Do you know what his idea was?

We can have another lively exchange of thoughts and ideas on Thursday, if you like.

I look forward to it. Ratch

 

I still think I have a good argument to make, and it could be looked at from either perspective. But both perspectives are valid, and no gain in this case can be obtained by going against a equally valid concept that is widely accepted.

Let's look into this other perspective of pressure being a vector. If it is valid, then maybe a better understanding can be gained by it. My understanding is that there are actually two "things" that are both called pressure:
1. The pressure property of a fluid.
2. The pressure that is applied to the surface of a submerged object.

The first is clearly a scalar because it exists in the fluid whether or not there is something to apply the pressure against. The second is the force per area applied to the submerged object. The area is considered a scalar while the force and pressure are considered vectors. The vectors are parallel and always point perpendicular to the surface.

Is this correct?

If so, the first thing to do would be to clearly define any relationship between them.

 

From post#1
because the phrase EMF means a voltage difference,

From post#47
Still don't know the difference between and EMF and regular voltage.

Ratch,

When you started this thread you clearly stated that you know what EMF is and that you were posting to help others understand.

You also decried the use of the term EMF

We have to wait until post#47 to discover that you don't actually understand EMF at all.

That is too much tail chasing.

If you were to say EMF is confusingly named and ask for explanation, I would agree with you and try to give a suitable account of what it is and how it came to be so named.

I do not know what your technical background is or where in the world you are, but your command of English is such that I guess it to be your first language.

When I took my first level secondary school exams I thought I knew everthing about geometry.

What a shock was in store when I started co-ordinate geometry.
So by my second level exams I again thought I knew all about geometry and now, vectors and scalars.


What a shock when I started my applied maths degree and started linear algebra, vector and scalar fields, Poisson's equation, the Navier-Stokes equations.......

After 35 years applying maths in engineering in several parts of the world , postgraduate qualifications and membership of an august Engineering Institution -

I am still learning.

So tell us what you don't understand and what level you would like the answer pitched at and we will try to help, don't just decry what others have said.

Otherwise this thread contains apparantly authoritative mis-statements and misconceptions by several posters and could well be misleading and so I think, reluctantly, should be closed.