Need help understanding voltage

1. Mar 4, 2013

remedemic

Greetings,

I am a high school student that is trying to learn about electronics and circuits for a hobby, and I just cannot understand voltage. Some resources are telling me that it is a "potential difference" between two points, while other resources are telling me that it is simply the electrical force being exerted at a certain point. With the first definition, I am under the impression that two points are needed to measure voltage, while with the second definition, voltage can be measured at one single point.

Could someone explain to me what voltage exactly is in the most basic form possible?
Help would be greatly appreciated! Thank you.

2. Mar 4, 2013

3. Mar 4, 2013

Ratch

remedemic,

Sure, voltage is the electrical energy density of an amount of charge. It is measured in joules per coulomb. Suppose you have a bunch of electons. They all have like charges, so they don't like to get together. In fact, they repel each other. It takes energy to force them into a finite space. The amount of energy (joules) it takes divided by the charge (coulombs) is the voltage. If you put additional electrons into the same space, it takes more energy, and the voltage will be higher. If you crowd the same number of electrons into a smaller space, more energy will be needed, and the voltage will again be higher. So voltage is a measure of the energy concentration per unit of charge (joules/coulomb). If one point is at a higher energy concentration (higher voltage) than another point, the electrons are going to move and spread out from the higher energy concentration (higher voltage) to the lower concentration (lower voltage), provided there is a conduction path.

Wrap your mind around the above and then ask more questions.

Ratch

4. Mar 5, 2013

jim hardy

Voltage is, as you said, potential difference and involves two points. That's why voltmeters have two leads.

Try this experiment - take an ordinary nine volt battery and place one terminal to the tip of your tongue. No perceptible sensation...
Then place both terminals on your tongue - it darn well stings!

Voltage is "electro-motive force", abbreviated EMF and that's why it is so often designated by "E". The British used to call it "pressure". It 'pushes' electrical energy along . Motive as in moving, Electro as in electric....

Observe that pressure too is a differential measurement. We just had a thread that touched on that - check " Physically Grounded " in the EE section... user dlgoff posted a really neat graphic on page 2.

old jim

5. Mar 5, 2013

sophiecentaur

Whilst you are perfectly right to talk about Joules per Coulomb, the terms "energy density" or "concentration" is pretty meaningless because PD had no relationship to volume or area.
Why not stick to a description that involves just the basic quantities involved? If you mean Energy then just say Energy and that's right. There may or may not be a spreading of energy or a concentrating of energy (as with EM fields) but that's not Volts or PD.

6. Mar 5, 2013

remedemic

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.

7. Mar 5, 2013

mearvk

I had to read it twice but I like your explanation Ratch. Electron density basically is what differentiates one voltage from another.

8. Mar 5, 2013

Ratch

sophiecentaur,

I never mentioned PD in my definition of voltage. However, charge carriers like electrons are a physical mass whose number is defined over a definite volume of space. The energy they contain is what it took to gather them into that definite volume of space. So if there is energy and charge together in a volume, that defines voltage. In other words, voltage is the concentration of energy per unit of charge.

Energy and charge are the basic quantities, and I gave a description of how they are involved.

I do say "energy" when I mean energy. The energy density per unit charge or energy concentration per unit charge is not a spacial density or spacial concentration. It is simply the amount of energy associated with a gathering of charge carriers no matter how close or far they are apart.

Ratch

Last edited: Mar 5, 2013
9. Mar 5, 2013

Ratch

mearvk,

Why thank you. I got it from the units in which voltage is defined.

A higher spacial electron density will surely define a higher voltage. But according to Coulomb's law, it won't be a linear one. I think it better to say that voltage is the energy density per unit charge.

Ratch

10. Mar 5, 2013

Ratch

remedemic,

You should have trouble. Voltage exists by itself without any link to resistance.

The correct application of the resistance formula you quoted is V=-IR. The sign is opposite to the voltage source, which I assume for this discussion is positive.

Ratch

11. Mar 5, 2013

dlgoff

Last edited by a moderator: May 6, 2017
12. Mar 6, 2013

jim hardy

Last edited by a moderator: May 6, 2017
13. Mar 6, 2013

Mordred

I really like that graphical representation. Thanks for providing it.

14. Mar 6, 2013

sophiecentaur

This is clearly a personal view of how you 'deal with' an abstract physical concept and you are , of course, entitled to your internal views of things. However, you have written about this from a presumed 'expert' standpoint. (That is how a beginner could view it and it could lead to serious misunderstanding - which would need to be undone before they could move on usefully)

If you think you can explain Voltage without using the term Potential Difference then you will be disappointed. Voltage is just an informal word for Potential Difference in almost the same way that people use the word 'Amperage' when they mean Current.

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 would be interested to know if you could quote me a text book that uses your strange form of definition. If you have read something of the sort in a manufacturer's literature then that is of no value. Likewise, for anything buried inside some non-academic 'folksy' post. This is a well enough established concept for it to be dealt with in a text book. They all agree, of course.

What is wrong with the basic definition of Energy per unit Charge? It is simple and to the point and is not open to any misinterpretation. You seem to think that the term 'concentration' helps in some way. It can hardly do so because you now say that the word concentration does not refer to anything spatial. That's even more confusing / meaningless. And then you introduce Mass into the argument, too.

15. Mar 6, 2013

Ratch

sophiecentaur,

I don't think voltage is an abstract physical concept. I explained my definition of voltage which anyone can evaluate and it does not contradict other definitions of voltage. What would the beginner need to "undo" to advance further?

I can and did explain it without using potential difference. I did not contradict what is meant by potential difference.

Voltage is not defined by only charges. Energy is taken into consideration also. Applying a varying voltage to produce a high or low current does not abrogate my definition of voltage.

Nothing strange about it. It comes from the units of voltage (joules/coulomb). They all say voltage units are joules/coulomb.

Nothing. It is what I am expounding.

If I put 1 kg of salt into a 1000 kg of water, that mixture will have a particular salt density and a salt concentration.

Did I mention anything spacial in the above analogy? What are you confused about?

I only mentioned mass in passing. I said that electrons do have mass. I did not use mass in defining voltage.

Ratch

16. Mar 6, 2013

mearvk

Thing I sometimes have trouble understanding is why it's potential difference and not just difference. Surely voltage implies that there are actual differences in charge density not just potential ones?

17. Mar 6, 2013

Ratch

mearvc,

I corrected you before about that phrase in post #9 of this thread. It is "energy density", not "charge density". The units are joules/coulomb. The amount of charge can be large or small, but the energy density or energy concentration specifies the voltage.

Here is what potential difference means. Its full name is electrical potential energy difference per unit charge or potential difference (PD) for short. Every gathering of charge causes two fields to form. One field is a vector field called the electric field. It is measured by the vector of force on a small test charge caused by the attraction or repulsion of the charge to be studied. Its vector field unit is newtons/coulomb or the equivalent unit of volts/meter. The other field is a scalar field called the energy or work field. It is the energy it takes to bring a test charge from infinity to within an arbitrary point from the charge (call it P1). Each point around the charge to be studied has a particular scalar energy value determined by the previous description. To find the potential difference, subtract the energy from one point (call it P1) from another point (say P2) and divide by the test charge amount. That gives you the PD or voltage in joules/coulomb or voltage between the two points. Voltage is measured with two points.

Ratch

18. Mar 7, 2013

sophiecentaur

I may be an old fashioned thing but, when I read the word 'density' in a definition, I expect the units to involve s-2 or s-3 in some form. Once you introduce 'density' into an idea you change things radically. Magnetic Flux and Magnetic Flux Density are not interchangeable terms, for instance.

The confusion that you have managed to introduce into this thread is well demonstrated in the post from mearvk, which you replied to with:
which is gobbledegook. Higher electron 'density' does not 'define' a higher voltage. In the case of a charged object, it is true that increasing the charge will increase the voltags (as in Q=CV) but that is a result and not a definition. W=QV does not include a 'density' term.

The trouble is that you don't even seem to realise the confusion you generate by this sort of 'explanation'. Explanations are supposed to help people along the best path towards understanding and not to divert them. There are enough problems when you take the conventional path and we can all do without added confusion. If you cannot show an example of where the word 'density' is used in acknowledged sources for this topic then you should avoid using it in such an authoritative way. It surely cannot help anyone.

19. Mar 7, 2013

mearvk

[Middle English potencial, from Old French potenciel, from Late Latin potentilis, powerful, from Latin potentia, power, from potns, potent-, present participle of posse, to be able; see potent.]

20. Mar 7, 2013

Ratch

sophiecentaur,

There is also line charge density whose units involves s-1. All the above are spacial densities.

Yes, a higher charge carrier or electron density will define a higher voltage, because more energy will be required to increase the electron density. Both formulas you present have voltage in their terms. Since voltage is an energy density, those formulas do include density.

I am just thinking out of the box. As I said before, all the "authoritative" sources say that voltage is energy per charge. What is confusing about that? That concept is surely not a mind-bending diversonary thought that will spin someone's mental state out of control.

Ratch

21. Mar 7, 2013

sophiecentaur

"Thinking out of the box" is a luxury. Someone who asks a serious question to help their understanding needs everything to be inside the box. Any statement that is obviously outside the box should, at least, be accompanied by some serious caveats and 'imho's.
Your posts on this have been streams of consciousness and not Engineering based.
Why do you end your last post with the correct and well established definition of Voltage as if you never wrote all the other non sequiturs - which was what I have been complaining about?
If you cannot quote any reputable source where this energy density nonsense is used as part of the actual definition of Voltage then you really should climb down and relegate your comments to 'chatting around the subject'.
Have some sympathy for the first timer in this field. He wants a 'definition' and not an arm waving word association game.

22. Mar 7, 2013

Ratch

sophiecentaur,

No, it is one way new innovations are devised and applied.

Why is that? I remind you that in post #1, the OP "just cannot understand voltage". All those inside the box explanations he read did not help him.

Nope, they should only be explained, defended and shown to be correct.

All thought is a stream of conciousness. I disagree that my post are not engineering based. You, for one, have not proven that to be so.

A non sequitur is a conclusion which does not follow its premise. I believe my conclusion does indeed mesh with the way voltage is employed.

Before you can make the above statement, you have to prove that energy density is nonsense, So far, you have not done so.

Read the first post again. The OP had resources coming out of his ears. He wanted understanding, not just another definition. I provided him with an explanation to help him understand.

To the OP:
Were you helped by my definition and explanation of voltage in post #3 ?

Ratch

23. Mar 7, 2013

milesyoung

I don't mean to butt in, too many cooks and all that, but I'm curious about something.

Ratch, how would you explain the voltage produced by electromagnetic induction, using your definition?

Edit: This was worded poorly. Maybe something more along the line of:
In the context of electromagnetic induction, how would you explain what voltage is?

Last edited: Mar 7, 2013
24. Mar 7, 2013

sophiecentaur

Do you know anything about the process of education (which is what this is about). Throwing random ideas at someone who is having difficulty is certainly not the best way of helping them. How are they supposed to know which is good stuff and which is dodgy?
You are using the wrong definition of a quantity that is well defined. The definition of the Volt does not include any reference to area or volume. That is your error. Why persist in it?
As the one who is proposing an alternative definition, it is up to you to justify it - as with all 'alternatives' to mainstream knowledge. I can only say that I have never read anything so whacky. All you need to do is give a simple link / reference to some supporting evidence.

There is just one definition of voltage and it involves Energy and Charge. I notice, you haven't ever suggested a way of actually working out Voltage with your alternative definition. I think it would be reasonable to expect a bit of Maths to support your new definition.
What sort of touchstone is that? How would he know whether or not he actually got the correct idea from what you have written - except by being given some supporting evidence? You could give someone a very convincing story about the Moon being made of green cheese and they might accept it. That would not make it true.

25. Mar 7, 2013

mearvk

This has gotten a bit heated. Maybe we can let it go since it's not even the OP that's persisting in the discussion.