Need help understanding voltage

  1. 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. jcsd
  3. davenn

    davenn 3,813
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    hi there
    welcome to PF :)

    here is another new thread discussing the same question as yours

    https://www.physicsforums.com/showthread.php?t=676135

    hopefully that will sort you out
    any further questions just ask :)

    cheers
    Dave
     
  4. 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
     
  5. jim hardy

    jim hardy 5,015
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    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
     
  6. sophiecentaur

    sophiecentaur 13,794
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    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.
     
  7. 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.
     
  8. I had to read it twice but I like your explanation Ratch. Electron density basically is what differentiates one voltage from another.
     
  9. 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
  10. 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
     
  11. 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
     
  12. dlgoff

    dlgoff 3,125
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  13. jim hardy

    jim hardy 5,015
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  14. I really like that graphical representation. Thanks for providing it.
     
  15. sophiecentaur

    sophiecentaur 13,794
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    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.
     
  16. 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
     
  17. 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?
     
  18. 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
     
  19. sophiecentaur

    sophiecentaur 13,794
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    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.
     
  20. [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.]
     
  21. 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
     
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