Treatise about Voltage

Discussion in 'Physics' started by Ratch, May 12, 2008.

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  1. Ratch

    Thread Starter New Member

    Mar 20, 2007
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    To the Ineffable all,

    While reading some of the questions and answers in this forum, I deduce that there seems to be a bit of confusion about just what voltage is. I hope to clear that up somewhat. Some sources claim it is the electrical potential (whatever that is) between two points in space or an electrical circuit. 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. Other descriptions include how it is measured (voltmeter), how it is calculated (resistance formula), but that does not explain what voltage is. Further attempts to define voltage involve comparing it to hydraulic technology such as the static pressure of a head of water. But voltage is not pressure. We know that electrical energy is involved with voltage, but voltage is not energy. To further confuse folks, we can have a very high voltage associated with a small amount of energy and vice versa. To understand what voltage is, and is not, we have to go back to basics.

    Electrical charges come in two flavors, positive and negative. Two or more like charges do not like to get together. They repel one another. To gather them together, energy has to be applied to the charges. So now the charges have this accumulated potential energy (P.E.)that was applied to the system to force the charges together. Now we can build up the same potential energy by bringing many charges somewhat close together, or we can bring few charges very close together. In both cases, we can have the same P.E. involved, but different voltages. Why is that? Well, the reason is that voltage is the energy density of the charge. In other words, it is the P.E. associated with the charges, divided by the number of charges involved. In MKS units, that is joules/coulomb. So voltage is not P.E., but it is proportional the P.E. associated with the charge. Twice the P.E., twice the voltage. When charge flows, thereby causing current to exist through a resistance, it dissipates its P.E. in the form of heat, and the voltage (energy density per charge) drops.

    Now let's talk about what voltage is not. First of all, voltage is NOT a vector quantity. It is a scalar quantity. The energy density of a charge has magnitude, but no direction. The hydraulic analog that some folks like to use for voltage is pressure, and that is a vector quantity. Voltage can increase and decrease, but so can any magnitude. That is not a direction. Sometimes magnitudes and phases of voltages and currents are compared to each other if they have the same shape, and are recurrent at the same frequency. This is not a vector comparison, it is a PHASOR comparison. The lines drawn to show this relationship are called phasors and have vector-like properties without being vectors. Questions? Ratch
     
  2. nigelwright7557

    Senior Member

    May 10, 2008
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    I have always thought of electricity like water.
    The pressure is volts and the flow is current.

    Sadly once you get past resistors being narrow pipes and capacitors being tanks, the theory falls apart with inductors.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Have a care when dismissing other people's models so lightly.

    All models are just that - models - and useful (true) only in the context for which they were developed.
    No model fully represents all aspects of the subject and many have characteristics of their own, not replicated by the subject.
    Nevertheless most of engineering is built upon modelling.

    In this forum users vary from around GCSE to PhD level and the models required for their discussions will be at greatly different levels of sophistication.

    Remember also that there is a big difference between what mathematicians call 'vectors' and what physicists mean by the term.

    I'm afraid your model of voltage is not even adequate for GCSE. Even at this level it is necessary to distinguish between an EMF and a PD - they are different - and to explain that if we distinguish postive and negative and any values at all we must also explain what is meant by zero or reference level.
     
  4. Ratch

    Thread Starter New Member

    Mar 20, 2007
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    studiot,

    I believe I gave reasons why some of the models and terms should be dismissed with respect to a definition of voltage.

    I gave and explained the definition of voltage, not a comprehensive course in electrical engineeering and how voltage relates to it with repect to modeling applications.

    What is GCSE? Acronyms should be defined once, unless they are so common that everyone knows them, such ad TV or ASAP. No matter what the level of sophistication, definitions remain the same and are the baseline of further study.

    I don't recall observing any conflict. Vectors have magnitude and direction, even in multi-dimensional space. I have seen non-vector constructs referred to as having "vector-like" properties. Voltage-current phasors are a good example.

    I did not propose a model and apply it to an application. I merely gave and explained the definition of voltage. However, positive could be defined as needing energy to gather the charges together, then negative would be the release of energy when they are scattered. Zero could be the state when the charges are so far apart that there is no interaction between them. Ratch
     
  5. Caveman

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    Apr 15, 2008
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    I didn't know either, but instead of posting and complaining, I googled it. FYI, General Certificate of Secondary Education.

    A vector to a mathematician is a matrix with only one column or only one row. They are equivalent in their origins, but are attributed different properties. Physicists generally think of magnitude and direction, but mathematicians may only consider it a list of numbers, or as coefficients in a system of equations.

    Voltage (technically called electric potential difference) is always defined between two points and is the line integral of the electric field over this path. Equivalently, stated from your energy point of view, it is the amount of work that a unit charge flowing from one point to the second can perform. So you see that there is a direction defined, but the potential doesn't have a vector quantity.

    It's a funny thing to figure out for people that haven't been taught it directly. Imagine you couldn't measure an absolute temperature at a point in a room, but could only measure between two points. This is like voltage. You always have to have a reference point, and you obviously are measuring in some direction, but the answer is just a number.

    This is why the pressure analogy isn't that bad. We are talking about water in pipes, so the vector quantity of it doesn't come into play. Water from a high pressure point will go to a lower pressure point. And it will follow the pipe. It's not completely accurate from a pure physics point of view, but it shouldn't be thrown out. What it does do well is give an intuitive feel of how electric potential works in a circuit.

    The point is use a model that helps you do what you are doing. GCSE level people generally don't have any understanding of what a line integral is, so it doesn't help. They may not have the physics background to understand the energy description either. Sure, they should get it eventually, but simpler models will help them move along with simple electronics. What is important is that they know they are simplifying it a bit. That, in general, is my only problem with these models.
     
  6. studiot

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    Here is an experiment that might be carried out in a Secondary School, to show the difference between EMF and PD.

    Take a 1.5 volt battery and a 1.5K resistor.
    Attach a voltmeter across the terminals of each.

    The voltmeter attached to the battery terminals reads 1.5 volts.
    The voltmeter attached to the resistor terminals reads zero volts.


    Connect each to a simple circuit - another 1.5K resistor.

    The voltmeter attached to the battery terminals reads 1.5 volts.
    The voltmeter attached to the resistor terminals reads zero volts.


    In the circuit, arranged by your teacher, there is a 1.5k resistor. (say a suitably biased transistor circuit with a 1.5K collector load)
    Note the voltage across it (say 4.5 volts)
    Replace this first your with your 1.5 k resistor, then by your battery.

    The voltmeter attached to the battery terminals reads 1.5 volts.
    The voltmeter attached to the resistor terminals reads 4.5 volts.


    This sequence has shown that there is a difference between the voltage across the resistor and the voltage across the battery.

    We call the voltage across the battery an EMF (electromotive force)

    We call the voltage across the resistor a PD (potential difference or potential drop)

    Both are measured in volts on identical voltmeters.

    A perfect EMF remains the same, regardless of the circuit conditions it is attached to or even if there is no circuit there.

    A PD varies according to the attached circuit and may dissappear if that circuit is removed.
     
  7. studiot

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    Vectors

    Unfortunately the term vector has been used with quite different definitions by various technical disciplines and by Hollywood.

    Biologists and epidemiologists use it to refer to an agent which carries something of biological significance. This could be a bug carried by a mosquito (the vector) or a chemical functional group carried by a particular molecule.
    Hollywood seems to have picked up on this definition.

    Statisticians use it to refer to a variable where they can separate the influence of that variable on a bunch of data.
    Somewhat similar really.

    Engineers and physicists call an ordered ntuple (pair, triple or whatever) of real numbers a vector.
    These can be presented as a list, line or column to show a geometric vector or something with magnitude and direction.

    Mathematicians, really take the biscuit for they have extended this definition to mean all sorts of (mathematical) things, some of which are not even numbers. But all mathematical vectors obey the 8 axioms of linear algebra and belong to a 'vector space'.
    Taken like this vectors embrace the whole of linear analysis and include such things as matrices, definite integrals, trigonometric functions, laplace transforms, rotations and symmetries. All of these may be operated on in linear fashion.

    This extension is unfortunate because mathematicians regard tensors as a special form of vector (the general form)
    Whilst
    Physicists regard vectors as a special form of tensors....
     
  8. Caveman

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    Apr 15, 2008
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    studiot,

    You have inadvertently proven your point even better than you realize.

    I come from a physics and EE theoretical background. They don't typically use EMF in the form you are describing it, so some of this is stuff that I have never learned. It is not incorrect, but rather another way of describing the same thing.

    But this shows very clearly exactly what you are saying about not throwing models away. By knowing many different ways of looking at a problem, you will have more tools in your belt to tackle it. Sometimes it can greatly simplify a tough problem by looking at it from a different angle.
     
  9. studiot

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    Yes Caveman,
    I'm trying to help understanding here with as homely an explanation as possible.

    Watertight definitions are realy difficult. Most leak somewhere.
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Studiot,

    Computer Programmers also use the term vector to refer to the memory address at which an interrupt service routine is located.

    hgmjr
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    And pilots understand it as the change in heading to put them on course to some destination.

    Want to try for definitions of the word "mole" next?
     
  12. Ratch

    Thread Starter New Member

    Mar 20, 2007
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    Caveman,

    I figured it meant something like that. How does it differ from GED (general equivalence diploma)? Anyway, the point is that one should not have to look up an uncommon acronym. Proper writing style suggests that an uncommon acronym be defined the first time it is used.

    Can't be. The dot product of two vectors is a scalar quantity. Can't get a scalor quantity by multiplying two matrices.

    The electric field is a vector, having both magnitude and direction. The charge has only magnitude, the path has only position. If the field is conservative, it does not matter what path the charge follows to go from point A to point B. The voltage is determined by the positional differences. It is another way of saying what I said earlier. Voltage is the energy density of the charge (joules/coulomb).

    No, you are measuring the energy density of the charge between two positions, not directions.

    You are talking about how voltage is used and applied. I am talking about definining voltage. Equating a non-vector quantity with a vector quantity even in an analogy is confusing

    Except I am not doing anything with voltage except defining it. Don't need any models for that.

    My definition and explanation does not and did not use line integrals.

    My explanation of the definition did not simplify or use a model. It just used two commonly understood words; charge and density. Ratch
     
    Last edited: May 12, 2008
  13. Ratch

    Thread Starter New Member

    Mar 20, 2007
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    Studiot,

    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
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
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    Po-TAY-to. Po-TAH-to.:rolleyes:
     
  15. Caveman

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    Apr 15, 2008
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    This is a forum. I'm just happy to limit textspeak. I could care less about acronyms. I'm already on the internet.

    Wrong. Multiply a 1xn matrix by a nx1 matrix, you will get a 1x1 matrix which is effectively a scalar.

    Actually by determining which point is the reference, you are determining the direction of the energy density. There are only two directions, positive and negative. It is not a vector, but it is related to one. Remember, the electric field is the negative of the gradient of the potential. So a voltage potential has a relation to an electric vector field.

    Then don't use the analogy. But it does help some people quite a bit.

    The basic explanation used in most physics and electromagnetic texts to describe exactly what voltage is does, though. And you ignored that I also said that they are not necessarily familiar enough with energy concepts to use your explanation.

    Yes, I understand it, but does everyone?

    I believe that sometimes the best way to learn is to get a vague and somewhat incorrect view, that becomes corrected and focused as you become more experienced.
     
  16. Ratch

    Thread Starter New Member

    Mar 20, 2007
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    Caveman,

    That still does not address what the acronyms mean, and why the user should have to look them up instead of the author defining them.

    Not wrong. The element of the matrix has the value the scalar would have, but is it still a matrix instead of a scalor, and therefore dimensionally incorrect. No self respecting mathmatician would happy with that.

    It takes two points to determine the total overall increase/decrease of the energy density of the charge. I would not call that change of magnitude a "direction". It only takes one point to determine a gradient, which is the instantaneous magnitude and physical direction, and therefore a vector.

    That relationship holds for every defined point on any scalar field. Each point can be the orgin of a vector that points in the direction of its greatest change. That relationship is the gradient as you pointed out. But that does not make a scalar field equivalent to a vector field. Empirically speaking, having more charges causes a stronger electrostatic field to form and therefore a greater energy density change while moving through the field.

    When defining voltage?

    Well, any text that uses pressure to define or explain voltage is just wrong. It might be OK to illustrate how to calculate the effects of voltage, but not to define it.

    Energy is the ability to do work. That is taught in the most elementary physics class. So is density. It doesn't get much simpler than that.

    Simple and straight forward is my way. Ratch
     
  17. Caveman

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    Apr 15, 2008
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    I'm not a mathematician, therefore I am happy with that.

    Still got to define zero. That's two points. It's all relative to a reference.

    Any text that uses pressure to define voltage is wrong. Any text that uses pressure to explain voltage is not necessarily wrong.

    Physics is optional in many programs. You sometimes must explain it even simpler, like by analogy.

    No one has said that your statement of a definition of voltage is wrong, just that there are others that are equally valid, and that there are analogies that provide suitable models for persons with less formal backgrounds. And no one disagrees that these models break down because they are fundamentally flawed, and need to be replaced by more suitable explanations as the person becomes familiar with the concepts required for the formal definitions.
     
  18. thingmaker3

    Retired Moderator

    May 16, 2005
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    I concur with Caveman. Defining "Voltage" as "Joules per Coulomb" is indeed accurate. But so is defining "Voltage" as "Amps per Ohm." Furthermore, defining "Voltage" as "Joules per Coulomb" is not useful in circuit applications - defining "Voltage" as "Amps per Ohm" is useful in circuit applications.

    The real problem behind this little communications glitch is the definition of "definition." I suggest you look it up, Ratch.
     
  19. m4yh3m

    Senior Member

    Apr 28, 2004
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    Roger Murdock: We have clearance, Clarence.
    Captain Oveur: Roger, Roger. What's our vector, Victor?


    Well that depends on what your definition of "is" is.


    I hope this thread doesn't confuse any of the neophytes... with all the "redefining" of words with each post, some of the readers might not take it with a grain of salt like the veterans, especially if they want to try and be "up to speed" with what they might assume to be "today's standards". I say dump it into the off topic area :p
     
  20. beenthere

    Retired Moderator

    Apr 20, 2004
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    The above is contradictory. Of course voltage is not energy. It is a measure of potential energy. Or is there something in your statement that implies that a difference in potential between two points (a voltage difference) will not cause current to flow in a conductor that joins those points?

    If voltage is not a measure of PE, then how do you explain the measurement of electrical power being defines as Volts times Current? If voltage can't measure energy, then it can't be used to measure power.
     
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