What is negative voltage?

Discussion in 'General Electronics Chat' started by coinmaster, Jan 20, 2016.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    I've only been dealing in electronics for a couple of months but one thing has kept bugging me.

    0v is a voltage potential of nothing i.e the electrons have no desire to move

    Lets say I have a 100v source and I hook it up to a 99v source.
    The 99v source will have a voltage potential of -1v relative to the 100v source and the 100v source will have a voltage potential of +1v relative to the 99v source.

    How can we have voltage below 0 potential?
    I would understand if 0volts was half way between infinite or if positive voltage was protons and negative voltage was electrons but seeing as how protons don't move and positive/negative voltages can still be attracted to each other I doubt it.

    What am I missing?
     
  2. Robin Mitchell

    Well-Known Member

    Oct 25, 2009
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    Voltage and current are all relative.

    For example, a battery has two terminals, + and -. The positive terminal has a positive voltage RELATIVE to the negative terminal. The negative terminal has a negative voltage RELATIVE to the positive terminal.
    0V is whatever you want it to be but all parts of the circuit will have a voltage and current relative to that point.

    Two batteries in series can give a split supply where the 0V is taken inbetween the two batteries, the positive terminal is taken from the + on one battery and the - on the other batteries negative terminal.
     
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  3. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Voltage is always relative between two points. In a simple circuit, ground is 0V and the other point is greater than 0V. Let's say the voltage is supplied by a 9V battery. So relative to the two terminals of the battery, we read 9V.

    This is when the ground lead of the meter is on the negative pole of the battery;the other lead is on the positive pole.

    Now reverse the leads so that ground is on 9V and the other lead is on the battery's negative pole. What do you read? -9V! That is because relative to the meter's ground lead, the positive lead is reading 9V lower.

    Understand?
     
  4. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Yeah but, lets say I have a rectifier and I put the positive end on ground and then I throw a multimeter across it.
    If the multimeter reads -99volts how does it know what it is relative to? How does it know it's not a value of 99 positive? You can't have -99v relative to zero because zero is already nothing.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Voltage is a measure of the potential difference between two points, so can be negative or positive.
     
  6. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    The difference between 0 and +99 and -99 is still 99. The question is how do you have 99 below zero i.e -99. 0 is already a voltage potential of nothing so you can only go up from there.

    If the multimeter is referenced to 0 then I can only see the difference being +99 in any scenario.
     
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Because the multimeter measures the voltage at the positive probe relative to the negative probe. You need two probes to measure and voltage is measured relative to the negative probe. If the positive probe is on a node that is less than the voltage at the negative probe, you have a negative voltage.

    "Zero" is not nothing; it is a value relative to another node.

    Let's try this with addition. If you add 1 to 0, what do you get? If zero was nothing, wouldn't you get nothing?

    Is the confusion regarding ground? Ground is merely a common reference point, to which voltages are measured. It does not have to be zero. Although it is typically thought of in that way.

    Put two nine volt batteries in series, positive pole to negative pole between them. Place your negative meter lead on the middle connection. In turn, place the positive lead on each batteries unconnected pole. One will read -9V and the other will read +9V.
     
  8. WBahn

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    Mar 31, 2012
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    The meter is showing what the voltage is at the positive probe tip relative to the voltage at the negative problem tip.

    Think of it like altitude. You can use all kinds of different references as your "zero" level. We often use mean sea level (MSL) and a positive altitude indicates a location that is higher than the reference. The altitude of Death Valley is a few hundred feet below that reference so it's altitude is negative. But we could also use the height above ground level (AGL), so that the ground is our reference, and if we are at a location where the ground is 5000 ft above sea level, then the elevation we report for the bottom of 1000 ft mine shaft would be either 4000 ft MSL or -1000 ft AGL.

    This is because altitude is ALWAY a differential measurement -- it is the height of one location relative to some other location that is being used as a reference . All that an altitude of "zero" means is that the height just happens to be the same height as whatever location you are using as a reference.

    The exact same thing is true of voltage -- it is ALWAYS a differential measurement -- it is the amount of potential energy, per coulomb, of at one location relative to some other location that is being used as a reference. All that a voltage of zero means is that charge at that location has the same potential energy as it would have at whatever location you are using as a reference.
     
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  9. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Okay I think I'm slowly starting to grasp it. I've always thought of voltage as some sort of universally set value on a chart from 0 to infinite like a meter or a gallon. It's hard to unlearn that.
     
  10. WBahn

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    You can have something measured in negative meters (altitude, for instance) or negative gallons, too. Consider a water reservoir that has an average amount of water in it. We can call this our reference and then express the actual amount of water in the reservoir as the number of gallons above (positive) or below (negative) this level. All this is needed for a negative measurement to be meaningful is for it to be relative to some reference, even if the notion of an absolute measure (the number of gallons of water in the reservoir) has intrinsic meaning (while absolute voltage and absolute altitude have no intrinsic meaning).
     
  11. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    It's hard to separate from the idea that voltage is not a set universal value of negative electron charge.
    The concept of ground being any random point is confusing without any scale of electric charge to work off of.
    I would say "oh well ground can be 50v then" but it can't be because 50v is nothing without a reference to compare it too, and there's no reference to compare it to without 50v. It's the chicken and the egg. If 50v is a measurment of electric charge between 50v and ground then ground must have a measurable amount of attraction in order to have a voltage.
    Arrgh head is spinning o_O
     
    Last edited: Jan 20, 2016
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Re: how do we have a voltage below zero?

    A negative voltage is a relative excess of electrons compared to some other point. If 0 V is no voltage. Negative voltage is an excess of electrons and positive voltage is a deficiency of electrons. Realize what voltage is. A negative ion is an atom with an extra electron, negative voltage. A positive ion is an atom with a missing electron, positive voltage. Voltage is a static condition of the atom(s).
     
  13. WBahn

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    Mar 31, 2012
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    First off, voltage is NOT a measure of electric charge -- it is a measure of the potential energy per unit charge relative to the potential energy it would have at some arbitrary reference point. The voltage is not a property of the charge, it is a property of the location whether there is any charge actually there or not. Just as altitude is a property of a location whether or not anything is actually there. And, like voltage, altitude is a measure of the gravitational potential energy, per unit mass, at that location relative to the potential energy that mass would have at the reference altitude.

    You get to arbitrarily define your reference -- and you do NOT have to declare it to be 0V (but that is almost always what is done). Just as I can stick a sign in the side of a mountain and declare that point to be at an elevation of 10,000 ft and then measure everything else consistent with that -- go up ten feet and I'm at 10,010 ft and go down 100 ft and I'm at 9,900 ft -- I can pick one point in a circuit and declare that the voltage at that point is 50 V and then measure everything else consistent with that. In both cases, what I am effectively doing is declaring some other unknown point as my zero reference. In the first case my zero reference is whatever point is 10,000 ft below the point I choice and for the circuit it is whatever point is 50 V lower than the point I chose.
     
  14. WBahn

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    Though there doesn't have to be electrons involved. A changing magnetic field induces an electric field, even in a vacuum and even billions of light years away from its source, and an electric field gives rise to a voltage field.
     
  15. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    Oh wow what a great explanation, that makes much more sense.
    So if you have 100v worth of excess electrons and 100v worth of missing electrons then the voltage potential between them should be 0, but I would imagine 100v of excess electrons would be dying to reach those empty slots so I guess it becomes zero though filling those slots?
     
  16. WBahn

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    Mar 31, 2012
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    The very statements "100v with an excess electrons," and, "100 V worth of missing electrons," are meaningless. Voltage is NOT a measure of charge!

    You need to study what the very concept of voltage is. It is the amount of work, per unit charge, required to move a charge from one location (at rest) to another location (at rest). If you were to take a hydrogen atom and pull the electron away from it to a distance of, say, 1 meter, that took a certain amount of energy to do. Relative to where it started (at the proton that was the hydrogen atoms nucleus), there is a certain voltage at that location. If you moved that electron to a new distance, say 2 meters, that took additional work and the voltage, relative to the location where the proton is, is at a higher voltage. Same charges involved, but different voltages. Voltage is NOT a measure of charge!

    Now consider a universe in which we just have the proton sitting at some location. We can calculate how much energy it would take to move an electron from that proton's location to any other location in the universe. That amount of energy is the amount of potential energy an electron would have if it were sitting at that location. That, by definition, is the voltage at that location. There doesn't need to be ANY charge there AT ALL and voltage is defined EVERYWHERE in the universe as being the potential energy, per unit charge, of any location relative to where that proton is located. But we could also leave the proton where it is and pick some point 10 meters to the right of it as our zero potential reference. All this does is add a constant to the prior voltage at every point in the universe (including where the proton is). If the voltage at some point in the universe is 10 V and if the voltage at some other point in the universe is 50 V, then if I take 4 coulombs of charge that is at the first point and move it to the second point, the total potential energy will go from 40 joules (1 volt, by definition, is 1 joule per coulomb) to 200 joules. The potential energy will have increased by 160 joules and that energy had to come from somewhere -- it is supplied by whatever force moved the charge from the first point to the second. Both of those energies are not absolute, they are simply relative to the energy that that 2 C of charge would have if they were located at the reference point.
     
  17. coinmaster

    Thread Starter Member

    Dec 24, 2015
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    But then in order for there to be any attraction to that unknown point it still must have a measurable level of electric charge associated with it.

    If you plant a flag on a mountain and claim it is 10,000 feet then there must be a reference to 0 feet and in order to claim 0 feet or 10,000 feet there must be a measurable and locatable difference between the two, there's gotta be substance in there somewhere.

    Ugh, give me a minute to ponder this.
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    No, it doesn't. It has nothing to do with attraction to a particular point. It has to do with the energy required to move a charge to that point.

    Imagine a 2 kg book sitting on a desk. Imagine a shelf located 2 meters above it. For simplicity, let's say that the gravitational constant at that location is 10 N/kg (which is the same as 10 m/s²). How much energy (work) is required to move the book to the shelf? It requires a force of 20 N acting through a distance of 2 meters, or 40 J (1 J = 1 Nm). On a per-unit-mass basis, that was 20 J/kg, so the potential energy of the shelf is 20 J/kg higher than the top of the desk. Notice that it didn't require anything to be located on the bookshelf to attract the book. In fact, it didn't require the book at all! Moving any object from the top of the desk to the bookshelf will require an expenditure of 20 J of energy for each kg of mass that is moved from the top of the desk to the bookshelf. This is due NOT to the book, but to the fact that the desk and the shelf are sitting in a gravitational field of 10 N/kg and because the shelf is located 2 meters higher than the desk.

    YOU get to arbitrarily declare ONE POINT to have whatever value of height (or voltage) you want.

    Imagine going to another planet that has never been visited or explored. It's reasonable to start mapping the planet's surface. You could pick the top of the tallest mountain you can see (knowing that there are probably other mountains that are taller elsewhere) and simply declare the elevation of that mountain to be 10,000 meters. There does not have to be any place on the surface of the planet that is at 0 meters to serve as your reference -- that mountain top, declared to be 10,000 meters, IS your reference. If the valley floor is 6000 meters below it, then the elevation of the valley floor is 4000 meters. If you insist on putting some marker at zero elevation, then just go find a spot that is 10,000 meters below your reference and stick a flag there. There may not BE such a spot -- you may discover that the lowest point on the planet is only 7000 meters below your chosen mountain top -- in which case you could always dig a 3000 meter deep hole at that spot and put your flag there. The point is that you never have to every find a point that is at zero elevation because it is always only the differences in elevation that matter.
     
  19. MrSoftware

    Member

    Oct 29, 2013
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    Think of voltage as a force. The absolute value of the voltage reading shows the magnitude of the force. The sign, + or -, shows direction.
     
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  20. crutschow

    Expert

    Mar 14, 2008
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    MrSoftware has a good simple analogy, to think of voltage as a force (on the electrons) between two nodes.
    The sign of the voltage indicates the direction the current will flow (direction of the force), nothing more.
    The magnitude of the voltage indicates how much current will flow for a given impedance between those two nodes.
    You can label one of those nodes "ground" if you like but that has no effect on the circuit, only the relative sign of the voltage.
     
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