Negative voltage explanation

Discussion in 'General Electronics Chat' started by pilotnmech, May 25, 2007.

  1. pilotnmech

    Thread Starter Member

    Feb 26, 2005
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    Is there anybody that can explain negative voltage and show illustrations? Is there any good material online where you can learn about negative voltage? Please help-this is driving me crazy!

    Thanks in advance.
     
  2. pilotnmech

    Thread Starter Member

    Feb 26, 2005
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    Let me clarify:
    I understand about grounds, and grounds are supposed to be a reference point, but I have a circuit here that has two six-volt batteries in series. The batteries are grounded in between them, and then I have a resistor shaped like a circle with a sliding contact that rotates clockwise to power a lamp. The wires come in from top & bottom to the resistance. They say at the 90 degree point from bottom each way in each direction, you have zero volts. It seems to me that if you leave the negative terminal of the bottom battery, it would go to the resistance. Some of it will split off either way on each side of the resistance, and the electrons will meet back up at the positive wire and go into the positive terminal of the top battery. It also seems that when you ground the lamp and in between the batteries, that becomes, in effect, another wire. So there would be no potential difference between each, and the lamp would never come on. I know I'm missing something, but I wanted to tell you what I'm thinking (which I know is wrong). Please help.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
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    Your resistor with sliding contact is called a "potentiometer" or "pot" for short. It functions as a voltage divider. From the perspective of the current carriers, it is the same as two resistors in series.

    By varying the setting (the position of the sliding contact or "wiper arm") we change the values of the resistors comprising the voltage divider.
     
  4. pilotnmech

    Thread Starter Member

    Feb 26, 2005
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    Can I get a better explanation than that? How about tracing the circuit that the electrons take? What effect does grounding the batteries have on the circuit?
     
  5. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    i m a bit too tired right to read the ckt description now
    wud it be possible to post the circuit diagram (a rough one wud do)
    i'll try to look into the matter.
     
  6. Dave

    Retired Moderator

    Nov 17, 2003
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    I recall the last time this question was asked, it resulted in a rather interesting (often heated) discussion: http://forum.allaboutcircuits.com/showthread.php?t=1892

    Dave
     
  7. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    A neegative voltage pole is stated as that because is more negative in potential than the ground, being therefore relative to ground. The voltage of a pole or node is potential difference between the pole and ground in the order specified (ground potential subtracted to pole potential, or V_pole = DeltaU_pole = U_pole - U_ground).
    When you are talking about to batteries connected to a common ground between them, you are in a situation where you have a double power supply, so you will have a positive pole, a ground pole and a negative pole. If you had the ground reference at the very bottom, you would have two positive poles, one 2 times more positive than the other when relative to ground. It is more a question of where the ground is.

    P.S.: Ground is normally considered at 0 potential, although in reality the potential of objects is unknown and cannot be evaluated, only their differences can.
     
  8. recca02

    Senior Member

    Apr 2, 2007
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    like it has been stated before.
    since negative is at a negative polarity (duh) with respect to ground.
    it can well be said ground is a positive polarity and -ve one acts as ground
    since potentials are relative concept absolute zero potential may not be defined.
    like mr cumesoftware just said.
    btw dont touch negative polarities since they may act as ground or else you wud act as positive polarity :)
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    I think that was the question in here. Sorry if I missed the point.
     
  10. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    This whole discussion is a complete red herring. Voltage is voltage and one side of any non zero measurement is greater than the other. One is positive with respect to the other, and one is negative with respect to the other. Potential functions in physics, including voltage, are largely unconcerned with absolute scales. Only temperature in degrees Kelvin (Rankine) has an absolute zero.
     
  11. Dave

    Retired Moderator

    Nov 17, 2003
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    I concur. The important aspect of voltage is the differential, i.e. if the voltages are -250V to -240V it is a 10V difference in the same way if the voltages are 500V to 510V. I would be interested in seeing this 'negative voltage' concept written in context - does anyone have any text/papers where it is stated?

    Dave
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Of course. As I said, the potential of objects cannot be determined in absolute terms. In order to simplify, it is considered a ground and all voltages relative to it. So when refering to 5V, it is 5V relative to ground. Is just a convention like any other.

    I think the issue here was having negative potential diferences relative to ground. It was not about potentials. That's why I put this in post scriptum.

    P.S.: Discussing potentials is like discussing who born first, the egg or the chicken.
     
  13. Dave

    Retired Moderator

    Nov 17, 2003
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    Sure. Given this is a question that has been asked several times before, I still would be interested in seeing 'negative voltage' written in context in a published text/paper - I would like to see why this confuses so many people when it is a relatively simple idea.

    Dave
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Now, going to the issue, any tension across the lamp is to be determined by the cursor terminal of the potentiometer. So when you have the potentiometer in the middle, the lamp will not light up, since the potentiometer acts as a voltage divider and so the cursor will be in a "voltage" in the middle of +V and -V, that is, 0.
    For example, if the cursor were 1/4, closer to positive, you would get (without the lamp, or considering it to have infinite resistance) 1/2V. If the cursor were 1/4, closer to negative, you would get (without the lamp, or considering it to have infinite resistance) 1/2V, and the lamp would emit the same light. But of course the lamp has resistance and the voltage across it in those situations will be smaller than stated.
    This situation is analogue to a Wheatstone bridge, replacing the two resistors in one side by the batteries (the known leg), and replacing the two resistors of the oposite side by the pot (the unknown leg). The lamp will be our galvanometer, acusing no current when the bridge is balanced.
     
  15. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    You can see this concept in datasheets, for example. See the LM7905 datasheet, for example, or the specifications of a computer PSU. It is a correct concept. You consider a ground, they you may have positive poles or/and negative poles. We may say negative voltage, but being voltage = potential differential, we have negative potential differential. Convening that the potential differential can be stated as the diference between the measured pole and the reference pole, why not having negative poles?
    Potential diferential does not have to be the difference between the largest more positive potential and the more negative one. In some cases can be referenced to ground as well (of course in calculus we should consider the differential as such). It is a question of reference, once more.
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Sorry, no. I'm away from my own workstation for at least another week. Once home again I can do so easily enough.

    And my bank account. :(
     
  17. pilotnmech

    Thread Starter Member

    Feb 26, 2005
    27
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    I thought I could get some real help. I guess I was wrong.
     
  18. beenthere

    Retired Moderator

    Apr 20, 2004
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    I think the subject got beaten to death. Try to follow - a voltage is only negative in relation to another potential. That is all there is to it. If you reverse the meter leads, it becomes positive with respect to the reference potential.

    Perhaps this will help: There is no such thing as an absolutely negative voltage.

    Hope that door doesn't pat yer fanny.
     
  19. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    what else can we help u with,
    its been written ten times already by 6-7 different members,
    nothing like negative voltage exist,its all relative,
    if something does not exist only material u'll ever find about it is sci-fi movies and novels,
    can u tell us what sort of real help do expect.

    abt the description can u post a circuit dia a rough one will do,
    i dont wish to construct one using the description (i just got free from exams)
     
  20. Dave

    Retired Moderator

    Nov 17, 2003
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    The electrons will take the path from the more negative potential to the more positive potential. The potentials can then be understood relatively as discussed before. The important aspect of voltage is the differential. Only when you put it into a circuit to the values become important relative to one another.

    I'm sorry you feel like that, but what you need to know about negative voltage has been covered in this thread. Perhaps you could take me up on my suggestion previous and show us an example of where the concept 'negative voltage' is used and we can try and help from there.

    Dave
     
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