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Intuition for positive, negative, and ground voltage

  1. Jul 11, 2012 #1
    Can anyone help me develop some insight as to what positive and negative voltage means?

    I've heard of the analogy to water flowing where amps are viewed as units of water flowing through a certain point per unit time and volts are viewed as how high the water is flowing downwards to the ground.

    Thus, I'm inferring that the positive voltage is how high it is relative to the ground but where is the negative in this picture?

    Also, in AC, since the polarities are switching back and forth, wouldn't that mean that the charges are going to ground as well? How would this provide useful work?
     
  2. jcsd
  3. Jul 11, 2012 #2
    I think intuitively positive voltage is moving towards your house and negative voltage is moving away from it. When the sine wave oscillates into the negative it requires less energy to move electrons towards the power company than it does towards your house. When the sine wave is positive the electrical difference now favors moving the electrons toward your house.

    I'm not entirely sure myself how the negative voltage is inherently electrically useful. But you can Google things like full-bridge rectifiers to see how they can be made to be.

    AC is more efficient, I believe, because the power company can just push/pull @ 60Hz instead of send a constant stream (DC) of electrons to your house. This pushing/pulling allows for greater distances which means fewer power stations, lower electricity costs, etc.
     
  4. Jul 11, 2012 #3

    jim hardy

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    wow there's no quick answer
    It is important that you grasp the concepts of force, work and energy as related to electric charge. The concepts are easy but the names are at first strange , mostly after long dead scientists.


    here's a hobbyist's introduction, written in plain language for other hobbyists.

    http://amasci.com/miscon/voltage.html

    take time to get those basics. else you'll have to un-learn the misconceptions that are so prevalent out there.

    as that site says most of us dont think much about static electricity beyond getting zapped by it.
    But if you've ever pulled apart styrofoam cups and had them stick together, or pulled the plastic bag off shirts from the laundry and had it stick to your hand, you have felt the attraction of opposite charges. That's voltage.
     
  5. Jul 11, 2012 #4
    The terms positive, negative, and ground are relative. Literally. Many fundamental concepts in electronics, because it is based purely on physics, are very straightforward.

    Instead of discussing 'positive' and 'negative' right off, let's just call it 'potential'. Each one represents a different point on a line. Remember in math, when you learned about negative and positive numbers:

    - <...5...4...3...2...1...0...1...2...3...4...5...> +

    It's no different here. Positive is any voltage above zero (as defined by ground), and negative is any voltage below zero (aka ground). So, the real question is, what makes 'ground' zero?

    That's the real question. Hold on, because here's a curveball: Ground doesn't have to be zero. What? But... ground is always zero! No, it isn't true. We never know what ground is. But we call it zero in relation to another voltage. Here's an example:

    Ground = 15V
    Positive = 25V

    A voltmeter will tell you that the 'potential' difference between those two points is +10V. So we only ever concern ourselves with the difference between them, not the actual value-- because that is relative to something else.

    Normally, in the real world, Ground is whatever voltage level the Earth is at-- Earth Ground. If that's 75V we'd never know it, but if we always based our voltage off that fundamental starting point-- anything higher is positive while anything lower is negative, Earth ground (for all intents and purposes) is zero.

    All charge is negative. The difference in potential is measured between two piles of negatively charged electrons. The physics of the universe, the very cosmic force of nature, always moves towards neutral (ground) charge. In other words, the pile with the most negative charge, keeps sending electrons to the nearest smaller piles of negative charges, until all piles are the same (so potential between them is zero).

    This is an incredibly strong force. Negative is always the larger pile of negatively charged electrons, positive the smaller, and ground is whereever we want it to be, between them based on (normally) Earth ground (simulated or real).
     
  6. Jul 11, 2012 #5
    In AC, charges do go to ground, 120 times per second. That's why if you measure an AC circuit with a DC voltmeter, it bounces around zero-- the average voltage it sees is zero. That's why you use an AC voltmeter (to rectify and obtain an RMS Value) to get a reading.
     
  7. Jul 11, 2012 #6

    jim hardy

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    that water analogy can work well but it must be used wih caution. It too easily leads one to the wrong conclusion that "ground" is somehow an infinite reservoir of charge,
    or that it is zero potential.

    Voltage is potential difference.
    There's no absolute zero voltage, like there is an absolute zero pressure and an absolute zero temperature.
    I think that's where people get mis-led by the water and pressure analogy.
    There's no absolute zero reference, your reference is wherever the black lead of your voltmeter is hooked.
    That's what mosfet was saying with "We never know what ground is. "

    For example, we don't know what is voltage between earth and sun. Nobody has carried a meter lead there to measure it.
    Maybe that's what is behind "Pioneer Anomoly".

    old jim
     
    Last edited: Jul 11, 2012
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