Let's say we have a 120 Volt AC source

  1. Trying this again from a different angle...

    Let's say we have a 120 Volt AC source. We'll call it a generator. This generator is not touching the earth. Let's just pretend that it's suspended in mid air with nothing at all connecting it to the ground (the earth). On this generator, there are two poles. If you put a volt meter across the two poles of the generator, your meter reads 120 Volts.

    You attach a wire to one of these poles, leaving the other one alone with nothing attached to it. You stick the other end of that wire into the earth. Does current flow?

    Why or why not?
     
  2. jcsd
  3. Re: Electricity

    Depends on what you're referencing as your 0 volt point.

    To clarify if there is a ( nonzero) potential difference between the ground and your "hot" line there will be an electric field, this electric field will cause electrons to flow thus generating a current.
     
  4. Re: Electricity

    The only reference in this scenario is the other pole. The earth is not referenced anywhere in this electrical circuit. If one meter lead is on a pole and the other meter lead is on the other pole, your meter reads 120V.

    I contend that if you put one meter lead on a pole (doesn't matter which one) and the other meter lead in the earth, you will read 0 Volts. Do you agree?
     
  5. Re: Electricity

    If your reference is the "other pole" then you would need to measure the voltage difference between the ground and the "other pole" and from there you could find the potential difference between the ground and the "first pole".

    The ground still might have a potential difference but you'd ned to reference it with the "other pole".

    The voltage at a single point is entirely relative and meaningless the only that is physical is a difference in voltage between two points.

    Ask yourself where the notion of potential difference come from. They arise because of electric fields. Electric fields carry energy and if we do work to place two charged particles at two different distances they will have two different potentials. It's the same if we are to lift two objects to two different heights there is a potential energy difference in the two objects.
     
  6. Re: Electricity

    I understand that voltage is a potential difference between two points... If you're talking about a "reference" then you're talking about either pole A or pole B, correct? What else could the reference possibly be? I'll just pick one for the sake of moving forward. Our reference is now pole A. You would measure a potential difference between the "reference" (pole A) and another point. I'm saying that the potential difference between the two poles is 120 Volts. The potential difference between pole A and the earth is 0 Volts. The potential difference between pole B and the earth is also 0 volts. Is there disagreement on this or not? If you are saying there is a voltage present there, can you explain why?
     
  7. cepheid

    cepheid 5,194
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    Re: Electricity

    Let's say that VA is the electric potential at pole A, VB at pole B, and VEarth of the earth, all measured relative to some common reference point.

    So you're saying that

    VA - VB = 120 V

    but

    VA - VEarth = 0 V

    AND (somehow!)

    VB - VEarth = 0V

    You see that this is impossible, right? If pole A and pole B are not at the same potential, then they can't BOTH have the same potential relative to Earth.

    If, for the sake of argument, VB = VEarth, then it must follow that VA - VEarth = 120 V (NOT 0 V)

    So, in this example, if you connected pole A to Earth, current would flow, since pole A has a positive potential relative to Earth.
     
  8. cepheid

    cepheid 5,194
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    Re: Electricity

    It could be ANYTHING you want. Pick a point in space, and define the electric potential to be zero there. All other electric potentials are measured relative to the potential at this point. This point is called "ground" or "earth" in a circuit, but it doesn't necessarily have to be physically connected to the Earth.

    Obviously it's convenient for this situation if the reference point is one of the poles in the voltage source.
     
  9. Re: Electricity

    This may be true. In fact, I ventured into a physics forum and everyone here is waaaay smarter than me and you certainly sound like you know what you're talking about, so I will concede that it is in fact true. I was wrong.

    But I think this difference is extremely small and probably hard to measure with common instruments. Lets say that your volt meter doesn't have a decimal point and it is accurate to only the nearest volt. I bet the display on your meter reads 120 Volts between the poles and 0 Volts between either one of the poles and the earth. Do you disagree with that?

    If we can't agree on that, let's change the example to something we can test easily ourselves. Let's get a battery and a volt meter and take some measurements. A car battery would be best, but we can use a little 9V battery for simplicity. If you measured voltage across the (+) and (-) terminals of the battery you would get the battery voltage (assuming it was charged of course). I bet we all agree on that.

    Now for the meat of it... I contend that you will get 0V if you measure between either one of the battery terminals and the earth (lets make sure the battery is isolated from the earth. If you can't levitate it, then I guess you could put it on a piece of wood or a wooden table or something to make sure there is no connection being made from the earth to either terminal :rofl: ). Stick one lead on the positive terminal and the other lead into the earth. What does your meter display? Now move the lead from the positive terminal to the negative terminal of the battery, (leaving the other lead stuck in the earth) what does your meter display?

    I contend your meter will display 0 Volts. Try it and let me know please...

    I would try this myself, but the earth around me is frozen solid with 3 feet of snow on top of it. :eek: However, I did try it in my house with my meter and a AA battery. My results were 1.6V between the terminals and 0V from either terminal to my tile floor. That's the best I could do.

    Does anyone disagree that this would be the case? If there is no disagreement on this, can we go back to the original post and pick up where we left off? If there is disagreement on this, lets see if we can get to the bottom of it.
     
  10. Re: Electricity

    no..you need a complete circuit...a connection to the other pole.....
     
  11. Re: Electricity

    Thank you!

    Okay... Now here is my question:

    I'm confused on "charge" I think. Electrons will always want to equalize. Correct? If there is a more positive charge on pole A with respect to pole B and there is a way to get there, the electrons will want to equalize and current will flow.

    Doesn't this mean that there is a deficiency of electrons on the positive pole?
    Doesn't this also mean that there is an overabundance of electrons on the negative pole?

    If there is a deficiency of electrons on the pole and you touch that pole to the earth (with a wire perhaps), why aren't the electrons from the earth being attracted to the pole with the deficiency?

    Is it because they are "equalized" in the earth already and they just want to hold on tight to where they are? It seems to me that if you had enough of a deficiency of electrons on that pole. the electrons in the earth would eventually be attracted to it and move there.

    But then we'd measure a voltage. :confused: So I just don't get it... I feel like I'm really close to understanding this but it's still escaping me.

    I'm pretty certain that I'm confusing charge and voltage and using them interchangeably when I shouldn't be... Or something. :rofl:
     
  12. Re: Electricity

    "...I'm pretty certain that I'm confusing charge and voltage and using them interchangeably when I shouldn't be..."

    I can still remember trying to figure out the difference MANY years ago.....most people have to think about each for a while when first confronting them....keep going and it will make sense sooner than you probably think....

    Try reading here:
    http://en.wikipedia.org/wiki/Electric_current

    charge (q) is measured in coulombs....a quantity of electrons.....and current is charge flow per unit time....q = i t.....

    Voltage is often described by analogy using water: it's the analog to water PRESSURE...more pressue makes water flow faster, move more volume of water and VOLTAGE pushes electrons HARDER....more current and charge will flow....
    V (or E) = IR...so for more voltage e or V for a given resistance, more current (I) is moved because there is greater ENERGY pushing each electron.

    try here for voltage:
    http://en.wikipedia.org/wiki/Voltage
     
  13. Re: Electricity

    very good question...I'm not sure I have a one sentence answer.

    The the electric potential in your statement has moved all the electrons it can...they are stuck in place....if you connect just one wire, no electrons will move...because the charge that can be moved already has been...further movement is opposed by the electrons and voltage potential in place....


    If the voltage difference is high enough current CAN flow even without ONE wire...that's what happens in a lightning storm when one part of a cloud is positive and another part is negative.....

    Are you familiar with a capacitor?, where equal charge is stored on either side of, say, parallel plates???...it's the electrical equivalent of what you example. Connecting either side to ground, individually, does NOT remove any charge from either side....electrons on the negative side (ungrounded) will oppose any more electrons being added if the positive side is grounded.....in other words, the potential (voltage difference) won't change untill a closed circuit is available.....
     
  14. cepheid

    cepheid 5,194
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    Re: Electricity

    I'd prefer it if you understood it, rather than just conceding the point, which tells me, "I wasn't convinced by your argument, so I'll just let it go." It would be a shame if you did that, because what you're saying is not impossible because of some hard-to-understand physics that I know and you do not. What you are saying is simply a mathematical or logical impossibility. You know arithmetic, therefore, you should be able to understand my argument. It stems from this:

    If x = y

    and,

    y = z

    then it must be true that:

    x = z

    Do you agree? Otherwise we have a logical contradiction. The same contradiction occurs in your scenario. You state that:

    potential of pole A = potential of ground (because you measure no difference between them) [1]

    And also potential of pole B = potential of ground [2]

    From this it MUST follow that:

    potential of pole A = potential of pole B.

    Since A and B clearly do not equal each other (it was GIVEN that they differ by 120 V in our example), statement [1] and statement [2] cannot BOTH be true. In particular, if one of the poles has the same potential as ground, then the other one MUST differ from ground by 120 V, since the other pole differs from the first pole by 120 V.

    Yes, I disagree, because, physics aside, it is a logical contradiction. (see above)

    I don't need to try it. Once again, we've started with the premise that the potentials of the + terminal and the - terminal differ by +9 V. Therefore, whatever you measure as the difference between the potentials of the - terminal and ground, the difference between the + terminal and ground HAS TO be 9 V higher than that. Otherwise, it would contradict our premise that the + terminal is 9 V higher than the - terminal.

    So, if you measure the difference between the - terminal and ground, and you happen to get 0.00512 V (5.12 millivolts), then the voltage between the + terminal and ground MUST be 9.00512 V (otherwise the difference between the terminals would not be 9 V). I really hope that you see that this must be true.

    Yeah, but that's because your tile floor is an insulator, and so the probe of the meter is not at the same potential as the floor. If you don't believe me, then let me ask you this: what does your meter read when neither of the probes is touching anything (or each other)? In other words, the voltage between the probes is "floating" (because the probes are not electrically connected and each one's potential could change independently by small amounts). I bet it displays 0 V as a default value. That's what my cheapo multimeter displays when the voltage between the probes is floating.

    EDIT: Or it could be that any initial imbalance in potential between the two probes is equalized by the flow of charge from one probe to another. Since the second probe is not connected to anything that can sink charge, those charges have nowhere to go and "pile up", balancing out any initial differences in potential.

    When we talk about "Earth", we mean something that acts like an ideal conductor (something that can be the source of or the sink for an arbitrary amount of charge without changing its electric potential). You'd have to dig a probe into the ground to get behaviour anything like that. Or you could just try measuring the voltage at each end of your battery relative to some conductor.

    EDIT 2: No, that probably won't work for any finite conductor, because of what I said in my first EDIT. Charges can flow freely between the two conductors until their potentials have equalized. But it will work in theory for any point that has a definite and constant potential and is not free to "float" up to some higher potential through the transfer of charge..
     
    Last edited: Feb 18, 2011
  15. cepheid

    cepheid 5,194
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    Re: Electricity

    Are you sure though? In this thread

    https://www.physicsforums.com/showthread.php?t=469342

    the OP had a similar question, and people were telling him that since the Earth can sink a LOT of charge without changing its potential, it could be very dangerous for a person to touch the positive terminal of a high voltage power supply. That person would be providing a path to ground for current, and the voltage across him would be maintained. Since the charges don't "pile up", there is no build up of an electric field to cancel out the one driving the current. In other words, there is no equalization of potential (this being the reason why current normally won't flow if there isn't a closed circuit).

    Are they wrong?

    EDIT: It looks like YOU were the one making this assertion, among others. So now I'm confused as to what your opinion is.
     
  16. Re: Electricity

    So I guess this is what I need to digest. I've got to process this for a little while.

    I do have a solid understanding of voltage and that if you had enough of it the air would conduct and you'd have an arc. I also understand capacitors and how they work in electronic systems...

    But I've been having detailed discussions about grounding and bonding of electrical systems and the way we distribute electricity in the united states. It's all about going back to the source. This is where most people (even electricians) go wrong... People think electrons want to make their way back to the earth. They do not. The only reason they EVER want to go into the earth in the first place is because we drive large metal rods into the earth with the specific intention of making the earth part of the circuit, giving it a way to get back to the source.

    I just keep getting hung up on this notion that if there is a large deficiency of electrons on a wire, the electrons in the earth will still never be attracted to it, no matter what the voltage is. With enough voltage applied, the arc would be between the two poles of the source and never between the earth and a pole. It's counterintuitive to me. But thanks for your answers, I will think on it some more.
     
  17. Re: Electricity

    You just have to watch nature at work in the heavens to see how wrong this view is. Old Ben Franklin must be turning in his grave.

    I think everyone will recognise that you can witness large discharges both between the clouds (poles) and the earth.

    Unlike Arial in the Tempest, electrons do not posess desire, which implies free will and choice.
     
  18. Re: Electricity

    I agree. I understand the logic and the contradiction you speak of. I also know how to read a volt meter and I trust that it is giving me the correct voltage.


    I really hope that you go try it. This is not a true statement.

    When I talk about earth, I am talking about the thing that my shoe touches when I walk outside. How about instead of the earth we use... umm... your car? the ground rod under your meter box? Use whatever you want. I still say your voltmeter reads zero unless your meter is touching both ends of that battery. (again... no decimal point. meter rounds to the nearest volt)
     
  19. Re: Electricity

    People. be patient uith a ignorant lay man.
    I have a question about static electrisity and Plank quanta "h'
    Let supose we have two electric unity charges far from each other 3.48181805*10^7 cm in static standing. The potential enrgy of these two charges will be E = h/1 erg.
    Now let supose that distance is plus 10 cm longer. I am confused about dilema:
    will be energy E< h/1 or will be 0 ?
    Please somebody help!
     
  20. Re: Electricity

    You bring up a good point Studiot. Why is it that neither of the poles in our 120 Volt generator have any desire to interact with the earth whatsoever, yet the electrons in that cloud are so excited to get to earth that they ionize the air and we see a bolt of lightning come down and touch the ground?

    Is it a matter of voltage? Let's take our suspended generator and crank it up to a million volts instead of just a wimpy 120 volts. Would that arc jump from one pole of the generator to the earth or would it jump from one pole of that generator to the other? I've got my money on the one-pole-to-the-other option. What about you?

    Excellent, excellent point! Something I know nothing about but am very interested in... Thank you.

    :rofl::cry::rofl:
     
  21. Re: Electricity

    Extremely high voltages have their own "set of rules"...try reading about lightning for example. I was recently told by a power company linesman they can use fiberglas poles up to something like 7,500 volts....don't quote me on that figure.....so current might flow through their bodies otherwise...."What about 25,000 volts and even 250,000 volts...I asked" "how do you work on that"...answer...." You have to shut the power off....
     
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