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What does more/less voltage mean?

  1. Jan 16, 2016 #1
    Hello again guys. I've been reviewing stuff from electromagnetism and I just recently read about voltage. I know it is a difference of potential, electric potential more precisely. Electric potential is just a property of electric fields and it is measured in Joules per Coulomb. 1 Joule per Coulomb is the energy spent in moving a 1 Coulomb charge across a potential difference of 1 Volt. However, when it comes to more or less voltage I can´t distinguish any difference. I can just say 12 V is just the same amount of charge being moved across 12 V and 12 J being spent by the electric field because of that. I know current is proportional to voltage but in many cases there are tiny currents caused by huge voltages so it is not a good reference to me. What does more voltage actually mean? By the way, please don't relly on the hidraulic analogy, it never explained anything to me.

    Thank you :)
     
  2. jcsd
  3. Jan 16, 2016 #2
    It depends on the resistance how much current you'll get. You can have a huge voltage difference, but if you have a high resistance in between, the current will be small.
     
  4. Jan 16, 2016 #3

    anorlunda

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    You say that you understand potential energy. Good.

    Mechanical things, like a barbell, have potential energy too. Compare lifting a barbell to your waist, or lifting it over your head. Do you see that the bar over your head needs more potential energy?
     
  5. Jan 16, 2016 #4

    Yes, i do get that but When it comes to height it makes me think as if greater voltages meant that greater electric fields were capable of transporting charges longer distances but I really doubt it.

    Thanks for your answer
     
  6. Jan 16, 2016 #5
    But in transmission lines cables have low resistance and yet currents are tiny to avoid energy losses due to joule effect. I was told that transformers can vary voltage and current parameters always conserving the same power.
     
  7. Jan 16, 2016 #6
    I forgot to say that transmission lines are really long and distance is resistance so yes, the little currents must be due to that as well.
     
  8. Jan 16, 2016 #7

    jim hardy

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    You already know the answer. You said it in first sentence: "I know it is a difference of potential, electric potential more precisely."

    i had same trouble and it stems from not thinking through this question: "What is electric potential?"
    https://en.wikipedia.org/wiki/Electric_potential
    Paraphrase the last part of that:
    "Potential is the work done in carrying a unit of positive charge from infinity to wherever you are measuring. "
    Tha's absolute potential and you can't get clear out to infinity to measure it. It's just a concept.
    So we have to settle for measuring the difference between absolute potentials of two points, neither of which we know,.
    But their difference is readily measured with an ordinary voltmeter.
    And that's why voltmeters have two wires.

    See ? Told ya you already knew.

    You will find it handy in your studies to form the habit of always specifying between what two points is the voltage of which you speak.
    For example V = 12 volts is ambiguous
    Vbattery = 12 is less so, it infers voltage across a battery
    Vgnd-Bat+ = 12 is even less ambiguous , it specifies voltage between gnd and battery positive
    It is useful to pick a convention for your subscripts, to further specify which meter lead goes where
    if first subscript means meter's negative lead
    Vgnd-Bat+ = -12 infers the battery's polarity is reversed , which might be an important detail !
     
    Last edited: Jan 16, 2016
  9. Jan 16, 2016 #8

    meBigGuy

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    Voltage is basically how forcefully electrons are pushed through a resistance. For a constant resistance, increasing the voltage will force more electrons through the resistance (more current), and vice versa. If you want less current, you need to decrease the voltage (or increase the resistance). The voltage is a potential in the sense that that there is potential energy (joules/coulomb) that can be accessed (or expended) when a path for current flow is provided.
     
  10. Jan 16, 2016 #9
    Ok, What do you mean by settling?
    From what I recall, infinity is applied to measure a potential difference between zero and a point closer to the positive charge (the big one) because potential differences are the magnitudes that matter to us, not just electric potential.
    How can multimeters measure such things being just concepts? Or at least how can you establish zero volts?
    Thanks a lot for your answer, I'm not far from finally understanding this.
     
  11. Jan 16, 2016 #10
    Yes, I know Ohm's law. I know current is directly proportional to potential difference and inversely proportional to resistance. I just wish I could relate more/less voltage to something else than current.
    Thank you
     
  12. Jan 16, 2016 #11

    meBigGuy

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

    Actually, I have a problem going beyond this:
    The voltage between two points is equal to the work done per unit of charge against a static electric field to move the charge between two points and is measured in units of volts (a joule per coulomb).

    So, if I take a coulomb of charge and move it through an electric field, it accumulates a voltage based on how much work I have done. Like stretching an elastic band.
     
  13. Jan 16, 2016 #12
    Your understanding of the definitions at play here seem sound but I am going to address your confusion regarding transformers, which might help you. You say transformers generate high voltages and low currents even though the power cable is very low resistance. Actually, it is true across the coil that the coil's resistance is small but this is not a resistive effect we are dealing with. It is an inductive effect. A totally different phenomenon is taking place than simple resistance. Voltage drops across inductors are related to the rate of change of the magnetic flux through the coil by Faraday's Law (this changing magnetic flux is caused by time varying currents through the coil).......not simply the product of resistance and current. To reiterate this using different terminology, in addition to resistance there is another property of a circuit called "reactance", which can also produce voltages in the circuit due to time varying magnetic fluxes, which are in turn caused by time varying currents.

    Now, the resistive drop in voltage across the transmission line IS consistent with Ohm's law. If we assume negligible reactance in the transmission line itself it will be a very small drop though since the current through the cable is low and the resistance of the cable is low.

    But again, the voltage across the secondary of the step up transformer is large even though the resistance of the coil is very small because the voltage across the transformer is NOT due to simple resistance. It is an inductive (or reactive) effect.

    How that helps.
     
    Last edited: Jan 17, 2016
  14. Jan 17, 2016 #13
    The voltage can be thought of as the amount of potential energy a one coulomb charge GAINS when it is moved AGAINST an electric field (much like the potential energy of a rubber band as it is being stretched but note that the equation for force and thus work/energy is different for a spring-like elastic band than for an electric field) or the amount of potential energy a coulomb charge LOSES when it is moved BY an electric field (much like the potential energy of a rubber band as it is being relaxed).
     
    Last edited: Jan 17, 2016
  15. Jan 17, 2016 #14

    davenn

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    Not sure where you got that idea from ? as these are bad and incorrect assumptions

    Dave
     
  16. Jan 17, 2016 #15

    jim hardy

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    So much rests in accepting the meaning of potential difference and relating that to absolute potential...

    Take me literally - we cannot go to a place infinitely far away to get a measurement.
    Now, i often use exaggeration to put across a thought.
    Absolute potential is, again, " the work done in carrying a unit of positive charge from infinity to wherever you are measuring. "
    Think about that.
    I said in another thread , ,,,
    '...when that concept finally sunk in i imagined myself walking all the way from Alpha Centauri to Miami Central High School , carrying a bucketfull of charge and tabulating the dynes of force every centimeter. (Nowadays it'd be Newtons every meter). '
    Summing those F X D's would give me the absolute potential of Miami Central High School, assuming for all practical purposes Alpha Centauri is close enough to infinity....
    so absolute potential is measured between here and infinity
    that is, it's the potential difference between here and infinity

    Since that's impractical to do
    we must accept we can only measure potential difference between locations within our reach
    I can 'settle' for that. "Settle for" means accept as reasonable compromise.


    i dont understand that sentence
    absolutely. We can only measure potential difference. And only between points we can reach.

    Grab a multimeter and a AA battery. You'll see in a hurry the meter reports about 1.5 volts potential difference between battery's ends.
    But we've no idea what is the absolute potential of either end.

    You said you don't like hydraulic analogies. Fine. They cause trouble if not used exactly right.
    But we get imprinted by our experience growing up that measurements measure absolutely not differentially.
    Tire pressure is described as let's say 35 psi. Not 50 psia inside the tire and 14,7 psia outside it, just the 35 psi difference. That's sloppy language. Fill a tire in Miami and drive to Denver it'll gain 2.5 psi.
    It's really as simple as realizing we are imprinted to the oversimplification that measurements are just a number not a difference. But ruler has two ends , a pressure gage has an inside and an outside, and a voltmeter has two leads..

    How to establish zero volts? Connect the black lead of your multimeter there. It's not zero absolute potential , it's your point of reference . If your black lead were long enough you could in principle hook it to alpha centauri and read absolute potential where you are with the red lead. But we always choose a more convenient reference point - local earth, chassis of our automobile, skin of an airplane, circuit common of an appliance, negative end of a AA battery ....
    The simple old fashioned metal flashlight is a good thought tool for demonstrating elementary circuit theory, especially "grounding" .

    Sorry to be so wordy - but when people show they're thinking about these things i try to share the struggles i had.
     
    Last edited: Jan 17, 2016
  17. Jan 19, 2016 #16
    Thanks a lot. I did not think about reactance in the first place. Reactance is also oposition tu current flow right? Reactance is directly proportional to inductance in a coil which is also directly proportional to the number of turns in it (if i´m not mistaken). Could I say then that the low current in the secondary is mainly because the secondary has a considerable number of turns?
     
  18. Jan 19, 2016 #17
    Thanks a lot for your High Quality answers Jim :) Now I understand this is about references rather than absolute values. However, I've heard that ground, or earth I think, has a potential of 0 V. Does that mean physical ground is only a common point of reference or does it really have such a physical property?
     
  19. Jan 19, 2016 #18

    jim hardy

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    Thanks for the kind words....

    "Ground" is probably the most mis-understood and abused word in EE.
    "Ground" is just a reference.
    Usually the term is referring to what is more properly named "Circuit Common", just a handy place to hook your voltmeter's black wire.
    That point may or may not be connected to "earth ground" .

    I don't believe we know earth's absolute potential. There is a field of about 100 volts per meter near the surface..... Is its net charge zero ?
    The best way to think of "Earth ground" is as just another wire that goes most everywhere.
    Like any wire it can have a voltage drop when substantial current flows, as around a lightning strike.

    We have had many many threads on ground. Try a search
    here's a not too ancient one where i wrote another of my rambles in post 10..

    https://www.physicsforums.com/threads/why-do-we-ground-an-electronic-circuit.810961/#post-5092341
    try a pf search on terms ground neutral grounding
    dlgoff has posted many many excellent pictures, try adding his name to your search

    Happy to see your interest - thanks -

    old jim
     
    Last edited: Jan 19, 2016
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