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Voltage. What is it really?

  1. Jun 22, 2012 #1
    Hi guys, I'm new to this forum and am not sure where I would ask this question so sorry if it's out of place :/

    But I want to know what voltage is exactly. I've asked lecturers, friends and i don't get it! I know that its measured in J/C, and that its the difference in potential energy between two places. But I don't know why voltage increases current (this is unlike the analogy with gravitational potential energy) and why as if it is just the difference in potential energy, surely the same amount of current would flow but for a longer period of time? Considering the fact that more voltage means more energy for, say, a circuit with a resistor. If the current increases doesn't that mean more energy is being used up quicker? And why? :/ This is doing my head in!
     
  2. jcsd
  3. Jun 22, 2012 #2
    Voltage is a potential difference. You create a voltage by applying an electric field. Electrons in the conductor experience a force due to this electric field, and are accelerated by it.

    Because the electric field is conservative, the potential difference is exactly equivalent to the work done on the electrons by the electric field. So, if you have one coulomb of charge and you apply a voltage of 1 V, then you will do one J of work on those charges, hence they move and produce a current.

    Get it?
     
  4. Jun 22, 2012 #3

    phinds

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    No, the amount of current at any given time is a function of the voltage and the load. For a given load, increasing the voltage means increasing the current.

    Look up Ohm's Law
     
  5. Jun 22, 2012 #4
    I get that part now, thanks. But why does voltage increase current for a given load?
     
  6. Jun 22, 2012 #5
    Oh i was confused about that. But can you explain why current is a function of voltage and load? Why does current increase as voltage increase in physical terms? Is voltage the "kinetic energy" of each charge which means it travels faster through the load? :/
     
  7. Jun 22, 2012 #6

    Drakkith

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    Imagine that the difference in potential is like pushing a box. With no voltage you have no force applied to the box, so it doesn't do anything. When you apply a voltage you apply a force to the box and it moves. However the box must fight friction, aka resistance, to get anywhere, so you have to keep that voltage applied. The higher the resistance the more voltage is needed to get the box to the same speed. The more force applied the faster the box goes.

    Now, electrons aren't boxes, so there are some differences. When you apply a voltage you have not just one electron, but a whole bunch that will move along the entire circuit. Increasing the voltage means a higher current if the resistance is kept the same because more force equals more electrons passing through a section in the wire over time (I don't know if it's just a larger number of electrons moving, or if they move faster or both, but the fact is that either way the current increases). Increasing the resistance of the circuit while keeping the voltage the same reduces the current, as fewer electrons can move with the same force applied.

    I'm sure I've butchered that explanation, but I hope it helped.
     
  8. Jun 22, 2012 #7

    sophiecentaur

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    Look into the mirror every morning and say to yourself "One Volt is one Joule per Coulomb".
    When ever you have a question like this, the answer lies in that simple statement.
     
  9. Jun 22, 2012 #8
    No wonder I'm having problems with Physics.

    All these years I've been saying "where'd she move the da*ned Dixie Cups this time?"
     
  10. Jun 22, 2012 #9

    Drakkith

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    Hmmm, not sure I want to know why you were staring in the mirror and saying that...
     
  11. Jun 23, 2012 #10
    It helped, but I'm having problems interpreting Voltage as a force along with its formal definition. If voltage is difference in potential difference, isn't it analogous with a rock being lifted 10 feet higher, and force wouldn't have changed (as in a gravitational field, g=ma, m=mass object, a=constant acceleration, independent of location), between the two locations?

    Unless, please correct me if I'm wrong. Continuing the analogy with gravity; In an electric field, Voltage is changed by increasing the strength of the electric field (like making the gravitational field stronger rather than lifting the rock higher?), and therefore the force also increases and so will the difference in potential difference at the same time (PE = m x g x h, with g increasing)? So Voltage is in physical terms the strength of the electric field being increased which also gives the charges electrical potential and "force" to use in a circuit? (This is the only thing I have at the moment lol)
     
  12. Jun 23, 2012 #11
    Yes this is essentially correct.
     
  13. Jun 23, 2012 #12
    OMG I am literally jumping for joy! (nerd).

    Why don't lecturers and teachers explain it this way? Or maybe I'm just a dunce....
     
  14. Jun 23, 2012 #13

    sophiecentaur

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    I think it's really bad, how electricity is taught at School level. These silly pictorial descriptions of current, voltage and resistance could, possibly, be justified for primary school but they do no one any favours when they need to get to real grips with the subject.
    Electricity is a totally abstract concept but, in today's world, the people who don't know much at all about anything, hold sway. They see it in simplistic terms and they (and we're talking about politico/educationists, here) seem to think it can be taught as such.
    You are not a dunce (afaik ) to have struggled with a nonsense explanation. You, and all recent generations of students have had to deal with the dreaded 'dumbing down'. You seem to be emerging out of the other end of the tunnel of dumb - welcome.
     
  15. Jun 23, 2012 #14
    If I remember correctly (a dorky analogy I just thought of):
    Suppose you are watching a series of electrons racing on a racetrack passing the finish line.
    Voltage is the amount of energy each electron possess as it crosses the finish line
    Current is the amount of electrons passing the finish line per second.

    So if you take a moment to put this into the "real world" then perhaps it will make more sense.
     
  16. Jun 23, 2012 #15

    sophiecentaur

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    So, if it's AC and no electrons move further than 1/100mm back and forth, how is that explained by your model?

    These models are a bit like trying to explain algebra in terms of piles of beans. If you're trying to understand electricity then you should be moving away from these simple pictures. They will only lead you astray, eventually. "Real World" only make sense when it actually applies to a situation.
     
  17. Jun 23, 2012 #16
    Well its pretty easy to visualize electrons oscillating back and fourth accross a finishline... but then again it was supposed to be a simple answer. If you want the real answer study quantum mechanics.
     
  18. Jun 23, 2012 #17

    Voltage is strength of electric field times length of electric field.

    An example:

    strength of electric field = 1000 V/m
    a 10 meters long piece of this electric field has a voltage 10 m *1000 V/m , between the two ends of this electric field.
     
    Last edited: Jun 23, 2012
  19. Jun 23, 2012 #18

    sophiecentaur

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    "real" answer?
    you mean 'another' answer, perhaps.
     
  20. Jun 23, 2012 #19

    sophiecentaur

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    It's difficult to relate the Fields in a circuit to Voltages. Where do you choose to imagine the two points to be in order to calculate the Volts per Metre? A circuit could be a 10km loop or a 5mm loop and the Volts would be the same. It would have to depend upon the separation of the terminals connecting the battery perhaps??? This is why we use Potential all the time and not Field. The Joules per Coulomb are not dependent on any fields that may be present around the circuit.
     
  21. Jun 23, 2012 #20

    I have improved my answer #17 by deleting lot of quoted text. Now you see I just added the length, which surely is an improvement.

    How does an ordinary volt meter measure the voltage between the floor and the ceiling (a large capacitor) ?

    Well, the connecting wires are of certain length and the same field that goes through the air, goes through the wires. Every electron in the wires is pulling or pushing the few electrons in the resistor through the resistor that is inside the volt meter.

    The volt meter measures the leakage of electrons, caused by the pressure of the stack of electrons in the wire.
     
    Last edited: Jun 23, 2012
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