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Confused about Voltage and Current

  1. Mar 20, 2010 #1

    FeDeX_LaTeX

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    Hello;

    What is the relationship between the two? Are they proportional or inversely proportional to each other? I ask this because my physics teacher said that they were proportional on one day, then inversely proportional on another day. I thought that more current = more voltage... am I right or wrong?

    Thanks.
     
  2. jcsd
  3. Mar 20, 2010 #2

    rock.freak667

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    Ohm's law states that they are proportional, V=IR. In what context did you hear inversely proportional?
     
  4. Mar 20, 2010 #3

    FeDeX_LaTeX

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    Well, the lesson yesterday was regarding generators. He stated that as voltage increased, current decreased (as one goes up, the other must go down). I didn't argue at that point because that confused me (as you said, I thought they were proportional by V=IR).
     
  5. Mar 20, 2010 #4

    Vanadium 50

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    Your teacher is trying to tell you when they are proportional, and when they are not. In resistors, for example, they are proportional. In transformers, they are inversely proportional.
     
  6. Mar 20, 2010 #5
    Regarding generators, the generating power of the generator is (ideally) equal to the product of the current and the voltage it produces, so increasing one means decreasing the other. Therefore you can transmit the equivalent amount of power with higher voltage and lower current or lower voltage and higher current -- if you want to increase both, you need to get a more powerful generator. Maybe that's what your teacher meant.
     
  7. Mar 20, 2010 #6
    The relation between the two depends on the device. In a resistor, the relation is linear, i.e. V = I*R, per Ohm's law.

    But for a junction diode, the relation ship is exponential/logarithmic. A diode behaves per the equation Id = Is*exp((Vd/Vt) - 1), or Vd = Vt*ln((Id/Is) + 1). Vt = nkT/q, which any reference text can elaborate on.

    For other devices, like incandescent light bulbs, another non-linear equation is used.

    So in conclusion, the relation between I & V depends on the device and application.

    Claude
     
  8. Mar 21, 2010 #7

    rcgldr

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    Voltate is a potential, the potential energy per unit charge within a field, with respect to some reference point, or the difference in potential between two points.

    Current is a rate of charge flow. 1 amp is equivalent to 6.24150948×10^18 elemental charges per second. An electron has -1 elemental charge.

    Voltage and current only have a relationship when there is current flowing through some type of electrical component, and the relationship depends on that component.

    Note that voltage can exist without current.
     
  9. Mar 21, 2010 #8
    ...and current without voltage.
     
  10. Mar 25, 2010 #9
    Lsos - under what conditions do you consider that a current will flow without there being a potential difference accross the conductor i.e a Voltage
     
  11. Mar 25, 2010 #10
    I'll answer, if it's ok. The answer is - superconductor. But I w/o V, as well as V w/o I, only happens under static conditions.

    With dynamic (time varying) conditions, I & V cannot exist independently. They are mutually inclusive, co-dependent, simultaneous, inter-related, etc. One cannot exist w/o the other. This issue is very old, but yet it keeps rearing its head. Current and voltage co-exist, and that is what we know. But because real world power sources, like batteries and generators, are designed and built to operate in the constant voltage mode, many assume that voltage is somehow more basic than current. It is not. Power sources could, and have been constructed for constant current operation. It isn't done because losses are much higher.

    The OP asked about the I-V relation. That would depend on the device under examination. Some devices are linear, some are not. That was what I conveyed.

    Does this help?
     
  12. Mar 25, 2010 #11
    That's what I had in mind, a superconductor.

    But also...even if you have a normal wire, doesn't it take some finite time for the current to stop flowing once you remove the voltage?

    I'm not so sure how electricity works at this deeper level, but it makes sense to me that it would not be instantaneous, and would depend on the resistance...
     
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