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What is a volt ?

  1. Jul 6, 2004 #1
    what is a "volt"?

    hi to all friends.

    what is a "volt"?

    can you pl explain the meaning of that word and also "watt" (i know it is used to mesuare the amount of energy but that is about all)
     
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  3. Jul 6, 2004 #2

    chroot

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    There are many ways to define a volt:

    A volt is the potential difference that will cause 1 ampere of current to flow through a 1 ohm resistance.

    A volt is the potential difference across a conducting material which 1 ampere of current dissipates 1 watt of power.

    The potential 1.43 nanometers from a proton is 1 volt.

    - Warren
     
  4. Jul 6, 2004 #3

    chroot

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    A watt is a power dissipation of one joule per second. It's also one volt-ampere.

    - Warren
     
  5. Jul 6, 2004 #4
    A volt is a measure of potential between two points. A one-coulomb charge will undergo a net change of one joule of energy if it moves from one point to the other.
     
  6. Jul 6, 2004 #5
    i now understan watt but not volt really.

    can you please put it in more simple way. i don't really understand what an ampeer is.
     
  7. Jul 6, 2004 #6

    chroot

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    Every electron carries a specific charge. A coulomb of charge is the total amount of charge carried by [itex]6.25 \cdot 10^{18}[/itex] electrons. An ampere is this number of electrons flowing past a given point in a wire every second.

    - Warren
     
  8. Jul 6, 2004 #7
    so! ampere is like bandwidth

    and

    volt is the amount of energy!
     
  9. Jul 6, 2004 #8

    russ_watters

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    No. Volts are an electrical force and coulombs are like mass. So, similar to Newtonian physics, power is force times distance per unit time - or volts times amps. Energy is power (volts times amps) times time.
     
  10. Jul 7, 2004 #9
    No, volts are not forces.

    What exactly is hard to understand using the definition I posted? Seems clear to me. I hope it's clear to most everyone else.
     
  11. Jul 7, 2004 #10

    robphy

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    I believe the correct analogy is that "charge [displaced]" (measured in Coulombs) is analogous to a [generalized]-displacement... not mass.
    Then, "voltage" [akin to an electromotive-force] (measured in Joules/Coulomb) is analogous to a generalized-force, in the Lagrangian spirit. "Current" is analogous to a generalized-velocity. By the way, "Inductance" is analogous to mass.

    For the beginning student, this is certainly too advanced to be properly appreciated unless the analogies are carefully presented in an appropriate example.
     
  12. Jul 7, 2004 #11
    im in year 11 and no background in Electricity what so ever. so can you plaese explain from start?
     
  13. Jul 11, 2004 #12

    Does this mean that 1 ampere will allow [itex]6.25 \cdot 10^{18}[/itex] electronsto pass in a second at any given point?

    if that is the case will a 2 ampere wire alow twice [itex]6.25 \cdot 10^{18}[/itex] electronsto pass in a second at any given point ([itex]12.5\cdot 10^{36}[/itex])
     
    Last edited by a moderator: Jul 11, 2004
  14. Jul 11, 2004 #13

    chroot

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    Yes, bayan, that's correct. A current of one ampere is defined as 6.25 * 1018 electrons per second passing a given point in the wire.

    I should note that there is no such thing as a "2 ampere wire" -- wires will carry however much current is dictated by the potential applied across them and their inherent resistivity -- at least until they melt.

    - Warren
     
  15. Jul 12, 2004 #14
    i have another question.

    1 ohm resistance is potential difference of one "VOLT" produces a current of one "ampere".

    does this mean that a 8 ohm resistance is the potential diffrence of 8 "VOLTs" produces a current of one "ampere".

    or

    does it mean that a 8 ohm resistance is the potential diffrence of 8 "VOLTs" produces a current of 8 "ampere". (although i think this is wrong)
     
  16. Jul 12, 2004 #15

    selfAdjoint

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    The formula is Ohm's law, E = IR, where E is in volts, I is in amperes and R is in ohms. So you can work out the answers to your questions from that.
     
  17. Jul 12, 2004 #16

    krab

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    Looks to me like you do not understand exponential notation. twice [itex]6.25 \cdot 10^{18}[/itex] is [itex]12.5\cdot 10^{18}[/itex], not [itex]12.5\cdot 10^{36}[/itex].
    This is vital, so study it first.
     
  18. Jul 13, 2004 #17
    thanx to all how answered. i think that i understand it now
     
  19. Jul 13, 2004 #18

    Janus

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    One stipulation: a watt equals a volt-ampere in DC circuits or purely resistive AC circuits, but not in AC circuits in which the load is at least partially reactive (contains inductors or capacitors.

    In the last case, the volt-ampere is a measure of "apparent power". To get "true" power (watts) you have to multiply the apparent power by the 'power factor' which is equal to the cosine of the angle of the phase shift between voltage and current due to the impedance (the combined effect of resistance and reactance) of the circuit.
     
  20. Jul 13, 2004 #19

    chroot

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    Yes, yes, Janus, but you need to crawl before you can compete in triathlons.

    - Warren
     
  21. Jul 13, 2004 #20

    DL

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    And, on that note, I'd like to ask a question I've been wondering about for a while.

    In my grade 9 Science class, the textbook/teacher explained that a "volt" was a measure of how much power each electron was carrying. So a 9-volt battery simply meant that each individual electron coming out of that battery would have 9 "volts" of energy.

    I'd been reading quite a bit ahead of myself in physics, and noticed that a unit being tossed around a lot was the "electron volt." So I asked my father, who has a Ph.D in Chemistry, what the difference was between an electron volt and a volt.

    He said that an electron volt was the amount of energy each individual electron carried in a current, while a volt was a measure of the total amount of energy in a current. So my school textbook claims that a volt is a measure of energy per electron, and my father claims that a volt is a measure of total energy (adding all the electrons together).

    So who's correct? My father is pretty out of the loop as far as chemistry goes, so he could well be wrong, but I wouldn't expect him to be on a seemingly basic issue like this.
     
  22. Jul 13, 2004 #21
    Voltage is a measure of the electical potential between two points. If there is one volt between two points, and one electron moves from one point to the other, it gains or loses one electron volt of kinetic energy.
     
  23. Jul 14, 2004 #22
    RE: "In my grade 9 Science class, the textbook/teacher explained that a "volt" was a measure of how much power each electron was carrying."

    Ugh. Not even close.

    Objects do not possess voltage, because the volt is a property of SPACE.

    Think of the term "elevation." An elevation of 1 meter above the ground exists regardless of whether or not an object is located at this elevation. Elevation is a property of space. (No general relativity, please.)

    Voltage is the potential difference between two points in space. It tells you how much potential energy would be gained or lost if a charged particle moved from the one point in space to the other.

    The particle itself does not possess the voltage.

    So if two points in space have a voltage of 1 volt, then a 1-coulomb charge would gain or lose 1 joule of potential energy if it moved from one point to the other (depending on which point had the higher potential).

    Any definition of voltage that assigns the term to the particle is incorrect.

    RE: "Voltage is a measure of the electical potential between two points. If there is one volt between two points, and one electron moves from one point to the other, it gains or loses one electron volt of kinetic energy."

    Potential difference is directly related to the potential energy, not kinetic energy. Otherwise the electrons in a circuit would not move at a constant drift velocity, but would accelerate uniformly, as if the circuit had no resistance.
     
  24. Jul 14, 2004 #23

    DL

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    So if I'm understanding this... a volt is a measure of the DIFFERENCE in energy between one point and another (the difference between the "altitudes" of two points, to use that analogy). And an electron volt is a measure of the amount of energy an electron has (how "high" the electron is).

    Is that remotely close?
     
  25. Jul 14, 2004 #24
    ----

    If I can read my physics book correctly the definition of a volt by units is
    volts per meter = force per unit charge where force has the normal meaning causing accelleration , but charge has to be defined in some way ( now I guess by the electron) so I'm leaving out any scaling constant .
    If you rearrange that it says volt = f.L / q now force x distance = work
    so I guess it's work per unit charge or energy per unit charge.
    So whoever said it was force does not seem correct.
    It is of no importance what type of energy this is ,potental or kinetic the charge either accellerated or was pushed against the potential it's equivalent.
     
    Last edited: Jul 14, 2004
  26. Jul 14, 2004 #25
    RE: "So if I'm understanding this... a volt is a measure of the DIFFERENCE in energy between one point and another (the difference between the "altitudes" of two points, to use that analogy). "

    No, points in space do not have energy, and voltage is not energy. (At least, not in a classical sense.) Instead, a volt indicates how much difference in potential energy a particle would have if it moved between the two points.

    Here is an analogy that might help.

    Think of a coke can that has a label of 16 fluid ounces. The can itself does not have 16 fluid ounces, and mass is not the same as volume. Instead, the label signifies the mass of coke that would be in the can IF THE CAN WAS FILLED.

    Here, fluid ounces is analagous to voltage, and mas is analagous to energy.

    While true, this is not a helpful definition.
     
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