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The Volt

  1. Dec 19, 2007 #1
    Wikipedia defines the volt as "The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power."

    Doesn't resistance always determine the dissipation of power? If you were to have a superconductor with no resistance then the current would flow but no energy would be dissipated. Does that mean theres no voltage even one terminal is charged differently to the other?

    I don't understand how dissipation of energy comes into the definition of the volt seeing as the dissipation of energy would vary depending on the resistance of the conductor while the potential difference between the two points would remain the same.

    If I had a circuit with a 7 volt battery. The resistance of the circuit is 1 ohm. There is a current of 7 amps flowing throughout the circuit which is dissipating 7 watts. Lets say I raise the resistance of the circuit to 2 ohms so there is a current of 3.5 amps flowing. Would this circuit still dissipate 7 amps due to the increased resistance?

    Anyhow in this case I changed the amount of amps but the voltage remained the same. If a volt is defined as the potential difference when 1 amp dissipates 1 watt how can the voltage remain the same when I alter the current due to resistance?
    Last edited: Dec 19, 2007
  2. jcsd
  3. Dec 19, 2007 #2
    Ok firstly, in the series circuit you describe we would have
    P=IV=7A*7V = 49Watts of power being dissipated

    If you double the resistance, and the voltage remains at 7V, then sure, there would be 3.5A of current.
    This of course means that less power is being dissipated,

    The power loss increases as the square of the current (Power can be re-written as P = I^2R) - which is why we have transmission lines in the hundreds of kV :smile:

    From the wikipedia article on 'Volt':
    The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.

    This is just basically saying Ohm's law
    In case one above, we had 7Amps dissipating into 49Watts.
    If we want one 'Wikipedia Volt' to be when one Amp dissipates to one Watt, we'd need 7 of these Volts to dissipate 7 Amps into 49 Watts
  4. Dec 19, 2007 #3
    de volt is simular to de pressure. I like to keep it simple.
    Last edited: Dec 19, 2007
  5. Dec 19, 2007 #4


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  6. Dec 19, 2007 #5


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    Using power = voltage * current (assuming DC), is one way of looking at it.

    1 Volt = 1 J/C (joule/coulomb), i.e. a potential difference of 1 V would induce a change in energy of 1 Joule on 1 Coulomb of charge.

    1 eV (electron volt) indicates the energy (1 eV or 1.602E-19 J) that a proton or electron would gain from passing across a potential of 1 volt.

    And by all means, check out the Hyperphysics reference cited by stewartcs.
  7. Dec 20, 2007 #6
    You know what i think. I think it is partly right that you are imagining the box and a person. Take a look at this: the resistance of air. When the engine is fired up, inside the engine will become a high pressure environment, whereareas the outside is a lower pressure environment than the inside of the engine. Therefore, it creates the opposite forces, and thats why when you see the afterburner of an aircraft is being fire, there are always a "skip" fire before it is fully being called "an afterburner". However, the thought of the fire pushes the inside of the engine is not wrong either. It is very logic that the fire burns every bit of particles of Oxygen just to make a leverage to be pushed away.
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