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Voltage/Battery Terminal confusion

  1. Jul 22, 2008 #1
    1.) Why can't you measure the potential difference across two batteries terminals, lets say a positive terminal from battery A and a negative terminal from battery B?
    Theoredically there is still a potential difference....therefore voltage.....right????
     
  2. jcsd
  3. Jul 23, 2008 #2
    You can... unless you are talking about two independent batteries (not wired together). If that is the case, then you have an incomplete circuit, thus no current flow.
     
  4. Jul 23, 2008 #3

    stewartcs

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    Because they are not connected as a circuit.

    No there wouldn't be. From an Ohm's Law point of view, since they are not connected as a circuit, no current can flow and no potential difference will exist.

    CS
     
  5. Jul 23, 2008 #4
    So there needs to be current induced by the voltage you are measuring????

    I was under the impression that EVERYTHING had electrical potential/voltage.
    Now this leads me to believe that a Voltmeter will NOT in fact read voltage, but moreover voltage drop??????
     
  6. Jul 23, 2008 #5
    It simply depends on the input impedance of the voltmeter. REAL WORLD voltmeters do not have infinite impedance so they require a current to flow in order to work.
     
  7. Jul 23, 2008 #6

    mgb_phys

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    They measure voltage DIFFERENCE. It's meaningless to say something has a voltage without saying what it has a voltage relative to.
    For batteries it is relative to the other terminal so your meter has to connect between the two terminals to measure the voltage between them.
     
  8. Jul 23, 2008 #7
    I understand all of this, but I was asking from more of a scientific view point.....
    Although I think this answers my question PERFECTLY well.

    To sum it up, its just that our typical modern day conventional Voltmeters cannot measure Voltage alone, but rather a "voltage drop" (the difference across a conductor) in reference to a circuit but dependent of the current induced in it..... Or as supernova suggested Impedence.
     
  9. Jul 24, 2008 #8
    If you want a voltmeter that takes no current (direct current, not alternating) once it's charged, then the electrostatic voltmeter is your friend.

    Don't expect to measure much below a kV or so though.
     
  10. Jul 24, 2008 #9

    chroot

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    The two batteries' terminals can have completely different voltages, measured with reference to some third object. For example, you could charge up one 9V battery's metal case with a Van de Graff (static electricity) generator so that its negative terminal was at 1000V with respect to the Earth. The battery's chemical potential will then cause its positive terminal to always be 9V greater than its negative. The positive terminal will therefore be at 1009V, with respect to the Earth.

    The issue arises with voltmeters, which, as has been said, are not perfect, and do not have infinite impedance. As soon as you connect the voltmeter to the two batteries, a very small current will flow, equalizing the charge on the batteries until the meter reads zero. This happens in the blink of an eye for any normal voltages, and appears to not happen at all.

    The issue of voltage being only a relative quantity is extremely important, but entirely separate from this mechanism.

    - Warren
     
    Last edited: Jul 24, 2008
  11. Jul 23, 2010 #10

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  12. Jul 23, 2010 #11

    sophiecentaur

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    Two batteries - or two lumps of metal, for that matter, can be at different potentials. A meter with a high enough input resistance can measure this but the act of measurement is likely to bring their two potentials together because some finite current will flow through the measuring device.
    The thing about a battery is that it has a source of energy which will keep the potential difference across its terminals even when you measure it or take current inot a load.
     
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