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Voltage questions

  1. Apr 12, 2015 #1
    I have a few questions about voltage. Is it always measured between two points, or can it be measured between two objects? What gives an object more electrical potential? Is it just free electrons? Can there be a voltage between two neutrally charged objects? Why does voltage drop occur through resistors? I've heard people say that it was because energy was being used, but if voltage is difference in electrical potential, then when current starts to flow I would thinks some of that would be lost to kinetic energy of the electrons.
     
  2. jcsd
  3. Apr 12, 2015 #2
    That's a lot of questions and I think I can help you a little....

    Yes, voltage is always measured between two points. Another way of thinking of voltage is like the potential for potential energy. A charged object generates a field of electric force whether a second charged object is there or not. If we assume the first object is a positive charge then moving towards it in the field, is an increase in potential. Much like when an object in a gravity field moves upwards and increases its potential energy. They both can the potential to do something with that energy. Two objects can have different potentials and therefore the difference in voltage becomes important. Charge is what drives voltage, ultimately and therefore a neutrally charged object can not generate a voltage per se.
     
  4. Apr 13, 2015 #3

    sophiecentaur

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    The extra KE of electrons, drifting through a conductor is effectively zero. The drift velocity is in the order of mm/s and their mass is minute (1/1800 of every proton and neutron in the atom). So please disregard the KE aspect of power transfer in electric circuits; it's not a good intuitive approach.
    The only time that Electron KE becomes relevant is in vacuum tubes, where each electron is accelerated to high speeds (sometimes, not far short of c) and they impact against an anode or collector, dissipating the kinetic energy all in one go. In those cases, there is no interaction with the vacuum they are flying through.
    The Potential Difference (Voltage) is the Energy (Joules) per Unit Charge (Coulomb) and the energy transferred to a hot wire will be due to the interaction of the flowing charge and the atoms (the whole lattice and not just individual atoms). This energy loss, through a resistance is measurable as a Voltage drop across it. A Volt Meter will measure the Potential Difference by measuring the current that flows through a very high resistance, connect across the terminals. (Many voltmeters, these days, use electronics to achieve the same thing - as in DMMs - but it usually boils down to the same thing).
     
  5. Apr 13, 2015 #4
    How exactly is voltage measured between two points? How can a point have more electrical potential energy than another?
     
  6. Apr 13, 2015 #5

    sophiecentaur

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    Measurement will always involve connecting a measuring device to both the points (/objects). A voltage can be measured by finding how much current is made to pass through a known resistance. (Very small current and very high resistance in a good conventional voltmeter) or be the effect of the voltage on the input of an electronic amplifier (as in a DMM). If you have loads of current available at the points to be measured, you can also find what the PD is by connecting a known resistance and then measure the heat output as the resistor gets hot.
    If there is an imbalance of charge then there will be a field between the two points, for instance. Work can be done as charges flow to balance the charges - which means there is a Potential Difference between the points (or objects).
    You can cause a Potential Difference between two parts of a circuit by chemical means (electrochemical action can produce excess positive and negative charge on each terminal of a battery and maintain that PD as current is drawn from a battery) or by induction as a conductor is moved through a magnetic field (as with a generator) or an alternating PD can be generated as a result of an electromagnetic wave passing near (or around) an antenna. (Your question is an extremely open one and there is no limit to how long the answer could be . . . . . . . .)
     
  7. Apr 13, 2015 #6

    jim hardy

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    What is your concept of "electric potential energy" ?
     
  8. Apr 13, 2015 #7

    meBigGuy

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    All points have some amount of charge (including 0) relative to other points. It's as simple as that. For a battery it is the positive terminal relative to the negative terminal. For a human body it is the forefinger relative to the doorknob (or your friend's ear).

    Work is done creating the charge difference.
     
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