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I Potential difference in an alternator

  1. Dec 26, 2016 #1

    In a current generated by an alternator, can we really say that there is a potential difference like in a battery?

    The magnetic field exerts a force on the electrons which makes them move. We can calculate the work done to the electrons in joules, and we can divide this energy by the amount of charge in coulomb which gives a voltage (J/C) but this energy calculated gives the kinetic energy of the electrons, not their potential energy...
  2. jcsd
  3. Dec 26, 2016 #2


    Staff: Mentor

    No, the energy you described is most definitely not the KE of the electrons. In an ordinary circuit the KE of the electrons is negligible.
  4. Dec 26, 2016 #3


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    Staff: Mentor

    Yes, there is a potential difference across the outputs of an alternator. Unlike a battery output, the potential difference varies rapidlywith time; how much variation depends on the details of the rectifiers and voltage regulator circuitry as well as the speed that we're spinning the alternator.

    And as Dale says, the kinetic energy of the electrons in a circuit is pretty much completely unrelated to the potential; this is true for a battery as well as for an alternator.
  5. Dec 26, 2016 #4
    But the magnetic field makes the electrons MOVE... how is it not kinetic energy? What is this energy?

    If it is potential energy, could you help me conceptualize how it is potential energy?
  6. Dec 26, 2016 #5


    Staff: Mentor

    The electrons do move and therefore do have KE, but they move very slowly and weigh very little so the amount of KE is utterly insignificant in ordinary circuits. You can do the calculation or find them done somewhere on PF.

    The fields themselves have an associated energy density. It is proportional to the square of the field, so if you double the field then you quadruple the energy density. The fields can also transfer energy to other locations or do work on matter. It is called Poyntings theorem.
  7. Dec 28, 2016 #6


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    Science Advisor

    Yes, the electrons are moved by the magnetic field, or should that be swept or lifted by the magnetic field, from a conductor of low potential to a conductor of higher potential. The electrons then have a higher potential energy, due to their change in electric potential or voltage.
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