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How does alternating current transfer energy?

  1. Sep 8, 2012 #1
    The specific problem I'd like to know is let's say you have Mr electron (E) in a the wire, denoted by "---"

    Lets assume 0.5hz AC for now. Below is a snapshot in time of the electron in the wire.

    Code (Text):
    1s ------E-------
    2s -----------E--
    Lets assume "-----" is the physical distance the electron (E) traverses along the wire for each half cycle.

    I realize it's probably an over simplification but if that is the general gist of things, my question is, obviously energy was required to move the electron towards the right. That in turn probably pushes another electron to the right as well.

    All good so far, but then when the electron comes "back", surely it needs to pull kinetic energy backwards down the wire?

    Hence the assumption is the total energy transferred is zero. Conservation of momentum?

    Obviously this is not correct, so wondering if someone can explain the flaw :p
  2. jcsd
  3. Sep 8, 2012 #2
    Electrons cannot traverse a wire without a source and a load. Your model is incomplete.
  4. Sep 8, 2012 #3


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    A saw transfers energy in both the push and pull strokes too...
  5. Sep 8, 2012 #4
    Something that i didnt figure out soon enough in my studies is that electrons have a drift velocity that is actually very slow (fractions of a mm per sec). Its only the energy of one moving electron that repels the one in front of it to move and so on. Just like at train. The mechanical energy used to rotated the conductor in the generator is then turning that energy into electrical in the form of moving electrons. So the energy is the moving electrons, doesn't matter which direction they are moving.
  6. Sep 8, 2012 #5
    And the generator is constantly spinning so it is not using the same energy to "push" the electrons as it is to "pull" them.
  7. Sep 9, 2012 #6
    The energy is not in the moving electrons, The mass and the speed of the electrons is so small, that the energy to get them moving is completely unimportant compared to the energy needed to build up the magnetic field because of the current in the wire.
  8. Sep 9, 2012 #7
    http://i.imm.io/DKrJ.jpeg [Broken]

    Still confused :/
    Last edited by a moderator: May 6, 2017
  9. Sep 9, 2012 #8
    OP, you have to ask a clear and definite question or no one will be able to help.

    Based on what you have said, here's my two cents-

    AC current, or any current for that matter, is produced from a potential difference (voltage source). Now, the voltage source is the thing that is supplying energy and is where your "transfer of energy" takes place. The battery supplies a potential difference forcing the electrons to move in one way, and then the battery switches its polarity and cause the electrons to move the other way. In both situations the battery supplies energy to the electrons to traverse through the wire.

    Now, I think you are asking how does AC current itself transfer energy outside of the system. The answer to this is that an AC current is usually put inside of a magnetic field, perpendicular to the field. We can imagine the circuit as being a loop of wire. The loop of wire then experiences a force due to the external magnetic field because of the moving electrons. This causes the entire loop to rotate inside. When the loop rotates so that it is parallel to the field, the currents direction must reverse in order for it to experience the force/torque in the same direction. This keeps the loop spinning in the same direction.
    Check out this post that gives a good explanation of this phenomena - http://answers.yahoo.com/question/index?qid=20080512175617AAnwZeG
    This spinning motion is what is used as energy, and is where the external transfer of energy may take place.
  10. Sep 9, 2012 #9


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    Imagine you have an under shot water wheel.. It doesn't matter which way the water is flowing the wheel will still turn and grind corn. The water looses energy whichever direction it's going.

    If you think about it in a DC circuit the electrons on one side of the load travel towards the load and on the other side they travel away from the load. In an AC circuit the direction alternates. In both directions there is a voltage drop across the load.
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