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How does current flow in AC?

  1. May 21, 2013 #1
    Just to be be clear about what Im asking, I would like to ask a few different questions 1st:

    Are these assumptions correct:

    1) In DC, current flows through a circuit due to the movement or flow of electrons. So in a DC circuit any one electron will do a complete lap ( or circuit ) of the circuit. ( The negative charge [or electron] moves in one direction whilst the positive charge [hole] moves in the other direction, giving rise to current flow [or charge flow])

    2) In AC, there is no net displacement of charge.

    3) The movement of electric charge in AC periodically changes direction (back & forth) along the line of flow. The AC source is creating an oscillation in the position of the electrons over time. Or in other words, the electrons simply vibrate on the spot and the mean position of charge carriers (electrons) is same.

    4) Current is the charge passing through a cross-sectional area per second taken in the conductor, and it is not affected by the mean position of electrons (which may remain same)


    If (1) is correct then it seems simple enough to understand [especially if you use a water analogy and have current = litres / second]. So I think Im ok on the understanding of DC!

    If (2), (3) and (4) are correct, then, as the electrons vibrate on the spot, they are still moving through some cross-sectional area per second and thus current flows.

    My question is:

    If in AC the electrons are oscillating, and as a result, the the sign is continuously changing back and forth, then how does the current actually flow [ say from left to right, through a cable]?

    Why doesnt the current constantly change direction along with the movement of charge?

    I hope I've made my question clear!
    Last edited by a moderator: May 21, 2013
  2. jcsd
  3. May 21, 2013 #2


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    Gold Member

    The current DOES change direction. Why do you think it is called ALTERNATING current?
  4. May 21, 2013 #3


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    Staff: Mentor

    Most of what you posted is correct. However, in conductors, where electrons move pretty freely in the conduction band, we do not talk about "holes" moving in the opposite direction of the electrons in the conduction band. We just keep in our heads that the "current" is due to electrons in the conduction band, and that their movement is in the opposite direction of what we call the "positive current direction". For a discussion of "holes", you need to look at conduction in semiconductors:


    And to answer your final question, the current does indeed vary back and forth with the AC voltage applied to the conductor.
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