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What does it mean to have a negative change in current?

  1. Nov 25, 2012 #1
    Does a di/dt<0 mean an increasing current moving from a lower potential to a higher potential (if we define the direction of current to be the flow of the positive charges)? Similar question with negative current i.e.; dq/dt<0.
    Last edited: Nov 25, 2012
  2. jcsd
  3. Nov 25, 2012 #2
    Good morning mathsciguy.

    You do realise that di/dt represents the slope or tangent on a graph?

    It may help to talk of negative going currents (negative di/dt) and positive going currents (positive di/dt) since there may also be negative currents (against the chosen circuit direction) and positive currents (with the chosen circuit direction).

    So the slope of a negative (or positive) current may itself be negative or positive.

    Of course the current may also be zero, especially if alternating.
  4. Nov 25, 2012 #3
    Yes I do of course, it's also the very reason that I'm having conflicting ideas. In the context of calculus (at least from how I knew it), a negative dq/dt would mean that there are decreasing charges as time goes but when the textbook says negative dq/dt also means charges moving in the opposite direction (whatever that direction maybe?) then things get a little perplexing for me. What does this time derivative actually mean? I'm sorry this might sound like a silly question.
  5. Nov 25, 2012 #4
    Think of a sine graph of current v time for alternating current.

    For half of each cycle i is positive
    For the other half i is negative ie going the other way.

    For both positive and negative halves

    Half of the time di/dt is positive and half the time di/dt is negative.
  6. Nov 25, 2012 #5
    So, at the negative part of the wave where the slope is negative, is that where the current is increasing and but goes the opposite direction (intuitively, becoming more and more negative)?

    That means there are different ways to interpret a di/dt or dq/dt that is negative.
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