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Current leading voltage or vice versa concept

  1. May 20, 2013 #1
    Hello,

    I was wondering if there is a conceptual explanation for when current leads voltage or vice versa for capacitors or inductors with AC voltages, or is it just the way the math pans out?
     
  2. jcsd
  3. May 21, 2013 #2

    tiny-tim

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    Hi Woopydalan! :smile:

    In a resistor, voltage is proportional to current, so there's no lead or follow.

    In an inductor voltage is proportional to rate of change of current, so when the current is at phase zero, the voltage must be maximum, at phase 90°, ie leading.

    In a capacitor, voltage is proportional to charge, so rate of change of voltage is proportional to current, so it's the other way round, ie current leading. :wink:
     
  4. May 22, 2013 #3
    This is a difficult idea to show in the early stages of learning AC theory. When I teach this I use a battery with an ammeter and connect this to an inductor. When the switch is closed it is clear that it takes some time for the current to rise to its steady value after the voltage is connected. In some way it seems reasonable to state that 'the current is behind the voltage'.
    Do the same with a capacitor and a battery and when the switch is closed the current is at a maximum and you have to wait for the voltage to rise to its steady value. So in some way the voltage is 'behind' the current.
    Not rigorous....but I find it a useful, non mathematical aid. ( some of my physics students do not do maths)
     
  5. May 22, 2013 #4
    I'm not sure what specific example you're referring to, but a voltage is a potential difference, and voltage per unit length of wire is a measure of the electric field strength, which is what causes the electrons to move and overcome resistance in the wires. For that reason I always think of voltage as the cause and current as the effect.
     
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