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Simple question re: Alternating Current

  1. Sep 28, 2009 #1
    Does a current need to change direction in order to be considered as an AC?

    What if the direction remains the same, but the amount of current which flows varies with time, as in a sine wave?

    Thanks
     
  2. jcsd
  3. Sep 28, 2009 #2

    mgb_phys

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    Regular AC current reverses direction because the voltage goes form positive to negative in a sign wave
    You could build a system where the voltage varied from 0 to a positive voltage and back again - so current was always in one direction.
    But this couldn't be a sin wave, it would be mod(sin) or sin^2

    Or you could add a DC offset to an AC signal so it was still a sin wave but went from 2*V to 0
     
    Last edited: Sep 28, 2009
  4. Sep 28, 2009 #3
    Yes, for a current to qualify as alternating (i.e. AC), it has to periodically change direction. In other words, the current is sometimes positive and sometimes negative relative to a defined positive direction at a point in the circuit.

    Normally the current alternates around a mean amplitude of 0A. However, you can move this mean amplitude up and down as you like by adding a DC offset. E.g. if you have a 2Apk-pk AC signal alternating around 0A and add a 1A DC offset, the current will remain in one direction, and you have a sinusoidally varying DC current.
     
  5. Sep 28, 2009 #4
    So would the laws of AC circuits apply in such a case.

    For example if in fact we do move the mean amplitude up so that we have a sinusoidally varying DC current and our circuit comprises of a simple capacitor, would the current in the capacitor lead the voltage?
     
  6. Sep 28, 2009 #5

    Born2bwire

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    Circuits follow linear superposition. So whatever components that you can break up the excitations into, you can solve for them individually and then add up the results to get the full answer (This does not always include nonlinear circuit elements like diodes and transistors since, for example, the individual voltage components may not be large enough for turn on voltages but together they could be. However, if we know what state they are operating in then we can properly apply superposition.). So if we can decompose our voltage signal into an AC signal with a DC offset, we could solve for the two signals independently and add them together. So there will still be a lagging/leading property to the signals due to capacitive and inductive elements.
     
  7. Sep 29, 2009 #6
    In case anyone's interested -- that's called pulsating DC.
     
  8. Sep 29, 2009 #7
    That is fantastic. Thank you all very much.
     
  9. Oct 2, 2009 #8
    pulsing current is just pulsing current. AC by definition means it alternates.
     
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