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Current/Voltage in an AC Generator

  1. Apr 25, 2005 #1
    I'm slightly confused about something. In an AC generator at one point in the revolution it produces a positive voltage or current, and then in the other half of the revolution it produces a negative voltage or current....what i'm confused about is which is it producing negatively - the voltage or current?

    also extra question: if a negative voltage is applied to a load..the load (whatever it is..) wont work. Correct?
  2. jcsd
  3. Apr 25, 2005 #2


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    Positive or negative are simply assigned for the purposes of reference. In fact, at one point about a century ago there was a guess made and its wrong! So physics/math is largely based on an incorrect assumption and tech schools teach electron flow. About the only difference is a negative sign.

    A light bulb for example doesn't care which way the electricity flows. For AC its irrelevant (except for grounding and safety) which lead is typically hooked up because its going to switch 60 times a second.

    So its positive voltage AND current for the first part of the cycle and then negative voltage AND current for the second part of the cycle. They work hand-in-hand and are in-sync like in a DC circuit when applied across a resistive load but can shift with reactive loads that have capacitance and inductance.
  4. Apr 26, 2005 #3
    so what is it if its positive voltage and negative current? or positive current and negative voltage?
  5. Apr 26, 2005 #4
    That is called a phase shift. Current can lag or lead voltage by 90 in either direction. There is an old mnemonic: "ELI the ICE man" that is used to remind people how inductive loads and capacitive loads affect the voltage/current phases. Voltage(E) Leads in an inductor(L) current(I) and Current(I) leads in a capacitor(C) voltage(E). There's also one about bad girls and victory garden walls, but that's for another thread...

    Anywho, a purly inductive load will only have a 90 degree shift as will be a purely capacitive load(neither of which actually exists).

    Here, I'll get ya started: http://home.planet.nl/~heuve345/electronics/course/course.html

    Enjoy, and good luck.

    I would recommend looking into phasors(and the Diff Eq leading up to said mathematical constructs) to understand and determine how these shifts occure and what the results are.
  6. Apr 26, 2005 #5


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    Hmm, I've heard the bad boys mnemonic about resistor color codes but not the one about lead/lag. PM? :smile:

    infamous - with a load like a light bulb there is no way to have a positive for one and a negative for the other. If you have your hand on a chair and push it moves one way, and if you pull then it moves the other way. Assign one direction as "positive" and the other as "negative" and you have a very similar setup, you push or pull and the chair follows your direction.

    The voltage either pushes or pulls and the current follows the same direction as the voltage at the same time (outside of the phase shift mentioned above) so just think of the voltage as the boss and the current does what it tells it to do without ever listening to hear "simon says"
  7. Apr 27, 2005 #6
    ok. thanks for your help guys!
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