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Does the direction of current inverse in ac?

  1. May 18, 2014 #1
    I'm learning AC and theory says that the polarity of AC inverses.. even the name says 'alternating current'.. now what about the live and neutral? does the current goes from neutral to live and and vice versa?
    Some people says that only the phase inverses, but the current is always from live to neutral, and direction of current never inverses..
    Is all these true?? can someone help me understand all these?? and why do transformers has only one color for both wires? like, transmission transformers also are pretty much same, i mean same principle.. i'm so confused..
  2. jcsd
  3. May 18, 2014 #2
    Alternating current doesn't necessarily mean it is centered at 0. You could have an alternating signal with a DC offset so that it always stays positive.

    However I think you are talking about regular mains voltage centered on zero. And yes, it does alternate going forward to going backward. It is rather easy to think about. If you had a battery run circuit and switch the terminals every second, the current would going 'backwards' every other second right?
  4. May 18, 2014 #3


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    Yes, the voltage AND the current reverses at line frequency (60Hz in US). I have no idea what the phrase "only the phase inverses" means to "some people". Phase relative to what?

    Additionally, the relative phase of the voltage and current can be non-zero, giving rise to "power factor". That is, you don't always get maximum current at maximum voltage. Wikipedia is a fine place to read about power factor.
  5. May 18, 2014 #4
    also, something that obvious that helped me when I was first learning...

    Electricity is like heat or water. It will always flow from High to Low. If at some point in time, neutral is 0V and the line is -5V, then the neutral is High and it has to flow from neutral to the line.
  6. May 18, 2014 #5
    if the direction has reversed, it means that the live became neutral?
  7. May 18, 2014 #6


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    The neutral wire is always a live wire (a current carrying conductor). It's said to be neutral because it's normally attached to ground at some point in a grounded power system and/or is a reference point for other wires IRT voltage.
    Last edited: May 18, 2014
  8. May 18, 2014 #7


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    Neutral is called such due to the way it is attached to the transformer or group of transformers.
  9. May 18, 2014 #8
    i know that the polarity will inverse periodically, phase and current directions would change.. yes but the live is the hot wire, current will flow to neutral.. but when the polarity inverses, the direction of current has changed, that is from neutral to live.. but how is it possible that current now goes from neutral to live??
  10. May 18, 2014 #9
    if you touch the 'live' wire and nothing else, will you get zapped? no.
    if you touch the 'live' wire and the 'neutral' wire, will you get zapped? yes.

    How can the 'live' wire be live if you touch it and nothing happens? Because current needs a return path.

    the neutral wire carries the return current. Current has to return in order to form a circuit. But the direction we say its going in is fairly arbitrary as long we stay consistent with our notation. The electrons don't care what the wire is labeled. They flow through them regardless. If voltage is applied one way, they go that way. If voltage is reversed, the electrons turn around.

    Its called neutral because it is tied to ground. If you touch the ground and the neutral wire, nothing happens because there is no voltage difference. But it still carries current when it is connected to a circuit.
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