Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does AC form a closed loop circuit?

  1. May 18, 2016 #1
    Some say that AC should form a closed loop circuit for electricity utilization but is there really an loop from the Power station(hot wire) to the Ground(neutral) and then from Ground to power station so that it forms a loop?
  2. jcsd
  3. May 18, 2016 #2


    User Avatar

    Staff: Mentor

    Power from a power station to your home does indeed form a loop, but for most of the way the power is three phase, which means it uses 3 wires instead of 2:

    Essentially, instead of having the electricity flow out one wire and in the other (like at your house), with no phase difference, there are three wires that oscillate "in" and "out" between them with a phase difference of 120 degrees.
  4. May 18, 2016 #3


    Staff: Mentor

    Yes there is a closed loop. No it does not go through the ground.

    But power transmission is three phase, not single phase like in your house. In three phase circuits, no return line is needed.
    How is that trick done? Look at the plot below. At every point in time, the three phases add up to zero, so there is nothing to return. Pretty clever trick don't you think? The advantage of no return is why they use three phase.

  5. May 18, 2016 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It isn't just one simple closed loop. There are three wires involved (ignore the ground / Earth conductor, which shouldn't be involved) and, at any time, the current flowing along any one of the three will balanced by the current flowing in the other two. Alternatively, you can say that the Sum of the currents in the three phases is Zero. Look at the above diagram and take one point on one of the sinusoids (i.e. one time). The other two voltages at that time will add up to that value. Take the simple example of when one is at a peak of +1, the other two will be at -0.5. It works anywhere else too. (Just supplementing the comments already in that post)
    That sounds needlessly complicated but the has some massive advantages with regard to the amount of copper (or other metals) needed to transfer a given amount of Power. Also, you can connect the three phases to a suitable Three Phase Induction Motor and you get a rotation with no commutator or lossy capacitors.
  6. May 19, 2016 #5
    To be honest I couldn't get anyone's replies since I'm totally weak in this subject.I would just like to know in layman terms what really happens after the voltage is produced from the generator.I only know that there is three lines in a socket and one is live and one is neutral and one is ground.Does the induced voltage produced from the generator goes to the live wire?Also after switching at the generator the voltage is reversed so do we feel that the voltage comes from ground and reaches the generator even though actually the voltage comes from generator itself since the voltage is negative.Could anyone help me.
  7. May 19, 2016 #6
    Let me blow your mind - in your breaker box you will find that the neutral wire is actually connected to the ground wire. And that ground wire is connected to a large pole driven into the ground somewhere in your yard.

    voltage always comes from the generator. negative voltage just means electrons are going to move in opposite direction. The generator can push electrons or pull electrons.
    Last edited: May 19, 2016
  8. May 19, 2016 #7
    Yeah that's true.Could you help me with how the charges flow from the generator to the electrical appliance and where the charges go after passing through the electrical appliance?
  9. May 19, 2016 #8
    in a generator, the voltage will be created across two poles. + and -. The electrons will flow from the - to the positive side. With AC the poles are flipping (in a way), so the electrons just "vibrate back and forth" you could say. In DC they would flow to the + side... in a battery for instance, the + ions would bond with the electrons, until it ran out of electrons on the one side so to speak, and your battery is dead.
  10. May 19, 2016 #9
    I could get that.Could you tell me more precisely how the electrons flow from generator to live wire and where does it(electrons) goes after it passes through the live wire?
  11. May 19, 2016 #10


    User Avatar
    Science Advisor

    The simple explanation for beginners is that the charge in the conductor is like a railway train that stretches all around the circuit, to form a complete loop. With DC the train of charges travels continuously in one direction. With AC the train of charges moves backwards and forwards at the frequency of the AC supply. The energy that moves the train of charges is applied at the generator, then removed at the load appliance.
  12. May 19, 2016 #11
    So if you have a magnet moving near a conductor (usually gas engine turns a rotor with coils in a magnetic field in a generator), the changing magnetic flux will push the electrons around on that conductor. Those in turn push the neighboring electrons and so forth, and that is how the voltage gets from the generator to the wire in your outlet. The pushing force wants to push electrons through things like lightbulb filaments, which resist the electrons. That resistance and current combination is the power. After passing the resistor in the circuit, the electron is free to flow back around the neutral wire to the other pole. (but in AC, it's just vibrating)
  13. May 19, 2016 #12
    Could you tell from where and to the "charges moves backward and forward"?
  14. May 19, 2016 #13
    Yeah this is where I'm stuck.Could you explain a little bit more about how the electrons vibrate in AC?
  15. May 19, 2016 #14
    Sure, in AC for one moment the voltage is positive and the electrons flow forward. The next moment the voltage is negative and the electrons flow backward. This happens very fast so the electrons don't actually move very far.I should say they want to push forward and back, not flow... The more flow means more current in the circuit.
  16. May 19, 2016 #15
    You're on my track.Does the electron really move backward or do we feel that electrons move backward?I'm asking this since the voltage becomes negative it's sure that the pull of electrons comes from the ground.So does the electron really flow from ground or whether the electrons that flowed forward to the ground comes back?
  17. May 19, 2016 #16
    In an AC circuit with a generator, The pushing and pulling is done through a coil of wire, so nothing is connected to the real ground (earth) per se. They are pushed and pulled along a single loop of wire which makes the circuit. However electrons can flow to and from the real ground (earth), which is what lightning strikes are
  18. May 19, 2016 #17
    Does the same electrons that are pushed through the live wire are pulled from the ground?
    Oh I forgot about the ground concept you've mentioned.I actually meant to say about 'neutral' wire.Sorry for that.
  19. May 19, 2016 #18
    Electrons are not pulled from the ground for typical AC circuits. It is possible to do this, but I think the answer to your question is electrons are not pulled from the ground.
  20. May 19, 2016 #19
  21. May 19, 2016 #20
    Yes, the same electrons get pushed and pulled back and forth. There are no other electrons available because it is a closed and isolated loop.
  22. May 19, 2016 #21
    Now I could get how charges in AC flows.I don't think asking this is a part of this question but could you tell how the current gets reversed in alternating current.Is this done by the commutator or is it done by the induced emf by Len'z law even though I couldn't understand this law till now.
  23. May 19, 2016 #22
    commutators are used to reverse current in the generator to keep generating DC (iirc), induced emf is what is going on with the rotating loop in the magnetic field. The changing magnetic flux causes positive and negative voltage which gives rise to AC.
  24. May 19, 2016 #23
    Oh so does that mean we need a battery to get an DC from an AC so that the current always get reversed?Also doesn't the current gets reversed in AC?I still couldn't get why we reverse the current again in AC to get DC?Could you help me.
  25. May 19, 2016 #24
    So the commutator works by reversing the AC current when it goes negative/backward. Thus the circuit always is positive/forward (DC). no you don't need a battery.
  26. May 19, 2016 #25
    I tried asking many sites of how a commutator reverses the direction of current but everyone said "it's hard to explain using words".Could you help me with this.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted