Does AC form a closed loop circuit?

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?

russ_watters
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:
https://en.wikipedia.org/wiki/Three-phase_electric_power

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.

anorlunda
Staff Emeritus
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.

sophiecentaur and russ_watters
sophiecentaur
Gold Member
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.

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.

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.

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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?

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.

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.
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?

Baluncore
2021 Award
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.

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)

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.
Could you tell from where and to the "charges moves backward and forward"?

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)
Yeah this is where I'm stuck.Could you explain a little bit more about how the electrons vibrate in AC?

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.

justin001
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.
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?

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

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
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.

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.

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.

russ_watters and justin001
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.
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.

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.

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 field causes positive and negative voltage which gives rise to AC.
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.

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.

justin001
So the commutator works by reversing the AC current when it goes negative/backward. Thus the circuit always is positive/forward (DC).
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.

so the black wire and neutral wire would be hooked up to the commutator as left side and right side. (it's a disk with two isolated sides). As the coil rotates in the generator it makes positive voltage for one half the cycle and negative voltage for the other half (in the coil). when the coil starts to make negative voltage the contacts (from the coil to commutator) on the disk switch sides so that the connection on the circuit is reversed and you have DC on the circuit side.

so the black wire and neutral wire would be hooked up to the commutator as left side and right side. (it's a disk with two isolated sides). As the coil rotates in the generator it makes positive voltage for one half the cycle and negative voltage for the other half (in the coil). when the coil starts to make negative voltage the contacts (from the coil to commutator) on the disk switch sides so that the connection on the circuit is reversed and you have DC on the circuit side.
Could you tell how the disk switch sides?That's where I'm stuck.

there are brushes on the coil which drag along the disk. They rotate with the coil, and are set up so that when the voltage is switching the brushes are going to the other side of the disk (left to right or right to left). Like running your two fingers around the rim of a glass (on opposite sides), where two sides are isolated

there are brushes on the coil which drag along the disk. They rotate with the coil, and are set up so that when the voltage is switching the brushes are going to the other side of the disk (left to right or right to left). Like running your two fingers around the rim of a glass (on opposite sides), where two sides are isolated
Oh sorry but I still couldn't get it.Is there any animation that shows commutator reversing the direction of current?

I couldn't find an animation but this picture shows the split disk and direction of rotation, with the connections.... maybe it will help? you can imagine the disk and wire rotating, you see the brushes touch different sides as it rotates

justin001
I couldn't find an animation but this picture shows the split disk and direction of rotation, with the connections.... maybe it will help?
Could you tell why do we need a DC power supply to reverse the current?

In this case it is a DC motor. In a generator, the motor would turn the coil and create DC where the power supply is.

justin001
In this case it may be a DC motor. In a generator, the motor would turn the coil and create DC where the power supply is.
Oh that's where I was stuck.Thanks for that.Now I could get the concept of commutator.I thought without battery you can't reverse the current.After you told that it creates a DC power I could understand the working of commutator.In AC motor does the motion occurs due to the repelling effect of the conductor and the coil that gets rotated?Does the same happen in DC motor?

AC motors have a few varieties, but essentially it boils down to the induced magnetic field interacting with a permanent magnetic field. They can repel or pull, I don't think there is a reason to prefer one over the other.

AC motors have a few varieties, but essentially it boils down to the induced magnetic field interacting with a permanent magnetic field. They can repel or pull, I don't think there is a reason to prefer one over the other.
Oh sorry I would like to know whether the coil rotates due to the repulsion between the permanent and induced magnetic field interacting?Also could you tell why the windings on a coil increase the magnetic field of the coil?