Does AC form a closed loop circuit?

[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?

 

[elegysix]

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?

 

[elegysix]

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.

 

[justin001]

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?

 

[elegysix]

yes the rotation is due to repulsion or pulling between the magnets. As for why the winding increase the magnetic field, i would say look up biot savart law. Basically the more electrons you have flowing in a loop, the stronger the magnetic field inside it becomes. So more windings is more magnetic field. I don't know why moving charges create magnetic fields, it's just a property.

 

[justin001]

yes the rotation is due to repulsion or pulling between the magnets. As for why the winding increase the magnetic field, i would say look up biot savart law. Basically the more electrons you have flowing in a loop, the stronger the magnetic field inside it becomes. So more windings is more magnetic field. I don't know why moving charges create magnetic fields, it's just a property.

I tried looking in wikipedia but couldn't get an image of the AC motor.s there any animation that shows the working of AC motor?

 

[sophiecentaur]

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.

Just forget about the ground for a minute. The ground is not worth considering when you are learning the basics. The basics are that a complete path ("loop") is required for current to flow. A battery, a bulb and two wires are the simplest circuit to consider. Connect these in a loop and the bulb will light.
Also, it really doesn't help to try to include electrons in your first understanding. People had never heard of electrons (or any other particles) when all the laws that govern circuits were invented so you should really put yourself back in the 18th / 19th century first and get an understanding of what they knew and loved. What happens to electrons is very complicated and there is really not a simple model of conduction that 'explains' it well with electrons. They can be a snare and a delusion, I would say. It is my strong opinion that, until you understand something of QM, electrons will only confuse your understanding of electricity. Enthusiastic and poorly informed educationists have dictated the electron theory of electricity be taught to 'beginners' and most of them couldn't solve the simplest circuit equation and few of them realize that the electrons flow through wires at an average speed of just a few mm per second. (So that's a rant for you! )
You ask where the charges go and where they come from. If you have a bicycle chain, you don't consider where the individual links come from. You just know that, when you move the pedal, the crank moves and pulls the 'next' link towards it. A link on the wheel sprocket is pulled off the tooth and makes the sprocket rotate. Same with simple electrical conduction. One charge in one end and an equal charge out of the other because they are all linked to each other like a chain. You could think in terms of compression and tension in the links / charges.
If you want to use an Earth Return instead of a wire, the Earth just acts like the wire it replaces. Charge flows into the Earth at one end and is squeezed into the component at the other end and flow into the bulb and so on.
Forget the Earth Wire in a 13A plug. I doesn't carry any of the current unless there is a fault somewhere and then a fuse or a breaker will cut the circuit.

 

[justin001]

Just forget about the ground for a minute. The ground is not worth considering when you are learning the basics. The basics are that a complete path ("loop") is required for current to flow. A battery, a bulb and two wires are the simplest circuit to consider. Connect these in a loop and the bulb will light.
Also, it really doesn't help to try to include electrons in your first understanding. People had never heard of electrons (or any other particles) when all the laws that govern circuits were invented so you should really put yourself back in the 18th / 19th century first and get an understanding of what they knew and loved. What happens to electrons is very complicated and there is really not a simple model of conduction that 'explains' it well with electrons. They can be a snare and a delusion, I would say. It is my strong opinion that, until you understand something of QM, electrons will only confuse your understanding of electricity. Enthusiastic and poorly informed educationists have dictated the electron theory of electricity be taught to 'beginners' and most of them couldn't solve the simplest circuit equation and few of them realize that the electrons flow through wires at an average speed of just a few mm per second. (So that's a rant for you! )
You ask where the charges go and where they come from. If you have a bicycle chain, you don't consider where the individual links come from. You just know that, when you move the pedal, the crank moves and pulls the 'next' link towards it. A link on the wheel sprocket is pulled off the tooth and makes the sprocket rotate. Same with simple electrical conduction. One charge in one end and an equal charge out of the other because they are all linked to each other like a chain. You could think in terms of compression and tension in the links / charges.
If you want to use an Earth Return instead of a wire, the Earth just acts like the wire it replaces. Charge flows into the Earth at one end and is squeezed into the component at the other end and flow into the bulb and so on.
Forget the Earth Wire in a 13A plug. I doesn't carry any of the current unless there is a fault somewhere and then a fuse or a breaker will cut the circuit.

Yeah you're right.I think it's the wave that actually provides the energy(voltage) and not the electrons.That's another place where I'm stuck.I couldn't get why "water in the waves aren't actually flowing to the shore but it's the wave".I think this would help in understanding electricity as I've read from documents online.Could you help me.

 

[sophiecentaur]

why "water in the waves aren't actually flowing to the shore but it's the wave"

I guess I could ask you why you would necessarily expect the energy carried in the wave would need to relate to flow of the 'substance'? If you jiggle a string up and down, you don't let go of the string but a wave (Energy) will travel along it. Intuition can often be wrong.
PF has loads of threads about how the energy flows along a wire and they range from highly technical to simple mechanistic. It's quite valid (imo) to treat it at many levels and a high level may be quite unnecessary for some purposes. We all yield to the temptation to show and tell too much of what we know, when someone asks a PF question and the resulting overkill can put a chap right off the subject (they can get scared off and never come back with responses).
Introducing Waves into electrical power flow, assumes a changing situation - either at switch on or every cycle of AC. For DC conduction (after things have settled down, what's needed is a replenishable source of charges on one terminal (battery / solar cell / dynamo etc) and a sink for those charges at the other terminal. The charges each have to be given an amount of Energy (the Potential Difference across the terminals). You still don't actually need to be talking of electrons here and, of course, there can be some instances where the charge carriers don't happen to be negatively charged (electrons). A steady Voltage (PD) across the terminals will produce constant DC through an unchanging Resistance and supply a Power of VI to the load.

 

[russ_watters]

I tried asking many sites of how a commutator reverses the direction of ccurren...

Commutators are not used in AC generators.

The wiki article on alternators has a simple diagram and explanation showing how a magnet rotating inside a coil of wire pushes the electrons back and forth. It really is that simple.

 

[sophiecentaur]

@Justin:
It strikes me that you are asking too many questions at once. Q and A will fail you here because you are darting from topic to topic. I have to ask what course, book or other source are you using, in addition to PF? There are answers to most of your questions in the Wiki pages about motors and generators. Why not try reading those pages - thoroughly and not skimming? You need a structured course of learning and even Wiki pages tend to have some structure to them.

 

[patellar-myotatic]

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?

Hi Justin!

The short answer is no. The current entering - and simutaneously returning from - your house only reaches the last power transformer near your house! It does not extend beyond that to the other transformers, much less the generators at a power station.On the other hand, each transformer completes a closed current loop. The power station is also part of a closed loop. Each loop is chained to the next by transformers until a load in your home is part of the last loop.

I see that you are asking for some really fundamental level of understanding. So it is important to start discussion with the basics.

With both current and voltage together, you can transport *power* and achieve *work*. You simultaneously need both current and voltage.There is no practical circuit, with current flow, unless there is a closed loop. When the loop is open, there is no current flow. For example, when you connect a single phase AC power cord to a device you are connecting at least two wires. The current on the two wires are equal but opposite. The total current (entering and leaving) adds up to zero at all times. If you disconnect one wire, either wire, the current stops. Without both current and voltage you cannot transport *power* or achieve *work*. The important feature of AC power systems is that they can use transformers to raise the voltage (and lower the current). Or vice-versa. Voltage up, current down. Current up, voltage down. A fundamental feature of transformers is that input and output can be easily isolated by design. The input current is separated from the output current. These currents are separate streams. Their waveforms may be in lockstep, but are comprised of a separate set of electrons.Single phase AC loads are supplied by just two wires. The power transformer outside a residential house is a single phase AC load using a three phase AC supply. Perhaps ony two wires of the three phase system run past the transformer.Between the transformer and your house is *another power circuit*. Treat it as if it is dependent upon but otherwise isolated from the three phase AC system. This isolation is typical of most power transformers. So the current entering - and simutaneously returning from - your house only reaches the last power transformer! If you would like, here is more complexity:Transformers work best on alternating current, AC.Circuits that provide electrical power do not need to be grounded. That is how we can utilize power on airplanes, and all portable devices *insulated* from the ground, like cars.(Grounding of massive utility power systems is done for safety purposes, not to allow utilization or generation. In most states, only one of the utility load conductors is grounded at regular intervals every 500 feet or so, just for safety. This is the *grounded conductor* AKA neutral. The highest voltage circuits, run for extreme distances, are not grounded this way. In California, by legislation, the grounded conductor is not a load conductor. This mandate increases the number of wires, used by the utility, by one.)Large power systems were designed by Nicola Tesla as *three-phase* AC. He did this so there would be continuous smooth power transmission within the period of time of one AC cycle, so that torque was an inheirent property of three windings, so transformers could be used, and the minimum of three wires could be used for long distance power transmission. With three phase AC, there are three powered currents that continuously add up to zero. This can be conducted by three wires or four, depending on design. The loads can be single phase, three phase or any mixture.Three phase loads *need all wires* to supply current (3 or 4). Disconnecting a single wire is always a Bad Thing to do, as the load is designed to require all three (or four).

 

[justin001]

Hi Justin!

The short answer is no. The current entering - and simutaneously returning from - your house only reaches the last power transformer near your house! It does not extend beyond that to the other transformers, much less the generators at a power station.On the other hand, each transformer completes a closed current loop. The power station is also part of a closed loop. Each loop is chained to the next by transformers until a load in your home is part of the last loop.

I see that you are asking for some really fundamental level of understanding. So it is important to start discussion with the basics.

With both current and voltage together, you can transport *power* and achieve *work*. You simultaneously need both current and voltage.There is no practical circuit, with current flow, unless there is a closed loop. When the loop is open, there is no current flow. For example, when you connect a single phase AC power cord to a device you are connecting at least two wires. The current on the two wires are equal but opposite. The total current (entering and leaving) adds up to zero at all times. If you disconnect one wire, either wire, the current stops. Without both current and voltage you cannot transport *power* or achieve *work*. The important feature of AC power systems is that they can use transformers to raise the voltage (and lower the current). Or vice-versa. Voltage up, current down. Current up, voltage down. A fundamental feature of transformers is that input and output can be easily isolated by design. The input current is separated from the output current. These currents are separate streams. Their waveforms may be in lockstep, but are comprised of a separate set of electrons.Single phase AC loads are supplied by just two wires. The power transformer outside a residential house is a single phase AC load using a three phase AC supply. Perhaps ony two wires of the three phase system run past the transformer.Between the transformer and your house is *another power circuit*. Treat it as if it is dependent upon but otherwise isolated from the three phase AC system. This isolation is typical of most power transformers. So the current entering - and simutaneously returning from - your house only reaches the last power transformer! If you would like, here is more complexity:Transformers work best on alternating current, AC.Circuits that provide electrical power do not need to be grounded. That is how we can utilize power on airplanes, and all portable devices *insulated* from the ground, like cars.(Grounding of massive utility power systems is done for safety purposes, not to allow utilization or generation. In most states, only one of the utility load conductors is grounded at regular intervals every 500 feet or so, just for safety. This is the *grounded conductor* AKA neutral. The highest voltage circuits, run for extreme distances, are not grounded this way. In California, by legislation, the grounded conductor is not a load conductor. This mandate increases the number of wires, used by the utility, by one.)Large power systems were designed by Nicola Tesla as *three-phase* AC. He did this so there would be continuous smooth power transmission within the period of time of one AC cycle, so that torque was an inheirent property of three windings, so transformers could be used, and the minimum of three wires could be used for long distance power transmission. With three phase AC, there are three powered currents that continuously add up to zero. This can be conducted by three wires or four, depending on design. The loads can be single phase, three phase or any mixture.Three phase loads *need all wires* to supply current (3 or 4). Disconnecting a single wire is always a Bad Thing to do, as the load is designed to require all three (or four).

That's a good explanation for three phase.Also a new information about the voltages reaches the "last transformer".I'll ask questions regarding this after I learn how AC generators work.

 

[justin001]

Commutators are not used in AC generators.

The wiki article on alternators has a simple diagram and explanation showing how a magnet rotating inside a coil of wire pushes the electrons back and forth. It really is that simple.

To be honest I couldn't understand it.It might be because I'm weak in this subject.

 

[justin001]

I guess I could ask you why you would necessarily expect the energy carried in the wave would need to relate to flow of the 'substance'? If you jiggle a string up and down, you don't let go of the string but a wave (Energy) will travel along it. Intuition can often be wrong.
PF has loads of threads about how the energy flows along a wire and they range from highly technical to simple mechanistic. It's quite valid (imo) to treat it at many levels and a high level may be quite unnecessary for some purposes. We all yield to the temptation to show and tell too much of what we know, when someone asks a PF question and the resulting overkill can put a chap right off the subject (they can get scared off and never come back with responses).
Introducing Waves into electrical power flow, assumes a changing situation - either at switch on or every cycle of AC. For DC conduction (after things have settled down, what's needed is a replenishable source of charges on one terminal (battery / solar cell / dynamo etc) and a sink for those charges at the other terminal. The charges each have to be given an amount of Energy (the Potential Difference across the terminals). You still don't actually need to be talking of electrons here and, of course, there can be some instances where the charge carriers don't happen to be negatively charged (electrons). A steady Voltage (PD) across the terminals will produce constant DC through an unchanging Resistance and supply a Power of VI to the load.

I accept that for a string while for a wave I think the water at least displace a few centimeters ahead isn't it?

 

[anorlunda]

To be honest I couldn't understand it.It might be because I'm weak in this subject.

You may have more luck with this YouTube video on AC generators. Other videos show how DC generators work.

 

[sophiecentaur]

I accept that for a string while for a wave I think the water at least displace a few centimeters ahead isn't it?

Out in open water, the individual particles of water actually move in vertical circles: up forward down backwards. No 'flow'. In deep enough water with small amplitudes, that' the way it goes. It's linear. In a shelving beach, the waves get steeper and break but there is also an undertow so even then there's no net flow. It's a combination of transverse and longitudinal waves and not how most people describe it.

 

[justin001]

Out in open water, the individual particles of water actually move in vertical circles: up forward down backwards. No 'flow'. In deep enough water with small amplitudes, that' the way it goes. It's linear. In a shelving beach, the waves get steeper and break but there is also an undertow so even then there's no net flow. It's a combination of transverse and longitudinal waves and not how most people describe it.

Could you tell whether undertow is a displacement of a water a few centimeters ahead?

 

[justin001]

You may have more luck with this YouTube video on AC generators. Other videos show how DC generators work.

Sorry but I couldn't play that video.It might be because You Tube is blocked in our college.

 

[sophiecentaur]

Could you tell whether undertow is a displacement of a water a few centimeters ahead?

You have to ask yourself what would be the result of a constant net flow in any direction.
Gravity waves (I.e. Waves on a liquid surface) have a longitudinal. component but that is a periodic displacement , only.

 

[justin001]

You have to ask yourself what would be the result of a constant net flow in any direction.
Gravity waves (I.e. Waves on a liquid surface) have a longitudinal. component but that is a periodic displacement , only.

Sorry but I couldn't get what you're trying to say.It might be because I'm totally weak in Physics.I would like to know whether there is at least a minute displacement for the waves that reach the beach?

 

[sophiecentaur]

Did you read the terms "transverse" and "longitudinal" in my earlier post? They are the key to this, I think. Google them or try the BBC Bitesize revision notes. I can only repeat that there can be no net forward flow of medium. The consequence would be a build up over time.

 

[justin001]

Did you read the terms "transverse" and "longitudinal" in my earlier post? They are the key to this, I think. Google them or try the BBC Bitesize revision notes. I can only repeat that there can be no net forward flow of medium. The consequence would be a build up over time.

Do you mean to say about this article: http://www.bbc.co.uk/education/guides/zgr8d2p/revision/1?

 

[sophiecentaur]

Yes. That is one way in. There are links to sound and light.
Calling water waves transverse is actually not accurate but they are often used as an intro because we can see them working. You need to get in the habit of reading around lots when you have a question. Only you know what level of info you need and you can select the most appropriate stuff.

 

[justin001]

Yes. That is one way in. There are links to sound and light.
Calling water waves transverse is actually not accurate but they are often used as an intro because we can see them working. You need to get in the habit of reading around lots when you have a question. Only you know what level of info you need and you can select the most appropriate stuff.

As the article says that "wave moves past you" does this mean that 'water moves past you'?

 

[sophiecentaur]

For a wave on a wiggled string, does the string move past you? Only if you let go and throw it. The 'Medium' is not the Wave.

 

[anaranjo]

There is an analogy for electron movements forward and backward in a conductor: the Newton's Cradle, or Newton Pendulum. The balls (electrons) do not travel. They just transmit the energy. Here a link of many:

 

[Salvador]

I felt maybe I could give a hand at explaining.But any way you need to know or learn if you don't know some of the basics because without them it's hard to explain as it sounds all like a language from Mars.
So in a typical AC generator you have three sets of coils in the stator and a rotating rotor with poles on it formed also by coils.The rotor coils develop a magnetic field creating poles on the rotor much like poles from a permanent magnet.the coils get their current from a set of brushes and sliding contacts called sliprings.Remember a DC motor or generator has what is called a commutator , but AC generators have slip rings.

Usually AC generators are three phase.The angles between phases represent the geometry of the AC generator were the coils are spaced with such angles in the stator.Look at my crude drawing and take for example one of the three coils on the stator, now imagine one of the rotor poles approaching the coil as it rotates , let's say it's the N pole.what you see in that coil is the voltage starts to rise in a sinusoidal fashion because as the rotor pole gets closer to the where the stator coil is the magnetic flux increases through that coil , so the voltage rises when the pole is exactly 1:1 with the middle of the coil that's were the flux stops increasing and the voltage has hit it's peak.Then as the magnet moves away from the closest position the flux lines decrease and the sinus starts to fall and the voltage decreases in the coil.This happens until the voltage hits zero volts and for a very brief moment there is no flux through the coil and no voltage in it but then the next pole , this time S pole again approaches the same stator coil so the magnetic flux starts to increase again , only this time it's the opposite pole so the voltage in the coil begins were the previous pole left it at zero volts and now it descends into the negative side of the sine wave, simply because its the opposite pole so the electrons (which you love) now flow the other direction into the coil.
As this pole again reaches the closest position and the flux is maximum through the coil the sine wave hits the lowest point as the pole moves away again the sine wave starts to rise again to get back to zero volts again and so one full AC cycle has ended and consisted of two half cycles.
two opposite poles rotating by was necessary to create this full cycle each of the poles made one half cycle but each half cycle was with opposite polarity because the poles themselves are opposite so the electron flow reversed direction.
If you understand how each cycle works then you can further see how the three phase AC generator works.It's all basically a matter of a static magnetic field rotating near by fixed coils and a change in magnetic field creates a voltage in a coil.A transformer works the same way it's just that it doesn't need to rotate because the current that makes the field is already changing so the field also changes.
In a generator you have physical movement which then through magnetism creates a changing current in a coil.This is also one of the reasons why in a typical AC generator the output frequency is dependent on it's rotor RPM , do you see why is that so Justin ?

Even in a typical DC generator the current (field) is changing in the output coils , just that the commutator and further addition of capacitors at the output smooths out the otherwise varying voltage/current.

The reason why single phase needs a return wire and three phases doesn't is because in three phase supply the return is managed by other phases because as you see in the workings of a generator , while one phase rises another one falls so to speak of or while the electrons flow in one way in one phase they flow the opposite way in another and this then creates a net flow of power , in a single phase electrons can't flow both directions at the same time in a single wire so two wires form a loop.Actually every loop must consist of atleast two wires, no matter whether it's AC or DC.And even when you think that in the ground case there is just one wire , no the ground is the other wire if it's used in a DC or AC situation , even though it's not common practice to use ground as a conductor because it's conductance varies from place to place.It's only used for safety , or as earthing point in case of accidents.As to the coupling between your house and the generator , No , the wires that power your house only run to the nearest transformer and in the national grid there are tens if not hundreds of transformers between your house and the generator of the closest electricity station.
Once you will learn this then you can start to go further and see that the electrons you are talking about are actually just a medium for something that is unseen to the naked eye , which is the EM field which also is the one that carries the energy , then you will also see that even though not directly via wires but your house is indeed coupled to the generator at the power station via the EM field because that is how every transformer can get power from one winding to another even though these two windings are not electrically or physically connected.

 

[the_emi_guy]

Justin,
Perhaps imagine what would happen if we did *not* provide return path.

Imagine delivering 1A of current from a generator to an automobile sized load without providing for a return path. Within 1 second we will have charged up the load to 1,000 megavolts (assuming 1000pf load to ground capacitance). The idea is that electric charges enforce equilibrium with enormous strength (using Sophiecentaur's analogy, the links in the chain are very strong).

(And even in this case it can be argued that there is still a return path in the form of displacement current through the capacitance)