Fascinating. I thought I understood this stuff but nope.
So, electrons bump (oscillate) through the phase wire which is connected to the power station generator many miles away?
Then after passing through a light bulb etc a few electrons bump along the neutral wire down into the earth?
And then bump each other back to the generator?
What is the third wire for - the ground wire?
The earth is mostly not involved at all. The three wires coming into your house are typically three different phases of AC: All three are hot, and all three carry current both ways at various times. They just switch direction at different times. There’s a graph a bit of a way down this page: Each of those colored curves on the graph represents the voltage on one of the wires as a function of time.
That’s not quite right.
There are only two phases coming into the typical house. The third wire is “neutral,” and is the electrical mid-point between the two phases (which are 180° out of phase with each other).
In a three-phase system, each wire is 120° out of phase with the other wire. Three-phase power is used in industrial and commercial settings, typically at 440VAC or 208VAC.
120VAC is derived from 2 phases of 208VAC (208 * sin(120) = 120).
Here’s the deal: there are three wires coming *from *the secondary winding on the transformer hanging out on the pole and *in *to your house. Let’s ignore the concept of “ground” for the moment, and simply label the three wires A, B, and C:
There is a sinusoidal voltage between A and B with a peak-to-peak voltage of 339 volts and a frequency of 60 Hz.
There is a sinusoidal voltage between A and C with a peak-to-peak voltage of 679 volts and a frequency of 60 Hz.
There is a sinusoidal voltage between B and C with a peak-to-peak voltage of 339 volts and a frequency of 60 Hz.
All of the 120 VAC stuff in your house (primarily regular wall receptacles and lighting) is connected between A and B or between B and C.
All of the 240 VAC stuff in your house (electric clothes dryer, central air conditioner, range, well pump) is connected between A and C.
And that’s all there is to it.
Now based on the system I describe above, what would happen if one part of your body was touching earth ground (which is more common that you might think), and a *different *part of your body touched conductor A, B, or C? Well, that obviously depends on the voltage between A and earth ground (if you’re grounded and touch A), the voltage between B and earth ground (if you’re grounded and touch B), or the voltage between C and earth ground (if you’re grounded and touch C). So based on the system I described above, what is the voltage between A and earth ground? Between B and earth ground? Between C and earth ground? You might think the answer is “0 V” for one of them, but in reality there is no way of knowing what it is. That’s because the secondary winding of the transformer hanging out on the pole is isolated from earth ground, for the most part. (You can sort of think of it as an “AC battery.”) But due to leakage currents in the transformer, it is possible for the secondary winding of the transformer to “float up” to the primary voltage of the transformer (7,500 V relative to earth ground), which would mean the voltage between A and earth ground (or B and earth ground, or C and earth ground) would be around 7,500 V. Instant death. Not good.
So how do we correct this problem? Simple: by connecting A or B or C directly to earth ground. So which one should we connect to earth ground? A, B, or C? Does it matter?
Well, if we connect A to earth ground, you will feel nothing if you’re grounded and touch conductor A, you will feel a peak voltage of 170 V if you touch conductor B, and you will feel a peak voltage of 339 V if you touch conductor C. If we connect B to earth ground, you will feel a peak voltage of 170 V if you’re grounded and you touch conductor A, you will nothing if you touch conductor B, and you will feel a peak voltage of 170 V if you touch conductor C. If we connect C to earth ground, you will feel a peak voltage of 339 V if you’re grounded and you touch conductor A, you will feel a peak voltage of 170 V if you touch conductor B, and you will nothing if you touch conductor C. Out of the three, the middle option (connect B to earth ground) is obviously the best, since it exposes you to the least voltage.
Wire A, by the way, is one end of the transformer’s secondary winding. Wire C is the other end of the transformer’s secondary winding. And wire B is the center tap of the transformer’s secondary winding.
My favorite image of AC power transmission is the way a Boy Scout uses friction to create fire from a stringed bow, a stick, and a notched block of wood. Imagine that at the power plant there is a Boy Scout furiously working a bow with an enormously long string which reaches all the way to your home, where you have it wrapped around the stick fitted into the woodblock. His back-and-forth motion supplies the energy that rotates the stick back and forth, creating enough heat hopefully to create a usable spark.
As mentioned before, the return loop from your house to the plant actually goes through the ground. And to extend the analogy, you might expect that a lot of energy is lost due to stretching of the string, which is in fact true of long-distance power generation for analogous reasons.
And as mentioned before, this statement is incorrect for almost all real power systems.
I think it is wrong for most modern installations in first world countries.* My own home, along with the rest of the neighborhood, is served by a single wire, ground return high tension line. Yes I am sure. I am a EE and marveled at it when I first moved in. The neighborhood is in the US, (Albuquerque, NM) and built out in the 1970’s, so they were still installing ground return power as late as that.
As dry as it is where I live, I suspect that the municipal water piping functions as the primary return circuit. When I moved in my house had no ground rods, only a connection to the cold water pipe. I suspect the whole neighborhood is the same.
It would not surprise me if some rural co-op power associations and other cut-rate utility operations still install single conductor service. It would make economic sense if you are running power to a few remote vacation cabins for example.
I really can’t imagine a generating station EVER using single-wire though…I can believe that it was done, but would have been really poor practice.
*Many residential areas of the US have rather primitive power grids by first world standards. Unless you are very near an industrial or agricultural area, it is frequently not affordable (If even possible to get a quote) to get 3 phase service, for example.
In simple terms, you have one wire coming out from the generator that goes all the way out to your house and then to the light bulb, then a return wire that goes all the way back to the generator. The path of electron flow isn’t through the earth.
In the real world, it’s a bit more complicated than that. Most power systems use three phase generators, which have three sets of coils instead of just one. Each set of coils is spaced 120 degrees apart from the next. The reason for this is that you divide up your customers, putting a third of them on each phase. So now you have six wires, right (three wires going out and three return wires)? Well, here’s where the benefit of three phase power comes in. If you tie all of your returns together, you might think at first that this one big return wire needs to be three times as big as the single wires were. But, if you take three sine waves 120 degrees apart and add them together, they sum to zero. So what actually happens is that all of the currents sum to exactly zero in the return wire. If there isn’t any current flowing through the wire, it isn’t doing anything and it might as well not be there. If everything were perfectly balanced, you could eliminate the big return wire completely. In the real world though, the loads from all of your customers will never be exactly balanced, so there will be some current flowing through the return wire, but it will be very small. So by using three phase, you go from having six big wires down to three big wires and one small wire for the return.
In the real world, power plants have multiple three-phase generators that are tied together. The power from these goes to a transmission substation close to the generators (it was right behind the generator building in the plant I worked in). Transformers step up the voltage to transmission levels (typically above 50,000 volts) to reduce losses in the transmission lines. Transmission lines transmit the power from where the generators are located to where it will be used. A distribution substation takes the incoming transmission voltage and drops it down to distribution voltage (typically somewhere between 3,000 and 10,000 volts). The distribution lines take the power out to the neighborhoods where people live. Most commonly, the three phases are split at that point and are balanced among the individual homes. A single transformer on the distribution line then typically feeds three or four houses. These transformers are typically of the “split phase” type, meaning they take a single phase and drop it down to 240 volts with a center tap. The center tap becomes the “neutral” and is grounded, and the other two connections become the two “hot” lines. You get 120 volts from either line to neutral, and 240 volts from line to line. At the breaker box in your house, the two hot lines will be roughly split so that each line powers half of the 120 volt circuits in your house. 240 volt breakers for things like a clothes dryer or an oven are connected from one line to the other.
Alternately, what is still used in some places is that all three phases are used for distribution. A three phase transformer is used to drop the voltage down to the residential voltage, and these three phases are divided up so that each residence gets two of the phases, as well as a neutral which comes from the center of the transformer. In this case, instead of a center tapped single coil, what you have are three individual coils with one end all connected together so that they form a Y shape. The mid-point of the Y is the neutral connection and thee three outside points are the three hot lines. Again, the line to neutral voltage is 120 volts, but the line to line voltage in this case is only 208 volts. This type of distribution isn’t very common, but it is still used in parts of NYC and in some other areas around the country.
Either way, you end up with two “hot” lines and one “neutral” line. It is possible to run power systems that are isolated from earth ground, and in fact these are actually safer since you can touch either line and be touching earth ground and not get shocked. In practice, though, isolated systems are very difficult to keep isolated. Hospitals use isolated power in operating rooms and other locations and they have to have these systems tested and inspected every year. For residential service, maintaining isolation is just too difficult, so instead of having mother nature create a randomly grounded system (which would be even more unsafe) we just intentionally ground the neutral.
Older systems used the neutral as the safety ground, but this can cause problems. Let’s say you’ve got some type of device with a metal case. You want to connect your metal case to the grounded conductor so that the metal case is always safe to touch. But if you use the neutral as your safety ground and the neutral breaks because of corrosion or some other problem, now when you turn the device on, electricity can flow from the hot, through the device, and to the case which is now floating and also becomes electrically hot. This makes the case an unsafe shock hazard. To prevent this from happening, we instead run a third wire and use it as the safety ground. So now each outlet has a hot, a neutral, and a safety ground. You connect the metal case to the safety ground, and now if the hot breaks, then the device just stops working. If the neutral breaks, the device just stops working (and the case doesn’t become hot when you turn it on like it did in the 2 wire system). And if the safety ground breaks, then the case just becomes disconnected and still doesn’t become hot. The only way the case can become hot is if you have multiple failures, such as the neutral breaking, the safety ground breaking, and the hot shorting to the case. So while it’s not completely impossible for the case to become hot and dangerous, it is much more difficult. This system is much safer, and it’s what we use today.
The safety ground and neutral are tied together at your breaker box, and these are then connected to earth ground somewhere very close to the breaker box. In older homes, the earth ground connection was required to be done through the cold water pipe, as this was a good ground that was always available. Then folks started using PVC pipe, and the electric code was changed so that you are now required to use a separate ground rod instead of the cold water pipe. However, you don’t want your plumbing to become a shock hazard, so your water pipes are also required to be grounded.
Excellent post, but minor nitpick, ECG. 
The voltage isn’t boosted to reduce losses in the transmission lines. The voltage is boosted to reduce the current. (For a given load, the higher we boost the voltage, the lower the current will be in the transmission lines.) Less current = smaller diameter conductors = lower material cost, less weight, smaller size, etc.
The only drawback to higher voltage is that we need better insulators. But that’s a lot cheaper than using conductors with greater diameter.
Sorry, but I need to nitpick your nitpick.
High voltage is used in transmission lines because it reduces the current, allowing for smaller conductors while still maintaing acceptable losses.
It’s pretty easy to show that transmitting 120V across the country would require enormous conductors. In order to make transmission losses (and conductor size) reasonable, the voltage must be stepped up.
So it’s both a floor wax and a dessert topping.
That’s exactly what I said. :dubious:
Then, you shouldn’t have nitpicked, ECG’s post, since what he said was correct, too.
I was assuming – perhaps wrongly – that ECG was stating that AC was “less lossy” in the transmissions lines themselves. This, of course, is not correct. In the transmission lines themselves, DC is less lossy than AC. However, if ECG was stating that the entire transmission line ***system ***was less lossy with AC (due to the use of transformers), then my assumption was incorrect.
In the old (Edison) DC power supply systems, voltage conversion was done by rotary converters: a electrical motor connected to an electrical generator.
In Melbourne, the DC power system was switched off about 10 years ago after somebody damaged the transmission line. They only had less than a dozen customers left, and it was cheaper to provide each customer with a rectifier connected to the AC mains, rather than repairing the DC system. They might of still had some Edison lamps, but the system was maintained to power old elevators.
I think the DC power system in Chicago was switched off some time in the 90’s. The hydraulic power system, (using water), some time in the 30’s or 40’s. When the hydraulic power system was switched off, a small number of elevators were converted to run on mains-pressure water, but I’ve never seen a working example.
Pretty much everything has been answered (which really takes the wind out of my sails because I was looking forward to explaining the basics of electricity). But just allow me to reiterate what has been discussed and verify it.
As has been stated several times, in AC circuits, the individual electrons do not move along the circuit. They “slosh” back and forth like waves. The electromagnetic field is what propagates along the length of the circuit, and this happens at a significant fraction of the speed of light. As another poster pointed out, you can think of it as waves in an ocean. In a wave, individual molecules of water are NOT moving from deep in the ocean up to the shore. Individual molecules of water are just moving up and down, up and down, up and down, but the energy is being propagated along.
In DC currents there really is a flow of electrons along the circuit, and if you “painted” one electron red you sure enough would be able to see that red electron circling around and eventually coming back to it’s starting point. Again, as already pointed out, this is called the drift velocity, and for common materials it is EXCESSIVELY slow. Even a very simple, small circuit, would take hours for one electron to make a complete pass through the circuit.
However, again, as already mentioned, this does not mean electrons are moving slow as molasses. In fact, they are zipping around very quickly, knocking into other electrons and bouncing around in a jumbled mess, at a very high speed. On average, however, they only slowly make their way around a DC circuit.
And if I may remain on my high horse for a minute longer, since we’re on the subject, allow me to continue. A battery as commonly understood does NOT “store” charge. A battery does not have some excess of electrons in it just waiting to come out and enter your circuit. A battery acts like a pump by creating a voltage difference across its terminals.
A capacitor also does not store charge, but it almost does, at least in more of a sense than a battery does. A capacitor is comprised of two surfaces separated by a dielectric, a substance through which the flow of electricity cannot “easily” travel (think air, for example, as a dielectric). For a capacitor to work, an excess amount of charge is built up on one plate by robbing that same amount of charge from another plate. If you hook up a capacitor to a battery and “charge” it, you aren’t adding electrons to the capacitor. You are simply taking some electrons from one plate and moving it to the other, until the “electric pressure” equalizes and your battery isn’t strong enough to pump any more charge from one plate to the other!
I know this is more than you bargained for but I hope it helps you understand a little bit more about electricity and circuits!
Also, I had totally forgotten about VanDegraff generators as one of the mechanisms where electrons really are moved over an appreciable physical distance in a short amount of time. But that is a really good example of real physical electrons moving quickly to build up a charge on a surface (or as the case may be, removing electrons from the metalic sphere which is how they actually work, I think). Thanks for pointing that out Chronos!
Now I have a question of my own to ask! How does an AC to DC converter work, in the sense that if AC circuits just slosh the electrons around, but DC circuits actually move the electrons from one point to another on the circuit… there seems to be a problem. What’s going on inside the converter? There seems to be a contradiction. On the AC side of the converter you have electrons sloshing back and forth against the converter, and on the DC side of the converter you have electrons actually moving away (or toward) it.
So what’s going on here?
Rectification uses Bridge diodes which act as 1-way valves - the diagrams on that article give an explanation that is easier than writing it.
The output from the bridge rectifier is lumpy, and generally uses a big capacitor to smooth the output to close to DC - this capacitor (sort of) stores the lumps and lets it out at a constant rate.
Nope. The snipped part is all wrong. The power company is taking all these electrons in your house and pushing them back and forth through the meter, selling you the same electrons over and over again, because the AC motor in your meter always moves in the same direction, regardless of which direction the actual electron is flowing. It only depends on which side of the meter they’re being pushed from. That way, they are charging you for the electron no matter which way they are making it move. Then they charge you per electron, even though it’s the same electron, just moving back and forth.
TL;DR explanation spoilered.
[spoiler]That was a joke.
What’s really happening, is that they are using the electrons to gather Phlogiston from the air in your house, and pumping it back to their power plant to power the combustion that runs their plant. And they’re charging you for the Phlogiston they are stealing from you. But there’s a way you can pull one over on them, though. Get outlet covers for all your outlets. That way, there’s no fresh Phlogiston-rich air in contact with any of their equipment. No Phlogiston gets transferred. They still charge you for the electron they’re moving, but they get no Phlogiston in return for their efforts. How does that help you? Here’s where the scheme comes into its own. You still have all your Phlogiston. You can use it to power a gas or diesel generator of your own (it pretty much works out the same mathematically, even if you’re using solar or wind, but the physics of the Phlogiston consumption is a bit more difficult to envision, conceptually, with methods that don’t involve actual combustion), and then you can push the electrons around. They can only measure electron movement with their meters. They can’t even tell who is pushing them. So, they just assume they are pushing them, and getting Phlogiston along with 'em. If you are using the Phlogiston to push the electrons, and pushing the electrons harder than they are, the meter runs backwards! You are now using the Phlogiston they were trying to steal to sell them that same batch of electrons, over and over. PROFIT!!!
The above is utter bullshit. I assume you pretty much knew you had it right in the post I’m riffing on, here, drewtwo99. 
[and The Onion used to be good, but they started to carry on jokes like the above, waaaaayyyyy too long. They haven’t been actually good for several years now.]
[/spoiler]
Now to answer the actual good question you asked in your next post:
Energy transfer between sides of the converter, via electromagnetic fields that connect them through a bunch of not easy to generally describe components (specific examples are not too hard to analogize, but you have to have the specific case to analogize it in your model). AC side sloshes, transferring energy via EM fields to the DC side, which carries the energy away by making electrons flow. DC to AC converters work just the opposite. Electron flow gives up energy, which causes the sloshing on the AC side via similar EM fields. It’s a bit more complex than that, but your general idea is well within the ballpark. It’s only a matter of what and how the components transfer the EM energy.
So the electrons are basically like galley slaves being whipped mercilessly by the power company - being forced to row back and forth, over and over for eternity.
FREE THE ELECTRONS!!!
What’s an orgy?
Seventh seat for a Galley Slave.Oar ‘G’
BTW, I woulda been calling for ‘FREE THE CARROTS’!!! And I know how to make a battery with a lemon! DON’T MAKE ME USE MY LEMON TO ZAP YOUR CARROT!!!