## LESSWRONGLW

I can give some partial answers based on my own models:

AC is used for transmission because transformers are ubiquitous and incredibly valuable at all stages of transmission, and transformers work using AC (you need a changing electrical field to generate a changing magnetic field). Transformers allow you to convert the voltage and isolate circuits. Isolation is important for safety, and voltage conversion is important to achieve the cross purposes of safety and efficiency. High voltage allows you to transfer more energy with fewer losses, but is far more dangerous to work with. This gets to your resistance question -- resistance / heat generation are related to the amount of current and the thickness of the material. To transfer a given amount of energy, higher voltage means less current needed for the same wire, which means less heat losses.

Why 50Hz (or 60 in the US)? As far as I know, this is largely arbitrary. I do know that subtle differences in the frequency are used for signaling grid load. https://en.wikipedia.org/wiki/Utility_frequency has a lot of info though!

As for metering, I have no idea how current meters (ammeters/watt meters) work, but I am pretty sure no net electrons are entering or leaving e.g. your house or your appliance. Electrons in a circuit should be conserved, they're just the means of transfer of energy.

1Long try3moTks. You mentioned isolation is important for safety. Can you elaborate some specific examples? As per my imagination, unless the threat has been predicted then the AC transformers are useless against sudden issues. Say, an abrupt surge will still propagate via its magnetic field before we can do anything.
3kpreid3moIsolation is not about surges, but about preventing current from flowing in a particular path at all. In a transformer, there is no conductive (only magnetic) path from the input side to the output side. So, if you touch one or more of the low-voltage output terminals of a transformer, you can't thereby end up part of a high-voltage circuit no matter what else you're also touching; only experience the low voltage. This is how wall-plug low voltage power supplies work. Even the ones that are using electronic switching converters (nearly all of them today) are using a transformer to provide the isolation: the line voltage AC is converted to higher frequency AC, run through a small transformer (the higher the frequency, the smaller a transformer you need for the same power) and converted back to DC.

Oh, I was too focused on the system function while forgetting that safety can primarily apply to human health too :)

# 8

What I know about it from high school and general articles on the net doesn't satisfy. Maybe because I have critical holes in my knowledge.

From what I think I know: we're having AC running in the lines. AC means if we zoom down, we'll see that an electron is zipping along this direction, and after 1/50 sec (or 1/100?) that very electron will zip back in the opposite direction, ideally back to the specific point we're looking at, because phases are supposed to be equal.

So how does resistance come into the picture at atomic scale? Conductors heat up after a while, so maybe that's because some of the electrons' kinetic energy gets transferred into the wire's temperature? Does this mean the electron slows down? But then does that mean electricity will somehow, sometime propagate slower than light?

Most if not all of our devices actually use DC, using relay(s) to get it from AC. From the only type of relay I was explained, the DC current the device receives seems to be on & off. This moment the electrons are moving forward, the relay allows them to flow into the device. 1/50s later electrons moves "backward" and it cuts the circuit so they can't flow back and the device doesn't have to lose electrons that way (but it doesn't gain anything either, thus my 'on & off' understanding). So my question is: is it detrimental to the device? Is it responsible for the flickering of lights & other stuffs? If so, is the number of 50Hz chosen for the main purpose of making that flickering imperceptible to us?

This lead to another big pondering. Why the fuck don't they just use DC from the source? There are some methods to transfer DC along big distances, they seem to be tried and probably true. Or the reason is simply because of inertia? That people are so used to AC and the systems for AC are all over the place, so switching is not cost-effective? Is there research on this very subject yet?

Last but not least, I wonder how exactly devices "consume" electricity. Like, is it that many electrons enter the device but fewer exit? If not so, how do counters count our consumption?

# 8

New Comment

gjm

### Feb 24, 2020

17

The speed at which electrical signals propagate is much faster than the speed at which electrons move in an electrical conductor. (Possibly helpful metaphor: suppose I take a broomstick and poke you with it. You feel the poke very soon after I start shoving the stick, even though the stick is moving slowly. You don't need to wait until the very same bit of wood I shoved reaches your body.)

The speed at which electrical signals propagate is slower than the speed of light, but it's a substantial fraction of the speed of light and it doesn't depend on the speed at which the electrons move. (It may correlate with it -- e.g., both may be a consequence of how the electrons interact with the atoms in the conductor. Understanding this right is one of the quantum-mechanical subtleties I mention below.)

When current flows through a conductor with some resistance, some of the energy in the flow of the electrons gets turned into random-ish motion in the material, i.e., heat. This will indeed make the electrons move more slowly but (see above) this doesn't make much difference to the speed at which electrical effects propagate through the conductor.

(What actually happens in electrical conductors is more complicated than individual electrons moving around, and understanding it well involves quantum-mechanical subtleties, of most of which I know nothing to speak of.)

It is not usual to convert AC to DC using relays.

It is true that if you take AC power, rectify it using the simplest possible circuit, and use that to supply a DC device then it will alternate between being powered and not being powered -- and also that during the "powered" periods the voltage it gets will vary. Some devices can work fine that way, some not so fine.

In practice, AC-to-DC conversion doesn't use the simplest possible circuit. It's possible to smooth things out a lot so that the device being powered gets something close to a constant DC supply.

But there are similar effects even when no rectification is being done. You mentioned flickering lights, and until recently they were an example of this. If you power an incandescent bulb using AC at 50Hz then the amount of current flowing in it varies and accordingly so does the light output. (At 100Hz, not 50Hz; figuring out why is left as an exercise for the reader.) However, because it takes time for the filament to heat up and cool down the actual fluctuation in light output is small. Fluorescent bulbs respond much faster and do flicker, and some people find their light very unpleasant for exactly that reason. LED lights, increasingly often used where incandescents and fluorescents used to be, are DC devices. I think there's a wide variety in the circuitry used to power them, but most will flicker at some rate. Good ones will be driven in such a way that they flicker so fast you will never notice it. (Somewhere in the kHz range.)

Sometimes DC (at high voltages) is used for power transmission. I think AC is used, where it is used, because conversion between (typically very high) transmission voltage and the much lower voltages convenient for actual use is easy by means of transformers; transformers only work for AC. (Because they depend on electromagnetic induction, which works on the principle that changes in current produce magnetic fields and changes in magnetic field produce currents.) I don't know whether AC or DC would be a better choice if we were starting from scratch now, but both systems were proposed and tried very early in the history of electrical power generation and I'm pretty sure all the obvious arguments on both sides were aired right from the start.

When a device "consumes" electrical energy it isn't absorbing electrons. (In that case it would have to accumulate a large electrical charge. That's usually a Bad Thing.) It's absorbing (or using in some other way) energy carried in the electric field. It might help to imagine a system that transmits energy hydraulically instead, with every household equipped with high-pressure pipes, with a constant flow of water maintained by the water-power company, and operating its equipment using turbines. These wouldn't consume water unless there were a leak; instead they would take in fast-moving water and return slower-moving water to the system. An "AC" hydraulic system would have water moving to and fro in the pipes; again, the water wouldn't be consumed, but energy would be transferred from the water-pipes to the devices being operated. Powering things with electricity is similar.

reallyeli

### Feb 24, 2020

9

Perhaps you already know this, but some of your statements made me think you don't. In an electric circuit, individual electrons do not move from the start to the end at the speed of light. Instead, they move much more slowly. This is true regardless of whether the current is AC or DC.

The thing that travels at the speed of light is the *information* that a push has happened. There's an analogy to a tube of ping-pong balls, where pushing on one end will cause the ball at the other end to move very soon, even though no individual ball is moving very quickly.

http://wiki.c2.com/?SpeedOfElectrons

cousin_it

### Feb 25, 2020

6

I think Bill Beaty's page on electricity might be what you're looking for. Here's a joking teaser which shows the kinds of questions he's trying to answer:

Electricity is quite simple: "electricity" is just the flowing motion of electricity! Electricity is a mysterious incomprehensible entity which is invisible and visible, both at the same time. Also, electricity is both a form of energy and a type of matter. Both. Electricity is a kind of low-frequency radio wave which is made of protons. It's a mysterious force which cannot be seen, and yet it looks like blue-white fire that arcs across the clouds. It moves forward at the speed of light... yet it sits and vibrates inside your AC cord without flowing forwards at all. It's totally weightless, yet it has a small weight. When electricity flows through a light bulb's filament, it gets changed entirely into light. Yet not one bit of electricity is ever used up by the light bulb, and all the electricity flows out of the filament and back down the other wire. College textbooks are full of electricity, yet they have no electric charge! Electricity is like sound waves, no no, it's just like wind, no, the electricity is like the air molecules. Electricity is like cars on a highway, no, the electricity is the speed of the cars, no, electricity is just like "traffic waves." Electricity is a class of phenomena ...a class of phenomena which can be stored in batteries! If you want to measure a quantity of electricity, what units should you use? Why Volts of electricity, of course. And also Coulombs of electricity. And Amperes of electricity. Watts of electricity and Joules, all at the same time. Yet "electricity" is definitely a class of phenomena; merely a type of event. Since we can't have an amount of an event, we can't really measure the quantity of electricity at all... right? Right?

And then he goes on to answer all the questions one by one, in a very straightforward way.

Jumpman

### Feb 24, 2020

3

The electrons in a current never move anything close to the speed of light (https://en.wikipedia.org/wiki/Drift_velocity). It is the propagation of the changes in the electric field caused by the electrons moving that moves at the speed of light. It is more like a tube full of marbles (a stretched analogy). If you push the marble on one end the marble at the other end moves almost instantly. The marble you pushed didn't move all that distance.

Yes, the heat in conductors is caused by the electrons kinetic energy. No, it doesn't really change the propagation speed of the current since that is the electric field propagating. There is certainly power lost there.

It is not easy to transmit DC over long distances (https://en.wikipedia.org/wiki/War_of_the_currents). Edison tried hard to push the adoption of DC going so far as to publicly electrocute elephants with high voltage AC as a PR stunt to scare people. You can find videos of this online if you want. It didn't work because it just so much more efficient to transmit AC voltage and use a transformers to step it down.

lincolnquirk

### Feb 24, 2020

3

I can give some partial answers based on my own models:

AC is used for transmission because transformers are ubiquitous and incredibly valuable at all stages of transmission, and transformers work using AC (you need a changing electrical field to generate a changing magnetic field). Transformers allow you to convert the voltage and isolate circuits. Isolation is important for safety, and voltage conversion is important to achieve the cross purposes of safety and efficiency. High voltage allows you to transfer more energy with fewer losses, but is far more dangerous to work with. This gets to your resistance question -- resistance / heat generation are related to the amount of current and the thickness of the material. To transfer a given amount of energy, higher voltage means less current needed for the same wire, which means less heat losses.

Why 50Hz (or 60 in the US)? As far as I know, this is largely arbitrary. I do know that subtle differences in the frequency are used for signaling grid load. https://en.wikipedia.org/wiki/Utility_frequency has a lot of info though!

As for metering, I have no idea how current meters (ammeters/watt meters) work, but I am pretty sure no net electrons are entering or leaving e.g. your house or your appliance. Electrons in a circuit should be conserved, they're just the means of transfer of energy.

MoritzG

### Feb 28, 2020

1

Why 50/60Hz? It has to be too low to be heard, to high to be seen, high enough for transformation, low enough for low induction losses, low enough for simple rotating machines. Trains can not use 50/60 so they went with 1/3 (16+2/3 Hz or 20 Hz)
Grid frequency is controlled to +-150mHz if that fails private customers might get disconnected/dropped.
The time derivative of the grid frequency is a measure of the relative power mismatch.