How does AC really work?

[RickieSalad]

Ok, I get DC, it's easy to understand. Electrons flow from one pole of the battery or power source through some load to the other. Easy peasy.

Well what about AC?  I get that the electricity changes directions back and forth at 50(or 60) Hz.  Well what is literally happening inside the conductor?  If I was an electron in the wire what would I see?  Do all those electrons flow all the way back to the generator and back 50 times a second?  After those electrons flow through some load they end up going to Earth? Then do they flow through the Earth back to the power station? It doesn't seem like AC works the same from a circuit perspective as DC, as in flowing in a circular way? Or maybe I'm seriously over thinking this?

[hamster_nz]

The odd thing is it is not usually the electrons that have the energy, they are just drifting around doing whatever feels right based on the local gradient of the electromagnetic field.... but then the field is defined by the distribution of charges and their motion....

And it gets weirder than that, as energy can move in just the electromagnetic field itself, without any electrons being there at all - like radio waves travelling in empty space, as does light.

At DC and low frequency AC it is much like a hydraulic system, where the fluid is incompressable and power appears at the other end of a wire  At high frequencies it becomes more like sound, where you get echos and reflections, and you are interested in how waves move in the medium.

Electrons are like water or air molecules in those analogies - a drop of hydraulic oil or an atom of air has no sense of the bigger picture, it is just doing its own thing and moving with the flow.

I am sure that didnt help you one bit, but ponder on it - a regular motion of charge can induce a wave in the electromagnetic field, and likewise waves in the EM field can cause a charge to move - this allows you to move power around, manipulate it and transform it without moving the actual charge carriers very far.

[amyk]

The short answer is the electrons vibrate back and forth, and averaged over a long period of time, remain in the same position.

There is a common misconception that the electrons "carry" energy somehow to the load as they move through it, but what's actually happening is they just transmit the force (voltage). This is the case for both AC and DC. (An electron beam may seem to be the exception, as in that case the electrons are actually carrying kinetic energy, but the current and resultant power transmission is still caused by the force on them.)

[bicc1306]

First thing came in my mind about AC is, it reverses direction. Where DC will only charge in one direction.

[Ian.M]

Directed electron movement in a wire is normally extremely slow. See http://c2.com/cgi/wiki?SpeedOfElectrons  No 'new' electrons from the power plant are likely to reach you in your lifetime!

However the effect of stuffing more electrons in the wire 'pipe' at the power station end travels down the wire at typically about 2/3 the speed of light.   Its a bit like a Newton's cradle desk toy.


Slowmo video

The balls in the middle jostle around a bit but never move travel down the wire significantly.  It takes something special to get electrons out of the metal and moving at high speeds, e.g. the electron gun in a CRT.

[helius]

Add to that the fact that increasing the current doesn't have much effect on the electrons' speed at all, since the random motion they exhibit due to their temperature is a million or more times higher than their drift through the electric field. This seems counter-intuitive, but the reason wires have resistance is that the acceleration by the electric field is never allowed to continue for more than a microsecond. The electrons collide with atoms very frequently and they lose that momentum each time. In a superconductor their speed can be much higher.

[Ratch]

Ok, I get DC, it's easy to understand. Electrons flow from one pole of the battery or power source through some load to the other. Easy peasy.

I wonder if you really do understand.  The charge carriers (electrons in a conductive wire) move in random directions at very high translational speed.  When a current exists in a wire, the average velocity of the charge carriers in the direction of the current is very slow.  This is called the drift velocity. It is analogous to a hose filled with marbles.  If you stuff a marble into one end of the hose, a different marble pops out the other end almost immediately.  The response is very fast, but the drift velocity of the marbles is very slow.

Well what about AC?  I get that the electricity changes directions back and forth at 50(or 60) Hz.  Well what is literally happening inside the conductor?  If I was an electron in the wire what would I see?  Do all those electrons flow all the way back to the generator and back 50 times a second?

The charge carriers jerk back and forth at the same slow drift velocity as they would at DC (defined constant).  It is like putting a marble in one end of a filled hose and then doing the same at the opposite end.

After those electrons flow through some load they end up going to Earth?

It depends on whether Earth in in the conduction path.

Then do they flow through the Earth back to the power station?

Tell me the route of the conduction path, and I will tell you where the charge carriers flow.

It doesn't seem like AC works the same from a circuit perspective as DC, as in flowing in a circular way?

AC (alternating cycle) works the same way as DC except that it changes direction at a defined frequency.

Or maybe I'm seriously over thinking this?

Nope, you are just asking questions that are nagging at you. 

Ratch

[Brumby]

If there is a single point I would like to highlight, it is this:

It doesn't seem like AC works the same from a circuit perspective as DC, as in flowing in a circular way?

AC (alternating cycle) works the same way as DC except that it changes direction at a defined frequency.

This is, perhaps, the single most significant point you need to 'lock in' with your thinking.

The practical uses of DC and AC do differ, but at the electron level and the basic 'circular way' they are hooked up they have the same requirements.

[Brumby]

The difference in usage of AC and DC come from the different responses of each of the components to a changing voltage and/or current or a steady one.

[Nerull]

If you had to wait for an electron to travel completely through a circuit, it would would take minutes to hours for a light to turn on after you flipped the switch. Electrons don't move down a conductor very quickly. In copper, at 60Hz AC, electrons barely move at all - a few micrometers, back and forth.

[Brumby]

Oh ... one more thing ......

There is nothing 'magical' about 'Earth' or 'Ground'.  Both terms refer to a conductor - plain and simple.

What they actually represent in a real circuit can depend on the specific arrangement and conventions used by whoever is describing the circuit.  Sometimes a chassis connection can be referred to as one or the other, even if it is not connected to a properly earthed conductor like a metal spike driven into the dirt under your meter box.  (Just be aware of this.)

If we get to the commonly used safety 'Earth' (or Ground) that is referred to with mains power - even then, this only works if there is a circuit available.  How does that happen?  It's actually very simple ...

On the last transformer in the chain of your electricity supply, on connection is labelled 'Neutral' and this connection is wired to a metal stake driven into the ground (as in soil, clay, whatever).  Because of the moisture and minerals in the soil, etc. under your feet, you now have a connection to a real 'ground plane'.  (Strictly speaking it's not a plane ... it goes down deep as well)

The advantage of this connection is that it goes everywhere, so just driving a metal stake into the ground will connect you to it.  The important thing is you MUST have a circular path for any current to flow.

[Melt-O-Tronic]

The short answer is the electrons vibrate back and forth, and averaged over a long period of time, remain in the same position.

. . . assuming a zero DC bias.