AC question

AL BENSER sez:

Possibly one more analogy:

2 pulleys, one at the power station, one at your home, and a rope is run around both, making a complete circuit. At your home, a fan is attached to the pulley. For purposes of this example, let's say it's not a rotary fan, but one of those found in stereotypes of mid-eastern 30's films, the kind that wave back and forth.

When someone at the power station pulls on the rope so that it moves one direction around the pulleys, the fan moves. Your home gets cooled. This is similar to the way DC works

When they pull on the rope such that it goes back and forth only a few inches, the fan still goes back and forth. Your home gets cooled. This is similar to the way AC works

In both cases, work is accomplished, and in neither case does the rope "go back to the power station".

Hope this helps...

The same electron doesn't actually return to the genertor, but electric current does. You can measure its magnitude and direction on both lines. At any point along the circuit, current in must equal current out, and you can follow that all the way around from the supply terminal to the return terminal of the generator. If current doesn't flow back into the generator, then cut one power line and everything should still work!

So if you push 10 inches of rope out, 10 inches of rope doesn't enter the other side? Cut the rope and see what happens. Suppose you have twenty feet of rope at the far end, not looped, and you pull it all in past the fan, don't you end up with a twenty foot coil of rope on the floor and no more ability to power the fan unless you walk back out with the rope and then pull it through again? So the rope must make a complete circuit for continuous power transfer.

In both cases, the thing that doesn't return is the power. Either the electrical power from the generator, or your mechanical effort moving the rope, are what provide power to the load. You eat food or the generator prime mover gets fuel in order to produce that power.

Ben Miller

ones that come back to it!! Then, what to you mean by 'the electric current does' go back'??. What IS electric current??? the "ampere", which is the unit to measure electric current, is simply "so many electrons flow per second". By the way, Mr Ampere was a Frenchman.

Electrons are indistinguishable from one another. Thus, you cannot authoritatively say that that. In quantum mechanics, it is this indistinguishability that leads to Fermi-Dirac statistics fundamental to a simple electron theory of metals and photoemission.

Bill

-- Fermez le Bush--about two years to go.

Excellent point. I should have said "The same electron doesn't necessarily return to the generator...", leaving open the possibility.

Ben MIller

In fact, it is unlikely that an individual electron will make the circle tour with AC as the "drift" velocity is low so that the electrons typically wobble around a bit. At least that is what appears to be the case -as Bill says, we cannot distinguish individual charge carriers- so why bother in cases where there are so damn many of them that statistical behaviour becomes indistinguishable from the deterministic behaviour upon which our circuit models are based? Rather than consider electrons be gross and consider energy flow which is what really matters.

I am quite surprise about all the philosophying on this issue!! Any electrons leaving the generator at the power plant CAN NOT reach any load because the generator output feeds only a step-up transformer, providing physical isolation.

Yes, the current "out" of the generator is and must equal to the current "in". But, the electrons making up that current can come from any source, copper, aluminum or any conductor.

Yes, the electrons are indistinguishable from one another. Quantum mechanics is irrelevant here.

| In fact, it is unlikely that an individual electron will make the circle | tour with AC as the "drift" velocity is low so that the electrons typically | wobble around a bit. At least that is what appears to be the case -as Bill | says, we cannot distinguish individual charge carriers- so why bother in | cases where there are so damn many of them that statistical behaviour | becomes indistinguishable from the deterministic behaviour upon which our | circuit models are based?

Is the drift rate equally low in an arc?

--------- Nope: but an arc involves ionization, both positive ions and electrons, etc - whole new ball game. Note that the current in an arc is generally limited by the rest of the circuit, not the arc.