# Re: Reason and eloquence

boots wrote:

"J.V.Ashby" <J.V.Ashby@xxxxxxxx> wrote:

boots wrote:

Perhaps you can answer two questions that puzzle me.

First, what exactly is "alternating current"?

current is what happens when electrons flow from one place to another,
usually down a wire (so one-dimensional). In direct current, they all
flow in the same direction all the time. In alternating current they
flow first in one direction, then in the other, then in the first
direction again (think of commuter traffic), with a regular
periodicity.

Yah, I knew all that stuff. But what exactly is "alternating
current"?

It's what I said. There are no deeper thoughts.

I'm looking for deeper thoughts here, maybe I asked the
wrong guy.
Or maybe I asked the wrong question. How far does each electron (or
hole, whatever) have to move during an alternating current?

Right, you want details. The answer to that one is, of course, it
depends. But let's do the sums. One amp, copper wire 1 sq. mm cross
section (lighting cable), 50 Hz.

One amp is one coulomb passing through a plane across the wire in one
second (or would be if it wasn't being turned back every 20
milliseconds). One electron is 1.6e-19 coulombs, so 1 coulomb is
6.25e18 electrons. (I'm hoping you're familiar with notation like 1e3
for 1000 - if not ask and I'll explain.)

So what volume of copper contains that number of electrons? Density of
Cu is 8.9 g/cc (damned CRC handbook) which is 8.9e3 kg/m^3 (9 tonnes,
sounds about right). That cubic metre contains 8.9e3 / 63.55 (atomic
mass of Cu) * 6.022e23 (Avogadro's number) *1e3 (because Moles are
defined in grams and I'm working in Kgs) atoms. Which is 1.1e26 atoms.
If each atom contributes one electron, then our coulomb of charge is
contained in 5.7e-8 cubic metres. The cross section of the wire is 1
sq. mm, or 1e-6 sq m, so the coulomb lies in 5.7e-2 metres of wire.
Thus, in direct current of 1 amp, the electrons would be moving nearly
six centimetres in one second. But of course, in AC they don't, they
get turned back after .02 of a second (at 50 Hz), so they only travel
1.14e-3 m, just over a millimetre.

I've just
never really understood this whole electrickcity thing, that's not to
say I can't wire up a fairly comples application, but it's rote and
good juju, not real understanding.

Second, given that there is a difference in electrical potential
between any two points in space, how does one grow a chip that will
sort them out and collect them into usable power?

I don't understand the question.

How can you not understand that question? Was it really that poorly
written?

Quite honestly, yes. What are "them"? Not the difference in electrical
potential which is singular (grammatically, not mathematically), so
presumably the two points in space. How do you collect points in space
into usable power?

Did you understand any of it, if so which part did you not
understand?

If I was asked to rewrite it to say what I think you were asking it
might go:

Given an externally generated spatially varying electrical field, is it
possible to produce a device to extract power from that field?

To which the answer is pretty obviously yes. But I think you may have
something else in mind, though I'm not sure what.

john

john

.

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