Re: Alternating current. question

From: JimC (jimc_at_yabba-dabba-doo.com)
Date: 06/14/04 

Date: Mon, 14 Jun 2004 14:18:11 GMT

"TimR" <timothy42b@aol.com> wrote in message
news:87af0be7.0406140234.47edec31@posting.google.com...
> "JimC" <jimc@yabba-dabba-doo.com> wrote in message
news:<Ffbzc.85558$kj1.13216@newssvr25.news.prodigy.com>...
> > "Paul Cardinale" <pcardinale@volcanomail.com> wrote in message
> > news:64050551.0406131639.572628c7@posting.google.com...
> > > Rushtown@aol.com (Andrew E. Smyth) wrote in message
> > news:<c40a7ddb.0406131037.12da4f38@posting.google.com>...
> > > > I live 300 kilometers from Hoover Dam. If I am using electricity now
> > > > that was generated at Hoover Dam (and which would be transmitted as
> > > > alternating current) does that mean that the current goes back and
> > > > forth from Los Angeles to Hoover Dam 60 times each second?
> > >
> > > No. It means that the direction of current hanges 120 times per
> > > second.
> > > Nothing has to travel from one end to the other in each cycle. Note
> > > also that although the direction of the current alternates, the
> > > direction of energy flow does not.
> >
> > An excellent point, the last clause in the last sentence.
> >
> > (I had written of "energy flow up and down the line." I really should
> > have said current or voltage wave. Of course a reactive load can
> > exchange instantaneous energy in both directions with source, but the
> > net flow of energy is unidirectional for a passive load with any
resistance.)
>
> Trying to resist - nope.
>
> Flow down "the line?" But there are two.

No, the energy moves toward the load on either line. That is,
as Penny pointed out, the usual Poynting vector relationship
S = E x H holds and significantly, works for direct current
as well.

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