I have always found it interesting that electricity propagates by the "skin effect". I have never found a good intuitive explanation for this before I met Frank Grimer. Once you are able to get over the cognitive dissonance of what he is saying, it all makes perfect sense.
Is it worth the effort? Probably not in this lifetime. ;-) http://tech.groups.yahoo.com/group/Beta-atmosphere_group/ Terry On 8/15/07, Horace Heffner <[EMAIL PROTECTED]> wrote: > I completely forgot about this question. I'm getting buried in > private email and mundane chores. Check out: > > http://en.wikipedia.org/wiki/Velocity_of_propagation > > Horace Heffner > > > On Aug 15, 2007, at 12:00 PM, Stephen A. Lawrence wrote: > > > > > > > Terry Blanton wrote: > >> On 8/13/07, Horace Heffner <[EMAIL PROTECTED]> wrote: > >> > >>> On Aug 13, 2007, at 3:51 PM, Terry Blanton wrote: > >>> > >>> > >>>> Interesting exchanges. > >>>> > >>>> If a pair of wires are 3 x 10^10 cm long; and, a potential is > >>>> applied > >>>> to the ends of the wires with a load resistor on the other end, > >>>> I have > >>>> two simple questions: > >>>> > >>>> 1) How much time passes before the first electron drifts > >>>> through the > >>>> load center point assuming the load is only 2 cm long? > >>>> > >>> The first current will appear in at most a few seconds. > >>> > >> > >> Wouldn't Poynting's theorem say almost exactly 10 seconds? Can we > >> understand by what mechanism? > >> > > Maybe! :-) Here's a go at describing the mechanism by which the > > current in the load starts flowing, long before the first electrons > > from the source have traversed the wire: > > > > When the source is connected to the ends of the wires, the > > electrons adjacent to the source start moving (relatively slowly!) > > along the wire. (From here on we'll just look at the wire > > connected to the negative electrode.) > > > > As electrons start to move into the wire from the source, it causes > > a "lump" or "wave-crest" of higher electron density in the "sea" of > > electrons in the wire, just like dropping a rock into a pool pushes > > water aside and results in a "wave-crest" of deeper water around > > the point where the rock landed. > > > > The "wave-crest" of higher electron density has a stronger E field > > than the lower electron density "sea" around it, and that causes > > electrons in front of the "wave-crest" to start moving away from > > the "wave-crest". In turn, that produces a higher electron density > > a little farther along the wire ... and so the "wave" propagates. > > > > The "wave" of higher electron density propagates faster than the > > electrons themselves are moving; again, this is not unlike what > > happens with water: The water in a wave train in the ocean moves > > slowly, while the wave crests move far faster. > > > > Modeling predicts, and observation verifies, that the "wave" of > > electrons will actually propagate along the wire at the speed of > > light, if the wire is in vacuum (which it presumably mostly is, if > > it's a light-second in length). > > > > The "modeling" I referred to is somewhat hairy and I don't recall > > enough to even start to explain it at this point, but the > > conclusion (which is born out by experiment) is that the wire will > > act as a "wave guide", with an electromagnetic wave traveling along > > outside it, at the velocity of light in the surrounding medium. > > > > Don't know if this helps... > > > > > >>>> 2) How much time passes before the first electron to leave the > >>>> source > >>>> of potential arrives at the load? > >>>> > >>> It will take a very long time. It depends on the free charge > >>> density > >>> in the metal, the wire cross sectional area, and the current. A > >>> very > >>> rough number for electron drift speed for estimating purposes might > >>> be 10 cm/h. Thats roughly 3x10^9 s, or 9.5 years. > >>> > >>> > >>> > >>>> Simple questions, eh? > >>>> > >>> Yeah, when you have a handy cheat sheet. 8^) > >>> > >>> Our very own Bill Beaty has a nice write-up on this subject at: > >>> > >>> http://amasci.com/miscon/speed.html > >>> > >> > >> Thanks, interesting document on drift. > >> > >> Terry > >> > >> > > > > > > Horace Heffner > http://www.mtaonline.net/~hheffner/ > > > >
