I remember reading a thread in which our late beloved Q.E.D. said that the speed of current energy in a conductor was limited to the speed of light in the insulation. Can someone explain this again or provide a link to that thread. I’ve searched on electricity, light, and Q.E.D., but there are so many matches that I’m lost.
I couldn’t find the thread with Q.E.D. in it though.
Wow, that’s an old thread. I don’t even remember writing those posts. Too many beers ago…
Well an EM field propagates as the inverse square root of the electric permittivity (e) and magnetic permeability (u). Those two values change based on the material the field is flowing through. For a vacuum you get c[sub]0[/sub], for other materials you get a fractional value of c.
Since the energy is transported by the EM field the rate of transport is dependent on the fields propagation velocity. Check out Poynting Vector which describes W/m[sup]2[/sup] of an EM field.
Thanks for the link, tr0psn4j. I’ll read that thread, and it probably describes the point, but I remember reading this within two years ago.
Whoa, that may be it, HKF. Thanks for that.
Best info on the web for really understanding electicity.
“Current energy” is nonsensical. You can talk about current, (charge flow) or energy flow, but these are different things. It is the energy, traveling as a electromagnetic wave, that is limited by the insulation properties.
Current is the rate of flow of charge…so many (one columb) electrons passing a point in 1 second=1 Ampere.
The average speed of these electrons is sloooooow though. On the order of 1mm per second or even less. Depends on current density in the wire. Bill Beaty has an article (link above) that details this.
If this seems wrong, consider this analogy:
Suppose you had a bunch of balls lined up. If you kick the one on the end, it will hit the next, and so forth. Eventually the ball at the other end will move from the energy of your kick, but no single ball moved the whole distance. The “wave” from your kick can move much faster than the balls.
Would someone try to explain to me why this would be true?
Wavelength is defined as speed of propagation / frequency.
If the speed of propagation in your antenna is different (less) than c[sub]0[/sub], then the wavelength *in the antenna *must be affected.
What happened to Q.E.D.?! (I just tried a keyword search, but his username is too short…)
Agreed. Back when I was teaching this material for an introductory physics course, I found this website to be invaluable. I particularly liked Bill’s analogy for circuits using a clothesline and pulleys here, as well as his analogy for capacitors.
I used both analogies extensively in my course, and both seemed to work well. Bill’s website certainly helped reinforce my basic conceptual understanding of electricity.
He died, unfortunately.
Well, I’m shocked. I had no idea that he’d died. I seem to recall that he’d been suspended for something or other a few months ago, and wondered if he’d ever returned after that.
I should read MPSIMS more often, I suppose, though I think that his passing was neither mundane nor pointless.
Rest in peace, Q.E.D. You will be missed.
So the frequency is fixed, but the speed is variable, making the wavelength also variable?
I messaged a mod to ask if we could get the announcement stickied in this forum so more of the people who knew him will see it. He had a big impact on this board, especially in GQ.
It’s an enormous board; nobody except the chronically unemployed can read the entire thing. You needn’t feel embarrassed for having overlooked that thread. IIRC, it was up for several days before I spotted it.
Astonishing—where has this been all my life? This page is to electricity what Brian Greene’s books are to theoretical physics—an explanation that I can at least partially freakin’ understand. Ten minutes spent on this site has revealed answers to questions which have been pestering me since before I was old enough to shave. You just got on my Christmas card list. I might even enclose a dollar if you’ve been good. A million thanks.
It’s one of the older sites on the web, and still wins awards.
I’m pretty sure I found my way to the Dope from a link on Bill Beaty’s site. He was apparently a doper, maybe still is.
Yes. The actual length you’d need would depend on the thickness of the PVC, from close to the vacuum value for a very thin layer, to the value Q.E.D. gave for the wire embedded in a large volume of PVC. For intermediate thicknesses, it would be kind of an average value (there’s no simple formula.)
Holy shit. I was completely unaware of this until I happened on this thread.
I’m really bummed out now, he was one of my favourite posters.
He also sometimes checks page traffic stats and finds mysterious traffic peaks coming from particular famous websites.
thanks for that great site!