On May 7, 6:48�pm, "fisico32" <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:> Hi Jerry, > thanks for the info. I have discovered a few things this week and I would > like to hear your opinion on them: > > �Most introductory physics and EE books(except Chabay, R. and Sherwood, B. > A., 2002, Matter and Interactions II) state that a DC current carrying wire > only generates a magnetic H field outside it. No electric field E. > That is not true. There is also a (weak) electric field E caused not by > volume charges in the conductor but by surface charges developed on the > wire surface by the battery. Energy flows outside the wires towards the > load (and also inside the wire from the outside, Joule effect). > > There is a short article on current in DC circuits by Ian Sefton, physics > educator:Understanding Electricity and Circuits: What the Text Books > Don�t Tell You. It is available athttp://sydney.edu.au/science/uniserve_science/school/curric/stage6/ph... > > I later found this book entitled "Electromagnetics Explained" by Ron > Schmitt. It gives a good explanation of what is going on in a circuit > powered by a RF source. > When the circuit is closed, surface charges develop on the two conducting > wires of transmission line (for example the twin lead line). > Such surface charges on, say, the top conductor, undergo an acceleration > producing a field kink. The charges on the other conductor, the bottom one, > experience a pulse �too being so near but in the opposite direction and get > accelerated too. As they accelerate they send out their own field kink > which pushes on the charges on the top wire. There is some sort of feedback > system. The charge movement in the two wires become coupled. > The pulse goes down the transmission line towards the load. This pulse > carries energy on the surface of the wires at a speed very close to that of > light. Eventually things reach steady state and two coupled electromagnetic > surface waves run on each conductor (the signal and return conductor) in > opposite directions. > Somehow (here I am not very clear how) these two oppositely propagating > cylindrical surface waves form the TEM wave that from the generator towards > the load and travels outside and in between the wires.Energy gets then > dissipated in the load. > > Books talk about current and voltage waves moving along the transmission > line. Those current and voltage waves are derivable from the fields E and > H. There are surely longitudinal currents along the wires. But energy is > carried outside the wires. In Ian Sefton article we find that "...Arnold > Sommerfeld (1952) has pointed out, metals are good conductors of current > but nonconductors of energy. Metals conduct current but space conducts > energy and the best conductor of electromagnetic energy is the vacuum...". > �Surface charge densities on the surface of the wires cause current and > fields E, H and the energy they carry. > > Does this story make sense? > > Tonite I am going to be reading the following: > > Barlow, H.E.M, "Surface Waves: A Proposed Definition", Proc. of the IEE > Barlow and Cullen, "Surface Waves", Proc. of IEE > Brown, M.m "The Types of Waves that may Exist Near a Guiding Surface", > Proc. IEE > Goubau, G. "Surface Waves and Their Applications to Transmission lines", > J.of applied Physics > Rosser, W. G. V., 1970, Magnitudes of surface charge distributions > associated with electric current flow, American Journal of Physics, 38, 265 > - 266. > > > > > > > > >On May 7, 4:05=A0pm, Jerry Avins <j...@ieee.org> wrote: > > > �... > > >> propagates down the cable at a velocity of sqrt(1/lc).... > > >Don't misread that. it's the same as 1/sqrt(lc). > > >Jerry > >-- > >Engineering is the art of making what you want from things you can > >get.The typical explanations stand with a foot in each of two incompatible worlds. The Waves and Fields approach is the One True Way, but the same kind of overkill as using relativity to describe the trajectory of a .22 caliber bullet. According to that Ohm's law is a fiction; Lumped constant components are fictions; Useful power travels in space (look up Poynting vector), the component of that vector that is normal to a conductor represents its losses, and other weird considerations. Circuit theory, on the other hand, is very useful for describing and planning complex networks, its fundamentally fictional nature notwithstanding. In order to pick and choose explanations from these incompatible descriptions of reality, you need to know the answers in advance to know if you're making sense. Consider that we compound the fiction of circuit theory by often assuming that conductors are lossless, that inductive and capacitive coupling can be ignored. We call them "second order effects" if we consider them at all. Yes, Virginia, there is no Santa Claus. But there might as well be. Jerry -- Engineering is the art of making what you want from things you can get.
balanced and unbalanced lines...
Started by ●March 16, 2011
Reply by ●May 8, 20112011-05-08
Reply by ●May 8, 20112011-05-08
On May 7, 7:29�pm, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote: ...> The funny thing, though, is that the velocity is about what > you would get if you considered the wave as following along > the spiral wound (high inductance) center conductor.I always wondered about that. I finally settled on its being both (based on no evidence), like is an electron a wave or a particle. Jerry -- Engineering is the art of making what you want from things you can get.
Reply by ●May 8, 20112011-05-08
On 05/07/2011 05:22 PM, Fred Marshall wrote:> On 5/7/2011 3:41 PM, Tim Wescott wrote: > >> >> If you're asking about reflections on transmission lines, then you have >> to concern yourself with the entire event, not just what happens when >> everything settles out. I'm not sure what you mean by "entire epoch", >> but if you mean "from the point where we disturb the system from steady >> state until the point where we declare it settled out", then yes, the >> math is being applied to the entire epoch. >> >> If you were to take a length of lossless transmission line, terminate it >> with a resistor at it's characteristic impedance, and connect a battery >> to the other end, you'd 'see' a current step starting at the battery >> end, with no current ahead of it and V/Z_o (Z_o = characteristic >> impedance) behind. This would propagate all the way to the load resistor >> at the cable's propagation speed (which is usually better than 65% of >> the speed of light, so you have to be on your toes), at which point the >> system would be in steady state once again. >> > > Tim, > > OK. Thanks. In other posts it seems to me dangerous to talk about "DC" > and then introduce batteries with switches. The "entire epoch" is what > you figured - it just isn't steady state. Only a minor point really.If I used the term then I'm guilty of being fast and loose with it. It's hard not to be, sometimes. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●May 8, 20112011-05-08
On 5/7/2011 10:33 PM, Tim Wescott wrote:> On 05/07/2011 05:22 PM, Fred Marshall wrote: >> On 5/7/2011 3:41 PM, Tim Wescott wrote: >> >>> >>> If you're asking about reflections on transmission lines, then you have >>> to concern yourself with the entire event, not just what happens when >>> everything settles out. I'm not sure what you mean by "entire epoch", >>> but if you mean "from the point where we disturb the system from steady >>> state until the point where we declare it settled out", then yes, the >>> math is being applied to the entire epoch. >>> >>> If you were to take a length of lossless transmission line, terminate it >>> with a resistor at it's characteristic impedance, and connect a battery >>> to the other end, you'd 'see' a current step starting at the battery >>> end, with no current ahead of it and V/Z_o (Z_o = characteristic >>> impedance) behind. This would propagate all the way to the load resistor >>> at the cable's propagation speed (which is usually better than 65% of >>> the speed of light, so you have to be on your toes), at which point the >>> system would be in steady state once again. >>> >> >> Tim, >> >> OK. Thanks. In other posts it seems to me dangerous to talk about "DC" >> and then introduce batteries with switches. The "entire epoch" is what >> you figured - it just isn't steady state. Only a minor point really. > > If I used the term then I'm guilty of being fast and loose with it. It's > hard not to be, sometimes. >Tim, Yeah. I'm a contradiction that way. I tend to be *very* literal and yet like to use "arm waving" terms when it makes sense to me. I just hope I'm clear when I'm doing that.... I know I'm a PITA with the literal bent at times. Fred
Reply by ●May 8, 20112011-05-08
On 05/08/2011 09:28 AM, Fred Marshall wrote:> On 5/7/2011 10:33 PM, Tim Wescott wrote: >> On 05/07/2011 05:22 PM, Fred Marshall wrote: >>> On 5/7/2011 3:41 PM, Tim Wescott wrote: >>> >>>> >>>> If you're asking about reflections on transmission lines, then you have >>>> to concern yourself with the entire event, not just what happens when >>>> everything settles out. I'm not sure what you mean by "entire epoch", >>>> but if you mean "from the point where we disturb the system from steady >>>> state until the point where we declare it settled out", then yes, the >>>> math is being applied to the entire epoch. >>>> >>>> If you were to take a length of lossless transmission line, >>>> terminate it >>>> with a resistor at it's characteristic impedance, and connect a battery >>>> to the other end, you'd 'see' a current step starting at the battery >>>> end, with no current ahead of it and V/Z_o (Z_o = characteristic >>>> impedance) behind. This would propagate all the way to the load >>>> resistor >>>> at the cable's propagation speed (which is usually better than 65% of >>>> the speed of light, so you have to be on your toes), at which point the >>>> system would be in steady state once again. >>>> >>> >>> Tim, >>> >>> OK. Thanks. In other posts it seems to me dangerous to talk about "DC" >>> and then introduce batteries with switches. The "entire epoch" is what >>> you figured - it just isn't steady state. Only a minor point really. >> >> If I used the term then I'm guilty of being fast and loose with it. It's >> hard not to be, sometimes. >> > > Tim, > > Yeah. I'm a contradiction that way. I tend to be *very* literal and yet > like to use "arm waving" terms when it makes sense to me. I just hope > I'm clear when I'm doing that.... I know I'm a PITA with the literal > bent at times. > > Fredhttp://xkcd.com/895/ -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●May 8, 20112011-05-08
