Peter T. Daniels ran out of electrical outlets; states rewiring entire house to be the only feasible solution.
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From: jimp on 30 Mar 2010 13:15 "Peter Duncanson (BrE)" <mail(a)peterduncanson.net> wrote: > On Tue, 30 Mar 2010 16:11:11 -0000, jimp(a)specsol.spam.sux.com wrote: > >>In sci.physics "Peter Duncanson (BrE)" <mail(a)peterduncanson.net> wrote: >>> On Tue, 30 Mar 2010 17:57:10 +1200, "PaulJK" >>> <paul.kriha(a)paradise.net.nz> wrote: >>> >>>> >>>>At every point in time and at every point inside the body >>>>of a resistive conductor the Ohm's law applies with 100% >>>>accuracy. It is applicable even when the conductors are >>>>never completely homogeneous and never absolutely linear. >>>> >>>>(Excepting conditions in the super conductive environment.) >>> >>> That assumes that the current results from a potential difference, a >>> voltage. >> >>Wrong, R=V/I is a definition. >> >>> If a closed circuit, a resistive ring for instance, is put in a varying >>> magnetic field an induced current will flow but no potential difference >>> will be created. >> >>True if, and only if, the "resistive" ring is an ideal superconductor >>with zero resistivity, otherwise there will be a potential difference >>between any two given points on the ring. >> > Are you absolutely sure of that? Yes. > If the resistive ring is of uniform thickness and electrical properties > and is uniformly "bathed" in a varying magnetic field how is one to > determine which two points to consider? > > If the ring is circular and you take two points at the opposite ends of > a diameter (A and B) and the current flows A -> B -> A ... what happens? > If there is a potential difference between A and B, say Vab, there is > also through the other half of the ring a potential difference Vba. If you could arrange to have such a field, and that is a big if, there would be no net current flow, which means I=0 and the whole thing is moot. What happens on the atomic scale is different, but R=V/I is a macro world relationship that doesn't address things like isolated electron movement. > It is many years since I considered anything like this but I think that > mneans that Vab = -Vba. The only obvious way in which that can be true > is Vab = Vba = 0. > > Obviously, attempting to measure any potential difference between two > points will modify the conditions and my result in a non-zero > measurement if the points are not diametrically opposite. What you are describing in the real world is called a transformer with a shorted winding and the smoke arising from such a situation provides visual confirmation the relationships R=V/I and P=I^2/R hold. -- Jim Pennino Remove .spam.sux to reply.
From: Hatunen on 30 Mar 2010 13:40 On Sun, 28 Mar 2010 20:18:39 -0400, dbd(a)gatekeeper.vic.com (David DeLaney) wrote: >PaulJK <paul.kriha(a)paradise.net.nz> wrote: >>Doctroid wrote: >>> "PaulJK" <paul.kriha(a)paradise.net.nz> wrote: >>>> Voltage and current are proportional to each other. >>>> It is therefore sufficient to say that damage is proportional >>>> to one of them. >>> >>> Only in materials and voltage/current ranges where Ohm's "law" is >>> obeyed. And if damage is occurring, it probably isn't. >> >>Ohm's law is always obeyed in all aparatus made by humans. > >... ... Okay, so you're not an engineer AND are not an experimentalist. >Meaning you're a theorist. This explains some things. > >(Tell the court, please, how long ago it was that humans made the first >material that had exactly the same resistance at every interior point, and >how to get current to go through an object in such a way as to have the same >current density everywhere inside the object...) As taught in high school science classes, Ohm's Law is the familiar V = IR. but in the real world Ohm's Law is valid for all kinds of cases of non-constant factors, writte in lower case, v = ir to indicate the values are non-constant. In gneral, Ohm's law is expressed e = iz, where, as I note in another post, z is impedance, which can be resistive or reactive and therefore contain an imaginary component, and e is the term usually used for "voltage". In fact, in the generic case the for Ohm's Law is a differential equation with time dependency. Obviously, some cases will be intractable to solution, but the fact that we can't solve it doesn't mean Ohm's Law doesn't remain true; there are approximation methods that can yield useful results. And, yes, Ohm's law is applicable to bulk materials as well as the idealized linear forms. This is all quite practical and not theoretical out in the real world. There are qutie a few eingineers that cope with this sort of stuff on a daily basis. In other word, Ohm's Law is always true and you should avoid making such statements unless you know what you're talking about. -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps *
From: Hatunen on 30 Mar 2010 13:41 On Mon, 29 Mar 2010 10:33:01 -0400, Doctroid <doctroid(a)mailinator.com> wrote: >And again, under conditions where the electric current is causing tissue >damage, Ohm's "law" doesn't really apply. Oh, it always applies. But it may not be terribly useful in some cases. -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps *
From: Hatunen on 30 Mar 2010 13:42 On Mon, 29 Mar 2010 12:58:25 -0400, Doctroid <doctroid(a)mailinator.com> wrote: >In article <hoql5j$rn5$1(a)news.albasani.net>, > "Skitt" <skitt99(a)comcast.net> wrote: > >> Doctroid wrote: >> > Peter Moylan wrote: >> >> PaulJK wrote: >> >>> Doctroid wrote: >> >>>> "PaulJK" wrote: >> >> >>>>> Voltage and current are proportional to each other. >> >>>>> It is therefore sufficient to say that damage is proportional >> >>>>> to one of them. >> >>>> Only in materials and voltage/current ranges where Ohm's "law" is >> >>>> obeyed. And if damage is occurring, it probably isn't. >> >>> >> >>> Ohm's law is always obeyed in all aparatus made by humans. >> >>> >> >> Except when it isn't - which is most of the time. >> >> >> >> It's true that a great many resistors are nearly linear, i.e. they >> >> can be said to obey Ohm's law to a reasonable degree of accuracy, >> >> provided that you don't go beyond their design limits. >> >> >> >> It's equally true that some resistive devices are *deliberately* >> >> made to be nonlinear. A semiconductor diode, for example, would be >> >> totally useless if it obeyed Ohm's law. >> > >> > Besides which, the subject under discussion was not human-made >> > apparatus, but humans. >> > >> > And again, under conditions where the electric current is causing >> > tissue damage, Ohm's "law" doesn't really apply. >> >> Ohms law applies quite well. It is the encountered resistance that is >> changing as the skin (of much higher resistance than the mushy and moist >> innards) is damaged. > >Your argument is circular. If you define "resistance" as the ratio of >voltage to current, possibly varying with time, voltage, or current, >then sure, Ohm's Law works; but then Ohm's Law becomes a tautology, >trivially satisfied by everything and predicting nothing. The "law" is >a useful law only in circumstances where voltage and current do not >alter the electrical properties of the material; then it becomes an >observation that current is proportional to voltage for all voltages and >currents within some large range of applicability. That observation is >not valid for the situation described here. You really don't know what you're talking about. -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps *
From: Hatunen on 30 Mar 2010 13:55
On Mon, 29 Mar 2010 14:55:32 -0400, "Otto Bahn" <Ladybrrane(a)GroinToHell.com> wrote: >"Doctroid" <doctroid(a)mailinator.com> wrote > >>> Resistance is the opposition offered by a body or substance to the >>> passage >>> through it of an electric current. >> >> Quantitative definition, please, not just a description. Here is a >> 1N4002 diode: How would you determine its resistance? > >Slowly increase the voltage across it until it goes "pop". >It's resistance is now pretty much infinite for any value >of voltage you're likely to apply. "Pretty much infinite". Is that an engineering term? That diode is designed for a maximum peak voltage of 100v (basically the maximum applied voltage before the heat generated in the non-infinite resistance will burn up the diode). From the data sheet: Ratings at 25 C ambient temperature unless otherwise specified. Single phase half-wave 60Hz, resistive or inductive load, for capacitive load current derate by 20%. * Maximum repetitive peak reverse voltage: VRRM 100 VOLTS * Maximum RMS voltage: VRMS 70 VOLTS * Maximum DC blocking voltage: VDC 100 VOLTS * Maximum average forward rectified current 0.375" (9.5mm) lead length at TA = 75�C: I(AV) 1.0 Amp * Peak forward surge current 8.3ms single half sine-wave superimposed on rated lead ( JEDEC Method) IFSM 30.0 Amps * Maximum instantaneous forward voltage at 1.0A: VF 1.1 Volts * Maximum DC reverse current at rated DC blocking voltage TA=25�C: IR 5.0 �A and TA=100�C: IR 50.0 �A * Typical junction capacitance (NOTE1): CJ 15.0 pF * Typical thermal resistance (NOTE2): R0JA 50.0 �C/W * Operating junction and storage temperature range: TJ,TSTG -65 to +175 �C Note: 1. Measured at 1MHz and applied reverse voltage of 4.V D.C. 2. Thermal resistance from junction to ambient at 0.375" (9.5mm) lead length, PCB mounted -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps * |