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From: barbara on 30 Mar 2010 14:06 On Tue, 30 Mar 2010 17:05:07 -0000, jimp(a)specsol.spam.sux.com wrote: >In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: >> In article <g2k987-ktn.ln1(a)mail.specsol.com>, jimp(a)specsol.spam.sux.com >> wrote: >> >>> In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: >>> > In article <7uq3r59afg17p63gsvitp4qbm6uh3032f8(a)4ax.com>, >>> > barbara(a)bookpro.com wrote: >>> > >>> >> On Tue, 30 Mar 2010 18:26:04 +1200, "PaulJK" >>> >> <paul.kriha(a)paradise.net.nz> wrote: >>> >> >>> >> >A homework for Doctroid and P.Moylan >>> >> >http://www.allaboutcircuits.com/worksheets/diode1.html >>> >> > >>> >> >just trying to help you to stop embarrassing yourself with >>> >> >high school physics. >>> >> >>> >> P'raps you should ask Doctroid what he is a doctroid of. >>> >> >>> >> BW >>> > >>> > See, the thing about high school physics is, it's taught by high school >>> > teachers out of high school textbooks to high school students. >>> > >>> > High school physics teachers usually do not have advanced training in >>> > physics. Often neither do the authors of high school physics textbooks. >>> > Even when they do, they recognize the necessity of presenting a >>> > simplified picture to high school students, who are not ready for >>> > differential equations, surface integrals, and deep questions about the >>> > meaning of physical law. >>> > >>> > So what gets taught is "Ohm's law is V = IR". The ones who go on to >>> > earn degrees in physics are taught there's more to it than that. (And >>> > some of them learn it.) The ones who major in English Lit or Business >>> > or Electrical Engineering? Might not. >>> >>> Since R=V/I is a definition, care to elaborate on when the relationship >>> does not hold? >>> >>> Your Nobel awaits. >> >> I refuse to enter into further public discussion with people with no >> reading comprehension skills. As before, you can write me at rsholmes >> dot physics dot syr dot edu, if you are genuinely interested in learning. >> > >From Fundementals of Physics by Halliday and Resnick: > >"The relationship V = i/R remains as the derfinition of the resistance of >a conductor whether or not the conductor obeys Ohm's law." So the conductor might not obey Ohm's law. Some of these mokes have been claiming that everything always obeys Ohm's law. BW
From: Brian M. Scott on 30 Mar 2010 14:09 On Tue, 30 Mar 2010 17:05:07 -0000, <jimp(a)specsol.spam.sux.com> wrote in <news:3jn987-38o.ln1(a)mail.specsol.com> in sci.lang,alt.religion.kibology,alt.usage.english,sci.physics: > In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: >> In article <g2k987-ktn.ln1(a)mail.specsol.com>, jimp(a)specsol.spam.sux.com >> wrote: >>> In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: [...] >>> > So what gets taught is "Ohm's law is V = IR". The ones who go on to >>> > earn degrees in physics are taught there's more to it than that. (And >>> > some of them learn it.) The ones who major in English Lit or Business >>> > or Electrical Engineering? Might not. >>> Since R=V/I is a definition, care to elaborate on when the relationship >>> does not hold? >>> Your Nobel awaits. >> I refuse to enter into further public discussion with people with no >> reading comprehension skills. As before, you can write me at rsholmes >> dot physics dot syr dot edu, if you are genuinely interested in learning. > From Fundementals of Physics by Halliday and Resnick: > "The relationship V = i/R remains as the derfinition of > the resistance of a conductor whether or not the > conductor obeys Ohm's law." Which clearly implies that V = I/R is *not* Ohm's law -- exactly as Doctroid said. My understanding, which could well be wrong, is that a conductor obeys Ohm's law precisely when a plot of V against I is linear, meaning that the resistance, as defined by R = I/V, is a constant independent of V and I. [...] Brian
From: Otto Bahn on 30 Mar 2010 14:11 "Hatunen" <hatunen(a)cox.net> 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? Yes, when we don't feel like calculating at what voltage arcing across the gap might occur. --oTTo--
From: barbara on 30 Mar 2010 14:15 On Tue, 30 Mar 2010 17:05:07 -0000, jimp(a)specsol.spam.sux.com wrote: >In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: >> In article <g2k987-ktn.ln1(a)mail.specsol.com>, jimp(a)specsol.spam.sux.com >> wrote: >> >>> In sci.physics Doctroid <doctroid(a)mailinator.com> wrote: >>> > In article <7uq3r59afg17p63gsvitp4qbm6uh3032f8(a)4ax.com>, >>> > barbara(a)bookpro.com wrote: >>> > >>> >> On Tue, 30 Mar 2010 18:26:04 +1200, "PaulJK" >>> >> <paul.kriha(a)paradise.net.nz> wrote: >>> >> >>> >> >A homework for Doctroid and P.Moylan >>> >> >http://www.allaboutcircuits.com/worksheets/diode1.html >>> >> > >>> >> >just trying to help you to stop embarrassing yourself with >>> >> >high school physics. >>> >> >>> >> P'raps you should ask Doctroid what he is a doctroid of. >>> >> >>> >> BW >>> > >>> > See, the thing about high school physics is, it's taught by high school >>> > teachers out of high school textbooks to high school students. >>> > >>> > High school physics teachers usually do not have advanced training in >>> > physics. Often neither do the authors of high school physics textbooks. >>> > Even when they do, they recognize the necessity of presenting a >>> > simplified picture to high school students, who are not ready for >>> > differential equations, surface integrals, and deep questions about the >>> > meaning of physical law. >>> > >>> > So what gets taught is "Ohm's law is V = IR". The ones who go on to >>> > earn degrees in physics are taught there's more to it than that. (And >>> > some of them learn it.) The ones who major in English Lit or Business >>> > or Electrical Engineering? Might not. >>> >>> Since R=V/I is a definition, care to elaborate on when the relationship >>> does not hold? >>> >>> Your Nobel awaits. >> >> I refuse to enter into further public discussion with people with no >> reading comprehension skills. As before, you can write me at rsholmes >> dot physics dot syr dot edu, if you are genuinely interested in learning. >> > >From Fundementals of Physics by Halliday and Resnick: > >"The relationship V = i/R remains as the derfinition of the resistance of >a conductor whether or not the conductor obeys Ohm's law." So the conductor might not obey Ohm's law. Some of these mokes have been claiming that everything always obeys Ohm's law. BW
From: Jerry Friedman on 30 Mar 2010 14:17
On Mar 29, 12:52 pm, "Skitt" <skit...(a)comcast.net> wrote: > Doctroid wrote: > > "Skitt" wrote: > >> Doctroid wrote: > >>> "Skitt" wrote: > >>>> Doctroid wrote: > >>>>> "Skitt" 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. > > >>>> Ohm's law describes the relationship between electrical current, > >>>> resistance, and potential in a circuit. That's it. Any member of > >>>> that circuit can vary at any given time and place, but the defined > >>>> relationship remains intact, forcing a change in another member. > > >>> If that is so, then please tell me how you define "resistance". > > >> 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? > > Ohm's Law does not provide a "quantitative definition". As I said, it > describes a relationship. When values of two of the variables are provided, > the value of the third can be obtained. I'd have said that Ohm's Law /is/ the quantitative definition of resistance. For objects in which current is proportional to voltage (with other things, such as the temperature, held fixed), the resistance is V/I. For others, such as diodes, the resistance can't be defined. -- Jerry Friedman |