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From: Evan Kirshenbaum on 30 Mar 2010 20:38 Hatunen <hatunen(a)cox.net> writes: > On Tue, 30 Mar 2010 14:11:42 -0400, "Otto Bahn" > <Ladybrrane(a)GroinToHell.com> wrote: > >>"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. > > There's no gap in a semiconductor diode.... Even after it's gone "pop"? -- Evan Kirshenbaum +------------------------------------ HP Laboratories |Never ascribe to malice that which 1501 Page Mill Road, 1U, MS 1141 |can adequately be explained by Palo Alto, CA 94304 |stupidity. kirshenbaum(a)hpl.hp.com (650)857-7572 http://www.kirshenbaum.net/
From: Brian M. Scott on 30 Mar 2010 21:52 On Tue, 30 Mar 2010 14:57:47 -0700, Hatunen <hatunen(a)cox.net> wrote in <news:2ps4r551hvchmctqaesvkufvudo300f1bt(a)4ax.com> in sci.lang,alt.religion.kibology,alt.usage.english,sci.physics: > On Tue, 30 Mar 2010 14:38:32 -0400, "Brian M. Scott" > <b.scott(a)csuohio.edu> wrote: >>On Wed, 31 Mar 2010 04:57:47 +1200, PaulJK >><paul.kriha(a)paradise.net.nz> wrote in >><news:hot6pr$fal$1(a)news.eternal-september.org> in >>sci.lang,alt.religion.kibology,alt.usage.english,sci.physics: >>> J. Clarke wrote: >>>> On 3/30/2010 10:27 AM, Doctroid wrote: >>[...] >>>>> Zener diode: >>>>> http://www.reuk.co.uk/OtherImages/current-voltage-graph-zener-diode.gif >>> Well? The semiconductors exhibit highly nonlinear >>> relationship between voltage and their resistance >>> resulting in nonlinear relationship between voltage and >>> the current. >>It seems to me that you (and several others) aren't paying >>attention to what Doctroid is saying: this clearly means >>that they don't obey Ohm's law as Doctroid (and, I might >>add, Halliday & Resnick) use the term. I quote H&R (1967): >> A conductor obeys Ohm's law only if its V-I curve is >> linear, that is, if R is independent of V and I. The >> relationship R = V/I remains as the general definition of >> the resistance of a conductor whether or not the >> conductor obeys Ohm's law. >>Even my old text, Kingsbury (1965), which isn't nearly so >>good as H&R, says the same thing: >> Thus one may say that [V = IR] is a defining equation >> for resistance, R, and that Ohm's law is a statement >> that resistance is independent of the magnitude of the >> current. Ohm's law is thus similar to Hooke's law in >> that it is valid for certain materials under certain >> limited conditions. > Hooke's law is the basis of all elasticity effects, and like > Ohm's Law can be applied to bulk materials. The fact that you > have to use the calculus to describe it doesn't alter that fact. > I'm beginning to think that engineers know things that physics > professors and authors don't. >>From a more recent text, Paul A. Tipler, _Physics for >>Scientists and Engineers_: >> Ohm's law is not a fundamental law of nature, like >> Newton's laws or the laws of thermodynamics, but >> rather an empirical description of a property shared >> by many materials. > Quite. And I doubt it would be called a "law" if Ohm had come up > with it today. Even Newton's Laws are no longer considered laws. And none of what you've said here is to the point.
From: Al in St. Lou on 30 Mar 2010 22:22 On Tue, 30 Mar 2010 14:52:26 -0700, Hatunen <hatunen(a)cox.net> wrote: >On Tue, 30 Mar 2010 14:11:42 -0400, "Otto Bahn" ><Ladybrrane(a)GroinToHell.com> wrote: > >>"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. > >There's no gap in a semiconductor diode.... You're forgetting his earlier post where he burnt it out: "...increase the voltage until it goes 'Pop.'" -- Al in St. Lou
From: PaulJK on 31 Mar 2010 00:57 Doctroid wrote: > In article <hos036$3mk$1(a)news.eternal-september.org>, > "PaulJK" <paul.kriha(a)paradise.net.nz> wrote: > >> Peter Moylan wrote: >>> PaulJK wrote: >>>> Doctroid wrote: >>>>> In article <homphq$91l$1(a)news.eternal-september.org>, >>>>> "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. >>>> >>> 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. >> >> I see you didn't have Ohm's law in your high school? >> What has nonlinearity to do with it? Absolutely nothing. >> >> 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.) >> pjk > > > I think the problem you are having is that you think Ohm's Law is "V = > IR". > > It's not. > > Ohm's law is the statement "There is a quantity R, defined by the > relation V = IR, which is independent of V and I." Often one says "V = > IR" is Ohm's law, because that's a convenient shorthand; but that > equation is merely a definition of R, and has no real content in the > absence of an observation that R doesn't depend on V and I. > > Which is true, for many materials, over a wide range of V and I, to a > good approximation; but not for all materials in all circumstances to > infinite precision. All materials obey V = IR, by the definition of R, > but not all materials obey Ohm's Law. I remember when the primary school teacher taught us to add and subtract numbers. We were not yet told about negative numbers so all subtractions were done with smaller numbers subtracted from the larger ones. Once the teacher made a mistake and wrote on the black board something like 13 - 18 = ? The class cried out: "No, no, no, you can't do that!", "Sorry" said teacher, "my mistake, you cannot subtract eighteen from thirteen". A couple of months later we were happily subtracting larger numbers from smaller ones and never looked back. I wonder how many months of study it would take you to stop parroting nonsense like "not all materials obey Ohm's Law". pjk > Anyone wishing to take this up with me further may write to me at > rsholmes at physics dot syr dot edu.
From: PaulJK on 31 Mar 2010 01:05
Otto Bahn wrote: > <barbara(a)bookpro.com> 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. > > Just because there's a few misbehaving conductors doesn't mean > the rest of us shouldn't obey Ohm's law. But it applies even to the conductor of Boston Philharmonic. pjk |