Peter T. Daniels ran out of electrical outlets; states rewiring entire house to be the only feasible solution.
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From: PaulJK on 31 Mar 2010 01:24 Brian M. Scott 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: No sorry, Brian, I believe we do pay sufficient attention. The thing is that what he and the below quoted "authorities" say about applicability of Ohm's law is to us a load of crock. The linearity of V-I curve has absolutely nothing to do with validity of Ohm's law. If V-I curve is non-linear it simply means that the conductor (such as a diod or a transistor) has variable resistance and the V-R and I-R curves are correspondingly non-linear. The high school boys and girls of sixteen do cope with that even before they do any calculus at a university. pjk > 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. > > 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. > > Brian
From: PaulJK on 31 Mar 2010 01:32 Hatunen 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 can view the P-N junction as a kind of an equivalent of a gap. It also breaks catastrophically when certain voltage is reached. pjk
From: Brian M. Scott on 31 Mar 2010 00:38 On Wed, 31 Mar 2010 17:24:00 +1200, PaulJK <paul.kriha(a)paradise.net.nz> wrote in <news:houigv$gc9$1(a)news.eternal-september.org> in sci.lang,alt.religion.kibology,alt.usage.english,sci.physics: > Brian M. Scott wrote: >> On Wed, 31 Mar 2010 04:57:47 +1200, PaulJK >> <paul.kriha(a)paradise.net.nz> wrote in >> <news:hot6pr$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: > No sorry, Brian, I believe we do pay sufficient attention. > The thing is that what he and the below quoted "authorities" > say about applicability of Ohm's law is to us a load of crock. > The linearity of V-I curve has absolutely nothing to do > with validity of Ohm's law. You're still not paying attention: in their framework this linearity *is* Ohm's law, so in that framework it's absurd to say that it has nothing to do with the validity of Ohm's law. In particular, when the term 'x satisfies Ohm's law' means that the V-I curve for x is linear, then it is perfectly true that not all materials are ohmic (follow Ohm's law). So far as I can tell, you and these authorities -- and scare quotes are *not* appropriate here -- do not disagree on the physical facts; you merely mean different things by the term 'Ohm's law'. If you're going to argue, you should at least argue over the actual point of difference, namely, what meaning should be assigned to the term 'Ohm's law'. [...] Brian
From: PaulJK on 31 Mar 2010 01:43 Hatunen wrote: > On Tue, 30 Mar 2010 15:46:32 -0400, "Otto Bahn" > <Ladybrrane(a)GroinToHell.com> wrote: > >> "Hatunen" <hatunen(a)cox.net> wrote >> >>>> Out of curiosity, is the resistance of charred skin the same as >>>> the resistance of normal skin? >>> >>> If it's charred I would imagine it's lower. >> >> I'd guess the absence of water would make it go up. > > Water, per se, is a pretty good insulator. Well, the flesh and blood will supply some salts and other chemicals which should make the moisture more conductive. What is most unfortunate is that the nerves are extra good conductors. Of course it's their purpose in life to conduct el-chem signals. If the electrical current reaches inside the human body it tends to predominantly run (according to the well understood and popular Ohm's law) along the nerves and does a lot of damage to the them. pjk
From: Jerry Friedman on 31 Mar 2010 00:56
On Mar 30, 11:41 am, Hatunen <hatu...(a)cox.net> wrote: > On Mon, 29 Mar 2010 10:33:01 -0400, Doctroid > > <doctr...(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. In this case, though, I don't see how you could try to model the current without J = E/rho (current density, electric field, and resistivity), the continuum version of Ohm's "Law". Certainly rho depends on position and time according to the amount of heat deposited, and maybe it's a tensor, giving a very interesting integro- partial-differential equation (a field in which my score is success 0, failure 1 [*]). But I'd say you're still using Ohm's "Law" unless conduction in charred flesh is known to be non-ohmic. [*] I was trying to numerically model pion flow in ultra-relativistic heavy-ion collisions using the Boltzmann equation with a density- dependent collision time. Another grad student in my group told me, long after I'd given up, that this is Somebody's Equation, notorious (in some circles) for intractability. The bright side was that I got to mention it in the acknowledgements of my thesis with the comment, "Le pion est l'âme des échecs." -- Jerry Friedman |