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From: Jerry Friedman on 30 Mar 2010 17:59 On Mar 30, 3:54 pm, Hatunen <hatu...(a)cox.net> wrote: > On Tue, 30 Mar 2010 11:17:42 -0700 (PDT), Jerry Friedman > > > > <jerry_fried...(a)yahoo.com> wrote: > >On Mar 29, 12:52 pm, "Skitt" <skit...(a)comcast.net> wrote: .... > >> 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. > > But it's done all the time, nevertheless. It has to be treated > infinitesimally, though, with calculus. People define resistance as the slope of the V-versus-I curve? What for, situations where you're going to be changing voltage only slightly and you want to approximate the diode as a resistor? -- Jerry Friedman
From: Richard Chambers on 30 Mar 2010 18:03 "Hatunen" <hatunen(a)cox.net> wrote in message news:nvo4r51gd2sfrjegh1q23jpkefbn1f8jmr(a)4ax.com... > On Tue, 30 Mar 2010 14:15:12 -0400, barbara(a)bookpro.com wrote: > >>>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. > > Even though the relationship remains as the definition. That > doesn't make much sense. > >>Some of these mokes have >>been claiming that everything always obeys Ohm's law. > > It is too bad that the authors didn't give an example of a > conductor that doesn't "obey" [*] Ohm's Law. I suspect it may > turn out to be less than it appears. We need to remind ourselves of what Ohm's Law really does state. From http://www.answers.com/topic/ohm-s-law we have the definition/explanation of the law as follows:- "The law stating that the direct current flowing in a conductor is directly proportional to the potential difference between its ends. It is usually formulated as V = IR, where V is the potential difference, or voltage, I is the current, and R is the resistance of the conductor." If this is Ohm's Law, then it is patently incorrect for many materials in which a current can flow. For example, a semiconductor diode operated within the range 0 to approximately 0.3V does not obey Ohm's Law as stated above. The current is related to the voltage, within this range, by the formula I = constant * V^n, (i.e. V to the power of n), where n is a number ranging from 2 to 4 depending upon the nature of the semiconductor. For a second example, a 4-volt semiconductor zener diode does not obey the above form of Ohm's Law if operated at voltages within the range 0 to 4.05 Volts. A very low current will result from any voltage in the range 0 to 4 volts, but the current will be dramatically higher than this for voltages in the range 4 to 4.05 volts. Ohm's Law is of limited truth for materials such as these. Why, therefore, has the law survived, and why is it still in use? It has survived because it is a useful desciption of current flow in a metallic conductor, and because metallic conductor are the most frequently studied types of conductor. All professionals know the limitations of Ohm's Law, and ensure that they use the law only when appropriate. Even with metallic conductors, Ohm's Law contains a hidden danger to truth. Electrical resistance changes with temperature. If we are studying the tungsten element of a light bulb, the resistance changes as the element gets hotter and starts to glow red, yellow, then white. If (for a 110-Volt lightbulb) we measure the relationship between current and voltage from 0 Volts to 10 Volts, we cannnot then linearly extrapolate our result to predict what the current will be at 110 Vo;ts. This finding is contrary to Ohm's Law, which claims that we ought to be able to extrapolate in this way. To attempt to apply Ohm's Law to a semiconducting diode is to waste one's time. It does not help one to analyse the behavious of the diode. Yes, you can define the resistance of the diode as V/I, but where does that get you if the value of resistance thus obtained is not stable? Furthermore, the absolute value of resistance, V/I, is not the same value as the incremental (or marginal) resistance, dV/dI. Any fool can divide a measurement of Voltage V by a corresponding measurement of current I, and call the result of this division the "Resistance R". Another contributor, earlier in this thread, has mentioned that any such analysis will usually lead to a circular argument, and be of little value. Richard Chambers Leeds UK.
From: Hatunen on 30 Mar 2010 18:22 On Tue, 30 Mar 2010 23:03:47 +0100, "Richard Chambers" <richard.chambers7_NoSpam_(a)ntlworld.net> wrote: > >"Hatunen" <hatunen(a)cox.net> wrote in message >news:nvo4r51gd2sfrjegh1q23jpkefbn1f8jmr(a)4ax.com... >> On Tue, 30 Mar 2010 14:15:12 -0400, barbara(a)bookpro.com wrote: >> >>>>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. >> >> Even though the relationship remains as the definition. That >> doesn't make much sense. >> >>>Some of these mokes have >>>been claiming that everything always obeys Ohm's law. >> >> It is too bad that the authors didn't give an example of a >> conductor that doesn't "obey" [*] Ohm's Law. I suspect it may >> turn out to be less than it appears. > >We need to remind ourselves of what Ohm's Law really does state. From > http://www.answers.com/topic/ohm-s-law >we have the definition/explanation of the law as follows:- >"The law stating that the direct current flowing in a conductor is >directly proportional to the potential difference between its ends. It is >usually formulated as V = IR, where V is the potential difference, or >voltage, I is the current, and R is the resistance of the conductor." I prefer not to use dictionaries for definitions of technical and scientific terms. Wikipedia is closer: "In electrical circuits, Ohm's law states that the current through a conductor between two POINTS is directly proportional to the potential difference or voltage across the two POINTS, and inversely proportional to the resistance between them, provided that the temperature remains constant."[Emphasis added] http://en.wikipedia.org/wiki/Ohm%27s_law Note that this definition does not require the points to be external. >If this is Ohm's Law, then it is patently incorrect for many materials in >which a current can flow. For example, a semiconductor diode operated >within the range 0 to approximately 0.3V does not obey Ohm's Law as stated >above. The current is related to the voltage, within this range, by the >formula I = constant * V^n, (i.e. V to the power of n), where n is a >number ranging from 2 to 4 depending upon the nature of the semiconductor. >For a second example, a 4-volt semiconductor zener diode does not obey the >above form of Ohm's Law if operated at voltages within the range 0 to 4.05 >Volts. A very low current will result from any voltage in the range 0 to 4 >volts, but the current will be dramatically higher than this for voltages >in the range 4 to 4.05 volts. > >Ohm's Law is of limited truth for materials such as these. Why, therefore, >has the law survived, and why is it still in use? It has survived because >it is a useful desciption of current flow in a metallic conductor, and >because metallic conductor are the most frequently studied types of >conductor. All professionals know the limitations of Ohm's Law, and ensure >that they use the law only when appropriate. > >Even with metallic conductors, Ohm's Law contains a hidden danger to >truth. Electrical resistance changes with temperature. If we are studying >the tungsten element of a light bulb, the resistance changes as the >element gets hotter and starts to glow red, yellow, then white. If (for a >110-Volt lightbulb) we measure the relationship between current and >voltage from 0 Volts to 10 Volts, we cannnot then linearly extrapolate our >result to predict what the current will be at 110 Vo;ts. This finding is >contrary to Ohm's Law, which claims that we ought to be able to >extrapolate in this way. > >To attempt to apply Ohm's Law to a semiconducting diode is to waste one's >time. It does not help one to analyse the behavious of the diode. Yes, you >can define the resistance of the diode as V/I, but where does that get you >if the value of resistance thus obtained is not stable? Furthermore, the >absolute value of resistance, V/I, is not the same value as the >incremental (or marginal) resistance, dV/dI. > >Any fool can divide a measurement of Voltage V by a corresponding >measurement of current I, and call the result of this division the >"Resistance R". Another contributor, earlier in this thread, has mentioned >that any such analysis will usually lead to a circular argument, and be of >little value. Most people wh do these things aren't arguing the philophy of it all; they have work to do with the results. Current and voltage are not arbitrary but are well defined in terms other than V = IR. This makes resistance the derived quantity with no circularity involved save for the kid in the back of the classroom who won't accept the teacher's explanation and keeps picking arguments. -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps *
From: barbara on 30 Mar 2010 18:25 On Tue, 30 Mar 2010 23:03:47 +0100, "Richard Chambers" <richard.chambers7_NoSpam_(a)ntlworld.net> wrote: > >"Hatunen" <hatunen(a)cox.net> wrote in message >news:nvo4r51gd2sfrjegh1q23jpkefbn1f8jmr(a)4ax.com... >> On Tue, 30 Mar 2010 14:15:12 -0400, barbara(a)bookpro.com wrote: >> >>>>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. >> >> Even though the relationship remains as the definition. That >> doesn't make much sense. >> >>>Some of these mokes have >>>been claiming that everything always obeys Ohm's law. >> >> It is too bad that the authors didn't give an example of a >> conductor that doesn't "obey" [*] Ohm's Law. I suspect it may >> turn out to be less than it appears. > >We need to remind ourselves of what Ohm's Law really does state. From > http://www.answers.com/topic/ohm-s-law >we have the definition/explanation of the law as follows:- >"The law stating that the direct current flowing in a conductor is >directly proportional to the potential difference between its ends. It is >usually formulated as V = IR, where V is the potential difference, or >voltage, I is the current, and R is the resistance of the conductor." > >If this is Ohm's Law, then it is patently incorrect for many materials in >which a current can flow. For example, a semiconductor diode operated >within the range 0 to approximately 0.3V does not obey Ohm's Law as stated >above. The current is related to the voltage, within this range, by the >formula I = constant * V^n, (i.e. V to the power of n), where n is a >number ranging from 2 to 4 depending upon the nature of the semiconductor. >For a second example, a 4-volt semiconductor zener diode does not obey the >above form of Ohm's Law if operated at voltages within the range 0 to 4.05 >Volts. A very low current will result from any voltage in the range 0 to 4 >volts, but the current will be dramatically higher than this for voltages >in the range 4 to 4.05 volts. > >Ohm's Law is of limited truth for materials such as these. Why, therefore, >has the law survived, and why is it still in use? It has survived because >it is a useful desciption of current flow in a metallic conductor, and >because metallic conductor are the most frequently studied types of >conductor. All professionals know the limitations of Ohm's Law, and ensure >that they use the law only when appropriate. > >Even with metallic conductors, Ohm's Law contains a hidden danger to >truth. Electrical resistance changes with temperature. If we are studying >the tungsten element of a light bulb, the resistance changes as the >element gets hotter and starts to glow red, yellow, then white. If (for a >110-Volt lightbulb) we measure the relationship between current and >voltage from 0 Volts to 10 Volts, we cannnot then linearly extrapolate our >result to predict what the current will be at 110 Vo;ts. This finding is >contrary to Ohm's Law, which claims that we ought to be able to >extrapolate in this way. > >To attempt to apply Ohm's Law to a semiconducting diode is to waste one's >time. It does not help one to analyse the behavious of the diode. Yes, you >can define the resistance of the diode as V/I, but where does that get you >if the value of resistance thus obtained is not stable? Furthermore, the >absolute value of resistance, V/I, is not the same value as the >incremental (or marginal) resistance, dV/dI. > >Any fool can divide a measurement of Voltage V by a corresponding >measurement of current I, and call the result of this division the >"Resistance R". Another contributor, earlier in this thread, has mentioned >that any such analysis will usually lead to a circular argument, and be of >little value. Yes, that contributor was Doctroid. BW
From: Evan Kirshenbaum on 30 Mar 2010 20:37
Hatunen <hatunen(a)cox.net> writes: > On Tue, 30 Mar 2010 23:03:47 +0100, "Richard Chambers" > <richard.chambers7_NoSpam_(a)ntlworld.net> wrote: > >> >>"Hatunen" <hatunen(a)cox.net> wrote in message >>news:nvo4r51gd2sfrjegh1q23jpkefbn1f8jmr(a)4ax.com... >>> On Tue, 30 Mar 2010 14:15:12 -0400, barbara(a)bookpro.com wrote: >>> >>>>>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. >>> >>> Even though the relationship remains as the definition. That >>> doesn't make much sense. >>> >>>>Some of these mokes have been claiming that everything always >>>>obeys Ohm's law. >>> >>> It is too bad that the authors didn't give an example of a >>> conductor that doesn't "obey" [*] Ohm's Law. I suspect it may turn >>> out to be less than it appears. >> >>We need to remind ourselves of what Ohm's Law really does state. From >> http://www.answers.com/topic/ohm-s-law >>we have the definition/explanation of the law as follows:- >>"The law stating that the direct current flowing in a conductor is >>directly proportional to the potential difference between its ends. It is >>usually formulated as V = IR, where V is the potential difference, or >>voltage, I is the current, and R is the resistance of the conductor." > > I prefer not to use dictionaries for definitions of technical and > scientific terms. > > Wikipedia is closer: > > "In electrical circuits, Ohm's law states that the current through a > conductor between two POINTS is directly proportional to the > potential difference or voltage across the two POINTS, and inversely > proportional to the resistance between them, provided that the > temperature remains constant."[Emphasis added] > > http://en.wikipedia.org/wiki/Ohm%27s_law > > Note that this definition does not require the points to be > external. But it does require that the current and voltage between the points be "directly proportional". To me, that means that there is some *constant* of proportionality, and that if one doubles, the other will necessarily double. I realize that my grounding in physics is much less than many here, but I had thought that that was precisely the point of contention here. -- Evan Kirshenbaum +------------------------------------ HP Laboratories |Usenet is like Tetris for people 1501 Page Mill Road, 1U, MS 1141 |who still remember how to read. Palo Alto, CA 94304 kirshenbaum(a)hpl.hp.com (650)857-7572 http://www.kirshenbaum.net/ |