What to call Ohm's law when it's not Ohm's law - Page 1
Forum section: electronics / beginners

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 02, 2022, 09:50:36 pm
content: I missed a question on a physics test once which asked if Ohm's law applies to diodes. I said yes, but turns out Ohm's law specifically stipulates that R is a constant: I(t) = V(t) / R. Wikipedia even mentions this explicitly, "If the resistance is not constant, the previous equation cannot be called Ohm's law,".So... what do I call it when I'm using not-ohm's-law to compute an effective resistance of a transistor or something? Is R(t) = V(t)/I(t) "the resistance law" the same as how P(t) = V(t)I(t) is "the power law"?

author: TimFox
author_rank: Super Contributor
timestamp: August 02, 2022, 10:01:19 pm
content: For a non-linear device such as diode, one could define the "average" resistance asR=V/I, but a more useful parameter is the "dynamic" or "incremental" resistanceR= dV/dI.Both of these parameters are functions of the current, while Ohm's Law states thatRis independent of current.An "ohmic device" is one that obeys Ohm's Law. Even resistors you buy from the store are not exactly linear, and have what is termed "voltage co-efficient of resistance" on a good datasheet.In physics, Ohm's Law relates vectors:J=sE, where the vectorJis the current density (A/m2), the vectorEis the electric field or voltage gradient (V/m), ands(should be Greek l.c. sigma) is the conductivity. In homogeneous materials,sis a scalar, but in inhomogenous structures (such as crystalline graphite), s can be a tensor.

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2022, 12:46:24 am
content: Quote from: Galaxyrise on August 02, 2022, 09:50:36 pmHello there,I knew what this thread was about when i saw the title because this question comes up so often and is so confusing to many of us and after years of working in the field i gained some insight into this strange measurement.The most general phrase is "non linear resistance". That simply means that there is some affect that causes the resistance to change. The affecting parameter can be temperature, voltage, current, just about anything.Now Ohm's Law is more of a rule than a law but it helps understand how constant resistances work in terms of voltage and current. Even though that sounds very simple, it is wise to look at some other type of rule that gives us more of an objective view of what this is all about. That is to start with a simple circle.The equation for a simple circle is:r^2=x^2+y^2and the most usual application of this is in solving for one of the variables x^2 or y^2 such as:y^2=r^2-x^2and of course the even more usual x or y when we take the square root:y=sqrt(r^2-x^2)We can ask about what this really means in terms of the variables.First we note that ALL of the three variables are LOWER CASE. That implies that they are ALL free to be any value they happen to be, and there is no constraint whatsoever on either of them.Now let's look at another form of this equation or 'formula".R^2=x^2+y^2Note that now the radius is upper case, while x and y are still lower case. What this usually implies in simple geometry is that R is constant and never changes. This small change makes a BIG difference. It means that x and y can only be certain values that satisfy this equation. This is not the same as with all lower case:r^2=x^2+y^2because r can change also, which means x and y only have to be constrained for a given r, and r can change to just about anything. In practice, r is often depending on some other parameter itself such as time 't' and then we would write:r(t)^2=x^2+y^2and here it is obvious that r(t) changes with time. This is much different than just 'R' which can NEVER change because it is a constant taken to be that way throughout any calculation using that equation or formula.Now back to Ohm's Law.The resistance is really taken to be constant and is usually written as:R=V/Iwhere everything is constant. So with a given R we can calculate for example the current 'I' as in:I=V/Rand what this means is that I will be constant with a constant voltage V and resistance R.That's the heart of Ohm's Law.The funny thing that plagues us sometimes though is what if we write it like this:i=v/rthen what?Well this implies that everything can change, including r.The kicker is that this also works, but it's not really Ohm's Law or more correctly Ohm's Rule.That's because Ohm's Law is complete without changing anything, even though i=v/r can change.Let me try to put this another way.If we have two constant resistances 2 Ohms and 3 Ohms, and put them into that equation, we get two equations:i=v/2i=v/3and quickly note that Ohm's Rule applies to BOTH of them individually, or you can say EITHER of them. That means that we dont need a different rule for i=v/r because we have one that describes any one single condition and it works just fine. However, it does not apply to both of them simultaneously.There are some disagreements with this however and that led to calling diodes "Ohmic" devices. That is mainly because diodes behave like resistances for one set of current 'i' and voltage 'v'. They dissipate power the same way as resistances for one set of values, unlike capacitors or inductors which store energy and are very different.To add to the confusion, there is also what is sometimes called "incremental resistance" i think someone already mentioned. This is the resistance you find for one particular set of 'v' and 'i', and how it changes over a short range of either 'v' or 'i'. For example for a device that has simple equation:r=v*2at a voltage v=1v for example we have r=2 Ohms, and for a voltage v=1.01 volts we have r=2.02 Ohms, and for a voltage v=0.99 volts we get r=1.98 Ohms. From this you can see that the resistance changed by an increment of 0.02 Ohms, either plus or minus. This gives rise to the APPROXIMATION that the resistance in the region of v=1 volt is 2 Ohms, and that works in a large number of real life applications even though it is not exact. This is the basic idea behind the diode characterization of being "Ohmic', because it behaves that way over a small range of voltage or current. Strictly speaking though, it does not 'obey' Ohm's Law because the resistance changes with v or i when a regular resistance R does not change with v and i only v and i change.Another example of an "Ohmic" device that does not obey Ohm's Law in the strictest sense is an ordinary incandescent light bulb. The filament gets hotter as the voltage is raised and that means the resistances changes. Many of these devices change resistance by a factor of 10 to 1, having lower resistance when cold and higher when hot. Again in the strictest sense they do not obey Ohm's Law because if v or i changes the resistance changes. They will often be referred to as "Ohmic" however because at one particular v and i they dissipate power in the same way that a regular resistor that does obey Ohm's Law does.So there you have it. You can call a device "Ohmic" even though it does not obey Ohm's Law over a wide range of v and i, although it does approximately obey the law over a small range of v and i.Ok so now for some more confusion.In AC analysis, we can note that there is an impedance associated with capacitors and inductors. The rule here is sometimes called "Ohms' Law for AC" even though there is no such law. It's called that for much the same reason, that over a small range of v and i and a third variable 'frequency' we get equations that look identical to what we see when we use the real Ohm's Law. For example:Z=V/Iwhere we see R being replaced with the impedance Z.This is not Ohm's Law either, but the equation is so much like that for DC and constant resistance that we tend to think of this the same way and so the rule "Ohm's Law for AC" came into being. As incorrect as it is, it does help to envision how impedances work in AC circuits and sort of makes it simpler to think about.Ok, so now you have the long and the short of it, but i wish i could say that is the end of the story. It turns out there are other 'rules' that are not applicable in all cases, and there are even a lot of misnomers that turn up a lot in electronics. For example, in some analysis you can think of a node that is both shorted to another node and at the same time not shorted to another node. This is usually called a "super node". I like to think of it as being something like an anisoelectric or anisoelectronic short, which we can get into another time.As you get into more math and related you will find that you end up running out of formulae and have to think of many things as strategies rather than rules. These are like 'rules' but they only help you to get to the actual formulae if they exist at all. Perhaps we should think of Ohm's Law as being a strategy rather than a hard and fast rule because its use varies with the application and of course the device in question.

author: ledtester
author_rank: Super Contributor
timestamp: August 03, 2022, 01:44:42 am
content: Quote from: Galaxyrise on August 02, 2022, 09:50:36 pmLet V be the voltage drop across a two-terminal device and I the current through it. Resistance is always a measurement of dV/dI. Note that we are assuming V is a function of I and therefore dV/dI is also a function of I -- i.e. it depends on the value of I that you perform the measurement at.For resistors and ohmic devices dV/dI is essentially a constant for all values of I. But in general V will not be a linear function of I, such as is the case with diodes.When you compute the effective resistance of a transistor or diode you are computing dV/dI at a particular value of I -- like at the DC operating point, for instance.I wouldn't be surprised if Ohm's Law is sometimes taken as the definition of resistance because, after all, it states R = V/I which looks like a definition. But it is only the definition of resistance for ohmic devices.

author: Brumby
author_rank: Unknown
timestamp: August 03, 2022, 01:50:52 am
content: Ohm's Law is a law.The simple expression of V=IR (and all its permutations) is correct - but that is predicated on the condition that any and all other factors - such as temperature - remain constant. With this understanding, it is absolutely reliable and eminently suitable for real world usage. (Nobody disparages Einstein's famous E=mc2- even though that is a special case of the full equation, which is a bit busier.)Where Ohm's Law does not apply is to non-linear elements - diodes being one.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 02:37:44 am
content: In somewhat extreme cases of real-world resistors I have had to deal with measurable non-linearity, which the manufacturer would specify as "voltage co-efficient of resistance".1. (More obvious). High-voltage resistors. I obtained sets from Caddock to make a 100 kV divider, where the voltage co-efficients of the 10 high values in series matched that ofthe one low value.2. (Less obvious). High-megohm resistors with only 10 V across them. Since the non-linearity is a function of the voltage gradient, physically small SMT parts can have a high dV/dx. I improved a circuit measurably by substituting premium 1206 parts for 0805 parts at 50 megohm (10 V maximum voltage), using parts that were specified for voltage co-efficient by the manufacturer.Ohm's Law is an approximation, which is quite often good enough for practical use, but is not a law of nature in the way that Maxwell's equations are.

author: magic
author_rank: Super Contributor
timestamp: August 03, 2022, 05:13:10 am
content: Diodes work by the diode law, obviously.Why would anyone even want to divide the forward voltage by the forward current of a diode in the first place?There is no meaning in the resulting figure whatsoever.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2022, 06:31:45 am
content: The Ohms law in the original form applies to metals at constant temperature and constant other conditions (e.g. stress, magnetic field). It is a material's law, similar to hooks law (elasticity) - so more like a good approximation for the behaviour of a class of materials but not a basic natural law like E=mC². There is not problem to the basic theroy of physics if one finds a metal that significantly deviates from ohms law - it would just be a strange exception in a special material. The somewhat nonlinear high value resistors are made from oxides or composites (thick film with a fraction of conductive particles in a non conductive matrix).Quote from: TimFox on August 02, 2022, 10:01:19 pmInstead of homogenous one should use isotropic. A perfect crystal is considered a good example for an homogenous material, but it can be anisotropic. In the case of the conductivity as a 2nd order tensor cubis symmetry would still imply isotropic conductivity, but a hexagonal crystal with less symmetry like graphite can be non isotropic.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 03, 2022, 06:57:18 am
content: Quote from: magic on August 03, 2022, 05:13:10 amThere are plenty of reasons. If you are designing analog circuits with small signal analysis, you can define R=dV/dI, and from that you understand the small signal equivalent resistance of the diode in that region of operation.You don't get to not adhere to Ohms Law, it pretty much follows from Maxwells equations. The problem is just if you can assume that R = constant over whatever variables, be it temperature, current, voltage, etc.In the case of it being dependent on voltage or current, R just becomes another variable, where you get something like V=I*R(I) with R a function of I. Perhaps R is proportional with current (so R(I) = a+b*I with a, b constants) and then you get V=a*I+b*I^2. In that case, you might call 'a' the resistance, and 'b' the current-dependent nonlinearity or something

author: armandine2
author_rank: Frequent Contributor
timestamp: August 03, 2022, 06:58:13 am
content: Quote from: Galaxyrise on August 02, 2022, 09:50:36 pmI had a similar question in marked lab sheet which I got wrong - the answer from my instructor was Ohms Law always applies, as V/R = I will apply in the  moment, so you don't need worry about the function of time and linear non-linear stuff.

author: magic
author_rank: Super Contributor
timestamp: August 03, 2022, 09:02:08 am
content: Quote from: TheUnnamedNewbie on August 03, 2022, 06:57:18 amYes, you can.I am still waiting for any reason to care about the ratio of instantaneous voltage to instantaneous current in a diode.The product is interesting since it's the power dissipation. The ratio, it's just unhelpful nonsense.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 03, 2022, 09:16:38 am
content: Quote from: magic on August 03, 2022, 09:02:08 amI literally said why: small signal analysis. Imagine you need to simulate/design a circuit that has that diode (be it physical or parasitic) in it's signal path. In this case, that ratio can be important as it gives you an approximation of the change in voltage you can expect for a small change in current or vice-versa. Where could this be important? Amplifier design where that diode might be part of the bias circuit and you need to figure out PSRR. Or in RF, where that diode might be part of the output impedance of a network, and thus this small signal equivalent resistance matters.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2022, 09:23:42 am
content: The small signal analysis cares about the differential resistance, so dU/dI, but not U/I. U/I has essentially no relevance for nonlinear parts like a diode.

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2022, 10:56:55 am
content: Quote from: Brumby on August 03, 2022, 01:50:52 amI remember there being a long thread about this, which turned into a big argument and eventually name calling. I can't remember whether it was on this forum or elsewhere.I think the point was Ohm's law is theoretically a law, but the resistance of some devices such as diodes varies with current, so it no longer makes sense to use R = V/I, P = I2R etc. Some pedantic people pointed out that normal resistors are non-linear to degree, especially when run close to the maximum voltage ratings, but that doesn't matter in most circuits.As far as the beginner is concerned: just use Ohm's law for resistive circuits. It's often safe to assume a diode has a voltage drop of 0.7V, but check the data sheet for the actual value for the current it's being run at, to be sure and note it's temperature dependent.

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2022, 01:05:42 pm
content: Quote from: magic on August 03, 2022, 05:13:10 amHi,There are at least a couple reasons.First is to calculate the dynamic resistance. You can actually make a gain controlled amplifier in this way.Second, for use in a log/antilog circuit. Diodes have curves that can be used that way. The complete range of I and V follow a log of sorts which can be used effectively although with some limitations.Be aware that it is very very hard to place limitations on electronic parts as to what kind of application they can be used in. I see transistors being used for other things besides transistors simply because they have the right characteristics for the job. A little ironic, as a diode :-)

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2022, 01:11:52 pm
content: Hello again,Here is a little writeup about this. Dont remember where i got this page long ago.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 01:41:38 pm
content: Quote from: Kleinstein on August 03, 2022, 06:31:45 amI stand corrected. Isotropic is the correct term, meaning that the resistance, etc., are independent of direction within the material.

author: madires
author_rank: Super Contributor
timestamp: August 03, 2022, 01:42:55 pm
content: I think there are two things mixed up. One is the classic Ohm's law and the other one is the concept of Ohm's law which can be applied in many situations. The tough question is where one ends and the other begins. What about NTCs, PTCs or LDRs?

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 01:49:16 pm
content: By the way, for a diode the (useless) valueV/Iis quite different from the dynamic or incremental derivative dV/dI.For a typical silicon device, at 1 mA current, the former is roughly 700 ohms, while the latter is roughly 26 ohms.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 03, 2022, 01:50:57 pm
content: Quote from: magic on August 03, 2022, 05:13:10 amThe context this actually came up was in discussing the human body, which is non-linear. It got pointed out that you can't use Ohm's law for a non-linear component and I wanted to reply that R(t)=V(t)/I(t) still works and you can approximate an Ohmic resistance for the human body over a narrow enough range of conditions; but didn't know how to justify that first part since many people see R=V/I as just another form of Ohm's law. This is addressed almost exactly by MrAl's excerpt, and seems that the "not Ohm's law" is being called the "definition of electrical resistance". Wikipedia calls it the "definition of static resistance" or "definition of dc resistance".Thank you for the replies, everyone! Fascinating tangential discussions in here

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2022, 03:15:44 pm
content: Quote from: madires on August 03, 2022, 01:42:55 pmNTC / PTCs are usually relative linear, as long as the temperature stays constant, as a condition also in ohms law. Including the self heating NTCs and PTCs become nonlinear resistors.With LDRs, the normal CdS types are relatively linear. With faster types there may be an relevant effect of charge sweep out, removing the photo-generated carries with more applied voltage. So they may become nonlinear (resistance going up with higher voltage).

author: armandine2
author_rank: Frequent Contributor
timestamp: August 03, 2022, 04:11:59 pm
content: Looking back at the old lab sheets - to see how I done

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 05:43:14 pm
content: To continue beating a dead horse:The equationV=IxRcan be used to extract a valueRat a given currentI, but for a non-linear device the equation predicts a voltage proportional to the current, which iswrong.The other equation, defining a "dynamic"R= dV/dI, is useful, since the small change dVresulting from a small change dIcan be predicted to a useful approximation for well-behaved non-linear devices.For example, calculating the change in emitter current due to a small change in base-emitter voltage, when the external circuit determines a quiescent value ofIEbefore applying the small change in voltage.

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2022, 10:13:23 pm
content: Quote from: madires on August 03, 2022, 01:42:55 pmHi,Yeah see, that's probably one of the best points in this thread.If we have an NTC it is non linear with temperature, but linear with constant temperature. So strictly speaking, it obeys Ohm's Rule (often called Ohm's Law) when the temperature is constant, but stops obeying when the temperature changes, but then obeys again when it stops for time.So we have these border line devices that may obey some times and not other times. So we can not really use the blanket phrase that it does not obey because it is non linear.The main idea is that it is hard to restrict electronic parts to any one application or any set of applications because one day a new application comes up that changes things a little. Have to keep an open mind and stay a little flexible.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2022, 10:35:24 pm
content: An NTC obeys Ohm's Law at a constant temperature.However, the power dissipated in the device will raise its temperature and change its resistance, which is a strong function of its temperature.With the thermal capacity of the device, its temperature will not change quickly.Siimilarly for PTCs, and also for LDRs whose resistance is a strong function of the illumination but normally do not self-heat very much.My problem is with computing the simple ratio ofV/Iof non-linear devices to get a useless parameter.

author: rooppoorali
author_rank: Regular Contributor
timestamp: August 04, 2022, 10:18:00 am
content: For diodes, we must see the I-V curves. Following are some good write-ups regarding diodes and their characteristics.https://www.khanacademy.org/science/electrical-engineering/ee-semiconductor-devices/ee-diode/a/ee-diode-circuit-elementhttps://www.theengineeringprojects.com/2018/05/introduction-to-diode.html

author: Sredni
author_rank: Frequent Contributor
timestamp: August 05, 2022, 02:53:17 am
content: Quote from: Galaxyrise on August 02, 2022, 09:50:36 pmYou can call it generalized Ohm's law.There is a vicious rumor that diodes are not resistors, but they are resistors. They are nonlinear resistors (if we neglect the dynamic parasitics, of course and assume the exponential curve in the VI plane is what characterize them).If you need to invoke the principle of authority to defend this assertion, you can always quote Chua, Desoer, Kuh, "Linear and Nonlinear Circuits". This is what I call "THE bible of circuit theory". It's not some high school or vocational school textbook for courses limited to the easiest circuits.If we turn a blind eye on memristors, we can see three types of circuital elements only: resistors (relation between V and I), capacitors (relation between dV/dt and I) and inductors (relation between V and dI/dt). We can introduce electric charge and magnetic flux and show the relations in terms of I, V, phi, and q if we wish, but that's it.So, a diode is a non linear resistor in the form of a one-port. A transistor can be seen as a nonlinear resistor in the form of a two-port (or just a three terminal elements, but two-ports are easier to digest, for some reason).An incandescent lamp is a nonlinear resistor whose resistance depends on the (V,I) point it's operating because that point dictates the power dissipated and the temperature it reaches. And yes, despite being nonlinear, we use the ratio V/I.Have you ever heard a phrase like "when cold, the filament has a resistance of x ohms, but when it's hot it shows a much higher resistance of Y ohms"? Well, that's the ratio V/I and not the differential resistance dV/dI.In another thread I gave other examples where these sort of quantities exists: hFE and hfe are one example. And if you are familiar with dispersion relations, w/k and dw/dk are another example (w is omega, the angular frequency, k is the wavenumber).

author: EPAIII
author_rank: Super Contributor
timestamp: August 05, 2022, 07:36:37 am
content: Interesting discussion.I do not think the word "Law" should ever be applied to anything in the scientific world. Science has observations. Science has theories. Science has more observations that either support those theories or bring them into question. But nothing in any field of science is ever absolute like the term "Law" would imply. I absolutely hate it when someone says, "The science is settled." Science is never settled. It is only our best current guess. And further observation will ALWAYS bring changes to those theories or completely new theories. That is the very nature and basis of ALL branches of science.With that off my chest, I have always considered Ohm's Law to be one of the better theories of science. Yes, there are devices that do not have a fixed or constant resistance. Diodes? Sure! Incandescent light bulbs? Also sure! But to my mind, these really are Ohmic devices in a true sense. It is simply that one MUST substitute r(x) for the simple r when the resistance changes due to some factor, x. So if you are solving for current it is not i but i(x).i(x) = v / r(x). i changes not only with changes in v, but also with changes of r as determined by x, whatever variable x might be. So, in this sense, Ohm's Law holds when the resistance changes with any factor by simply including the expression for that change in the basic equation of Ohm's law. No magic, just include all the variables.And if x happens to be the Voltage, theni(x) = v / r(v)Again, no magic. v just appears more than once on the right side of the equation. Of course, in order to properly solve this you need to know the correct function to use for r(v) and that may not always be easy to write down.When I first thought about the complex impedances of capacitors and inductors it gave me pause. But then if a DC Voltage is applied to a capacitor, it starts with a zero Ohm impedance; there is no resistance to the flow of current. But then, as the plates charge up to the applied DC Voltage, there is a reverse or back Voltage which effectively, over the charging time, changes that impedance/resistance from zero to infinity, going through all values between the two. The current is initially very high because i = v / r does apply and, with a constant applied Voltage, the very low value of r gives a very high value for i. But as the charge builds up and the back Voltage increases so does the resistance. So i = v / r continues to apply on an instantaneous basis and the current decreases. When the capacitor is fully charged and the back Voltage equals the applied Voltage, the resistance becomes infinite and i = v / r still CORRECTLY gives the current as zero. The DC version of Ohm's Law holds perfectly, ON AN INSTANTANEOUS BASIS, throughout this entire process. Ohm's Law never fails, even for the smallest increment of time you can imaging. It is simply i = v / r. It is just that r changes with time to make it so.A similar argument can be made for a constant DC Voltage that is applied to an inductor. Ohm's Law applies strictly on an instantaneous basis when the effect of the induced Voltage on the resistance is included.In my mind, that is what the AC form of Ohm's Law is all about. And it is just as precise as the DC form, but harder to envision. And all the math with complex numbers (a + bi) is just a way to simplify the calculations in real world AC circuits. But it is never introduced to the students in that manner.Thus, I see Ohm's Law as being very precise for both DC and, using complex numbers, AC. I am sure there are places where Ohm's Law does break down and become inaccurate but ordinary devices and circuits are not among them.What to call Ohm's Law when it is not Ohm's Law? I can not even imagine an example of this. And until one is found, the question is pointless.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2022, 09:56:04 am
content: For everyday use science and especially physics has pretty much settled. There is a long known in consistency between quantum mechanics and general relativity - so we know that our current best therories are not perfect and 100% as they are incompatible. However on there own they worked out fine and for normal use there is not problem - that starts with sub µm back holes or siminar strange things that are very small and very heavy.R = U/I is just a definition of resistance, not ohms law. The ohms law states that the resistance of metals is independent of the current as long as other parameters are constant. So it tells that metals (without the effect of self heating) are linear resistors. In that sense an ohmic resistor has to be linear. If it gets notably nonlinear there is nothing left of ohms law.Relating current to voltage or on the microscopic scale current density to the electric flield the ohms law is a materials equation and on it's own not a theory. Initially it was just an empirical observation, that turned out to be quite good. There are than theories (e.g. Drude / Sommerfeld) that want to explain resistivity based on the structure / chemistry. These theories than usually also explain why ohms law holds and most reistors are linear. These theories can also give more detailed material laws, that may include the exffect of temperature, strain and similar. E.g. to a first approximatio the resistance of simple metals goes up about proportional to the absolute temperature.

author: magic
author_rank: Super Contributor
timestamp: August 05, 2022, 10:06:00 am
content: Quote from: Sredni on August 05, 2022, 02:53:17 amYes, and it has nothing to do with nonlinearity. You can push 10A through a lightbulb and still enjoy your low resistance if you keep it cool.There is absolutely no use for the ratio of V/I. The only thing you can do is to calculate it and then immediately go back to either V or I by multiplying/dividing by the other. It's 100% useless.Here's a much more interesting problem I can solve using resistance:- measure the voltage drop of 680Ω resistor at 1mA to be 0.68V- know that the voltage at 10mA will be 6.8VAnd another interesting problem I can solve, knowing that diodes aren't resistors:- measure the voltage drop of a diode at 1mA to be 0.68V at room temerature- know that the voltage at 10mA will be 0.686~0.69VYour definition of "resistance" doesn't help with either of these problems, because it makes no sense for the latter and only adds unnecessary confusion to the former.

author: armandine2
author_rank: Frequent Contributor
timestamp: August 05, 2022, 11:25:43 am
content: I may have found another term albeit a bit general "Electric Characteristics"- from Hans Kammer ? Brussels Workshop on TI NSpire data acquisition calculator / school experiment workshop.Edited to make more sense to the author

author: Sredni
author_rank: Frequent Contributor
timestamp: August 05, 2022, 02:25:55 pm
content: Quote from: magic on August 05, 2022, 10:06:00 amIt has ALL to do with nonlinearity. The nonlinearity of an incandescent bulb is the consequence of the fact that the resistivity of the material increases with temperature. Have you ever drawn the VI curve of a light bulb from experimental data? You set the voltage (or current), wait for temperature to stabilize and measure the current (or voltage). You end up with a lot of points on the VI plane that follows a square rootish shape. That nonlinear shape is the non linear characteristic of the nonlinear resistor.QuoteAnd it would not be incandescent. What you do by placing an active cooling system is to create a new component that has a different VI characteristic (most likely still nonlinear, but even if it was linear, it would not be the incandescent bulb you started with).QuoteWell, I just gave you an example of its use. You are free to disregard it, if it makes you feel better.QuoteR = V/I adds confusion to the problem of finding the voltage drop at 10 mA when we know the voltage drop at 1 mA? What confusion?And as for the diode:This is just simple math applied to the nonlinear characteristic of the nonlinear resistor that is the diode. It's done routinely with exponential curves. Ever heard of the Newton's cooling problem? You know the temperature of the liver at time t1 and you can recover the temperature it had at time t0 - (or, as they do in CSI, determine the time of death computing the time the temperature was normal body temp).The exponential curve of the diode is just a nonlinear curve of a component whose behavior is described by the values of current and voltage (and not their derivative or integrals).