Bulbs connected in series. Physics doesn't add up - Page 1
Forum section: electronics / beginners

author: Electrump
author_rank: Contributor
timestamp: September 28, 2024, 01:30:38 pm
content: Dear communityI am a beginner learning the basics of electronics. Currently I read "Easy Electronics" by Charles Platt. I am making the experiments from the book to make sure I get the same results. I am on the page 9 and already have a question I cannot answer myself.The book says when two identical components are in parallel, then the current doubles. Consequently, when two identical components are in series, they create twice as much resistance for the current and there will be half as much current. Sounds logical.I have proved experimentally that "when two identical components are in parallel, then the current doubles" and I have no problems with that. However, something weird happens "when two identical components are in series", because they actually do not create twice as much resistance as promised. In fact, the resistance increases less than 50%!In other words, the question is: Why is the resistance not doubled when I put two identical cmponents in series?Below are details of the experiments, images and drawings. Although I tried to be as thorough as possible, I imagine there may be some gaps, so any relevant questions and comments are welcome.Equipment usedDC Power Supply2x 2.5V bulbs2x bulb holdersNotesFor unknown reason the resistance if the same type bulbs significantly differs - from 1.3 to 4 Ω. I selected 2 bulbs with 1.9 Ω.Bulb holders are identical.Experiment 1One 2.5V bulb consumes 178mA. Verified with both bulbs.Experiment 2Two 2.5V bulbs in parallel consume 353mA. 176.5mA per bulb which is pretty close to the current for one bult from Experiment 1(178mA)!Experiment 3What will happen if bulbs put in series? 175mA as half of the current? Half of that like 87.5mA? Wrong! The current is 127mA. And this figure puzzles me. How is it possible?

author: Grandchuck
author_rank: Super Contributor
timestamp: September 28, 2024, 01:59:08 pm
content: Incandescent lamps are non linear. The resistance goes way up when they are lit.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 28, 2024, 02:01:53 pm
content: You have independently discovered light bulbs are not nice linear resistors.Tungsten (the filament material) has a pretty high positive temperature coefficient, the hotter/brighter it gets the higher the resistance, so the bulb conveniently sort of finds it's own operating point for a given voltage across it and requires no driver/ballast.With identical bulbs in series the voltage and current will be equally divided, but as you've run into the current/resistance value is very dependent of the filament temperature. You'll note they don't glow as brightly in series.This is slightly outside of your question but do also keep in mind measuring low ohm values accurately is tricky business. With a basic DMM at least check for zero (short test probes together, observe zero or note the offset) and make sure you can get a repeatable reading.

author: Sorama
author_rank: Frequent Contributor
timestamp: September 28, 2024, 02:09:24 pm
content: Your experiment proves that the resistance of the bulbs when the voltage is halved (bulbs in series) is 19,7 ohms, so not linear in function of voltage.You better take 2 resistors (watch the power dissipation): they are lineair.Edit: yugo was quicker than me…

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 02:55:24 pm
content: Ohm's Law is an approximation that is accurate for "ohmic" devices, i.e. those that obey Ohm's Law.Devices sold as "resistors" obey the equation closely.Other conductive devices (e.g., diodes) disobey this "Law".In your experiment, at equilibrium, there is a large deviation from the linear relationshipV=I Rfor each bulb in your different connections.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 28, 2024, 04:22:34 pm
content: Quote from: TimFox on September 28, 2024, 02:55:24 pmThe light bulbs obey ohm's law just as the resistors do. Just that the temperature of the light bulb changes so much from room temp to over 4000F when it's lighted.

author: Electrump
author_rank: Contributor
timestamp: September 28, 2024, 04:58:16 pm
content: It came as a surprise to me that not all components strictly obey the Ohm's law. I will repeat the experiment with a two and maybe even three resistors to see how obedient they are. Thanks to everyone for the quick and comrehencive replies!

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 06:04:45 pm
content: Quote from: BeBuLamar on September 28, 2024, 04:22:34 pmIf the "resistance" parameter depends on the current in a strong way, then the object does NOT obey Ohm's Law.If it did obey Ohm's Law, you could predict directly the current as a function of voltage (or vice-versa).There is a weaker law, where you have a differential parameterRAC= dV/dI, whereRACis reasonably constant over a narrow range of current (and therefore temperature) that allows you to calculate small changes of current or voltage for small changes of the other variable.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 06:10:01 pm
content: Quote from: Electrump on September 28, 2024, 04:58:16 pmA component that obeys Ohm's Law is called "ohmic". Wires, good resistors, etc. are examples of such components.As an elementary example of a two-wire device that is far from ohmic, consider a normal silicon diode.Even normal resistors, operated at a constant temperature, will show a small non-linear effect, called "voltage co-efficient of resistance".Wirewound and metal film resistors have small voltage co-efficients (sometimes found on a datasheet), but carbon resistors and thick-film resistors can have measurable co-efficients, even when the temperature does not change.The physics behind Ohm's Law is complex, and part of "solid-state physics" in the scientific textbooks.This is a page from a good manufacturer of high-resistance resistors that we used before I retired for critical applications:https://srt-restech.de/pdf/eng/TD_VCR_En.pdf

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 06:33:14 pm
content: Quote from: Electrump on September 28, 2024, 01:30:38 pmThe resistance of an incandescent lamp is temperature-dependent. When connected in series, the voltage across each lamp is reduced, leading to lower power output and consequently a lower operating temperature compared to parallel connections, where each lamp receives the full supply voltage. This variation in temperature results in corresponding changes in the lamps' resistance.Quote from: Electrump on September 28, 2024, 04:58:16 pmNo, Ohm's Law is a fundamental principle, and it applies universally. However, the effect that you see is due to the fact that the properties of certain components, such as resistors or incandescent lamps, can change with varying voltage or temperature. For instance, in components like incandescent lamps, the resistance increases with temperature, which in turn affects the current and voltage relationship. This can give the impression that they do not strictly follow Ohm's Law, but in reality, the law is still valid - it’s the component parameters that are changed dynamically.

author: RJSV
author_rank: Super Contributor
timestamp: September 28, 2024, 07:06:08 pm
content: Now, you see, you have obtained a very good and hopefully lasting memory, of this. If others mention 'non-linearity' your memory system will recall all this, and you can be emphasizing that, if you answer, with confidence. You understand the other person's question quite well,... maybe even making comment about the wrong assumption, led you astray a bit as well!By the BTW, I'm 70 now, and often 'pleased' to learn some, simple, technicality that I'd never heard. So questions, and learning really should be at any age, any level of knowledge.It's the ones that have finished learning, that are missing out, especially if complacent. (Those folks are in government, seemingly everywhere).Thanks for good question !Any EEVBLOG member can always message me, with question.

author: tooki
author_rank: Super Contributor
timestamp: September 28, 2024, 07:14:00 pm
content: Quote from: TimFox on September 28, 2024, 06:04:45 pmWell, I mean... strictly speaking, the incandescent lamps do obey Ohm's Law. The fact that their self-heating changes their resistance doesn't mean Ohm's Law doesn't apply. If you heated the tungsten filament with a directed acetylene torch flame, its resistance would rise just as much as if it reached that temperature through self-heating. Conversely, if you used highly effective cooling to prevent the filament from warming up, its resistance would remain low even with current flowing through it.

author: RJSV
author_rank: Super Contributor
timestamp: September 28, 2024, 07:17:09 pm
content: The explanation, in physics and molecular terms, is that increased temperature activity consists of faster and likely more widespread movements, of each particular particle, and that creates a kind of opposition to other particle (electrons) travel or out of the more active region.Can think of it like passing through a 'party' of dancers, vs a quiet library where everyone is passive and more stationary. The odds of collision are less, in the library, as you make your way through, to other side.That's thermal energy.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 07:18:09 pm
content: Quote from: radiolistener on September 28, 2024, 06:33:14 pmNo. Ohm's Law is not a fundamental principle, and does not apply universally.It is a good approximation for many useful situations.If the constant parameter "R" is not a constant, then the equation is not linear, and Ohm's Law is linear.Note that my discussion of voltage co-efficient of resistance is not due to temperature changes: in my work with precision analog electronics, we had to be careful with physically small 50 megohm resistors, with voltage < 10 V, because at that voltage gradient (V/mm) in small sizes standard SMT resistors had too much deviation from linearity, but the 2 microwatt power dissipation did not change the temperature.Remember: Ohm's Law (for engineering) statesV=I R, whereRis a constant parameter.In physics, it is usually stated as a vector equation:J=sE, whereJis the current density (vector),Eis the electric field vector, ands(usually lower-case Greek sigma) is the conductivity. More generally, s can be a tensor if the two vectors are not parallel (in an anisotropic medium).

author: IanB
author_rank: Super Contributor
timestamp: September 28, 2024, 07:24:17 pm
content: Quote from: Electrump on September 28, 2024, 04:58:16 pmA very nice experiment you can do, is to take a single bulb and vary the voltage across it in increasing steps, and record the current each time. Then plot a graph of the results.VmA0.000.5?1.0?1.5?2.0?2.51783.0?

author: RJSV
author_rank: Super Contributor
timestamp: September 28, 2024, 07:37:05 pm
content: Since there is discussion of thermally related nonlinearities, maybe your next question concerns transistors ?With BJT or bipolar type, PNP or NPN; they have non-linear inputs, base to emitter. Because that type transistor responds to the Bas Emitter current, and fairly complicated effects of voltage, there are 'regions' that change, accordingly:Voltage applied one way (direction) or the other way, will alter these regions, within the block of silicon. That's what give the BJT type the ability to control a larger change in output,...even if 'temperature' never changes.A capacitor might have changing values as well, depending on applied voltage, and not on temperature. So in extreme cases a capacitor might have a slight amount of nonlinearity.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 07:53:17 pm
content: I can't quote my solid-state physics textbooks here, since they are not available online, but Wikipedia's article is pretty good.https://en.wikipedia.org/wiki/Ohm%27s_lawSpecifically, it notes near the beginning: "More specifically, Ohm's law states that the R in this relation is constant, independent of the current. If the resistance is not constant, the previous equation cannot be called Ohm's law, but it can still be used as a definition of static/DC resistance. Ohm's law is an empirical relation which accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However some materials do not obey Ohm's law; these are called non-ohmic."When measuring a material, it is practical to keep its temperature constant, for example a thin or thick film deposited on a ceramic substrate can have its temperature externally controlled, or lab-standard resistors are often kept in a circulating constant-temperature oil bath. If the ratio between voltage and current, at constant temperature, deviates from a constant, that can be considered a voltage co-efficient.For an ohmic material with a substantial temperature co-efficient (e.g., carbon), if you maintain the temperature (or the self-heating is not large enough to matter), then the resistor is ohmic with a temperature-dependent parameterR(T), whereTis the temperature.A "light bulb" is more than its filament, since it is mounted in a bulb, and it is difficult (not impossible) to maintain the filament temperature (around 1000 to 3000 K) in its normal operating range, therefore thecomponentis non-linear.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 07:56:14 pm
content: Quote from: RJSV on September 28, 2024, 07:37:05 pmMany popular ceramic capacitor dielectrics (such as Z5U) areverynon-linear over their safe voltage range.A good manufacturer will graph the capacitance as a function of voltage, and the effect is huge.That value is useful to estimate the AC current at a given DC bias voltage.NP0/C0G ceramic capacitors have both very low voltage and temperature co-efficients of capacitance.

author: IanB
author_rank: Super Contributor
timestamp: September 28, 2024, 07:58:03 pm
content: Quote from: Electrump on September 28, 2024, 01:30:38 pmDear Electrobump,I just wanted to mention that this is one of the best first post, beginner questions we have seen on this forum!It contains all necessary information about the question, presented in a well organized manner, and you followed up promptly in response to the answers.Please do stick around!

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 28, 2024, 08:05:10 pm
content: Two additional pointers:- You could have had a sniff already that something is different than expected even with a single bulb. You measured 1.9for a single bulb and measured 0.18A or so with 2.5V. If you plug those values in Ohms law you'll notice a few minor issues- I noticed your supply is at 2V. Make sure to check all your variables ;-)You've stumbled on the difference between textbook and beginners approaches and real life. You'll encounter much more of this, and some of it will bite you in the ass from time to time.Other examples: a textbook wire is perfect zero. Often that approximation is good enough, sometimes it's not. A diode is, as a first approximation, equivalent to a check valve. Untill you figure out it won't open in the pass direction untill you have 0.7V. And then you figure out before 0.7V it already leaks a very little bit. And then you notice that that 0.7V will increase slightly with increased current. And then you figure out that...Electronics is the art of choosing the right abstraction level ;-)

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 09:54:41 pm
content: Here are the calculated values based on your experiments.As you can see, Ohm's law holds true in all cases. The variation in results is due to the change in the lamp's resistance as its temperature changes.You can verify this with a digital multimeter by measuring the voltage across the lamps. The measured values may differ slightly from the calculated ones, as I assumed both lamps have identical parameters, whereas in reality, there may be slight variations. The exact values can be confirmed with the DMM.You can also verify this through an additional experiment: connect a single lamp and set your power supply to 0.9995 V. The current should read 127 mA, and the power should be 127 mW, as calculated from your experiment with two lamps in series. This will further confirm that Ohm's law is still valid.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 10:04:23 pm
content: In your table, the calculated parameterRdiffers between the two circuits, and is not constant if you change the applied voltage.See my quotation above on applicability of Ohm’s Law.Ohm discovered his empirical relationship for a wide range of practical conductors and configurations, but there are many exceptions, as I have already discussed.IfVis an arbitrary function ofI, then the ratioV/Iis not constant and Ohm’s Law whereRis constant, is not obeyed.

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 10:24:27 pm
content: Quote from: TimFox on September 28, 2024, 10:04:23 pmYes, you're correct that the R can vary with changes in applied voltage, especially for components like incandescent lamps, where resistance is temperature-dependent. However, R can still be accurately calculated from the voltage drop across the component.To confirm the calculated value of R, you can perform an additional experiment with a single lamp by applying the same voltage drop as observed in the series experiment. If the measured resistance matches the calculated value, this will confirm that the calculation of R based on Ohm's Law is correct, despite the temperature variations affecting resistance.The same principle of Ohm's Law applies to electromagnetic waves, where the magnetic field, electric field, and wave impedance of the medium are interrelated in a similar manner. Just as in electrical circuits, the relationship between voltage, current, and resistance can be seen in the context of electromagnetic waves, where the electric field (E), magnetic field (H), and wave impedance (Z) are connected through the equation:\${Z} = \frac{E}{H}\$This demonstrates that Ohm's Law is not only applicable to conductive materials but also holds true in the propagation of electromagnetic waves through different media.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 10:37:10 pm
content: If I slowly increase the current through the bulb, and the voltage does not change proportionally, then the bulb is not linear, the resistance is not constant, the device is not ohmic, and Ohm’s Law does not apply.Yes, you can calculate the non-constant ratioV/Ias a function of current or voltage, but the actual Ohm’s Law states that the ratio is constant.This is well-described in the Wikipedia article I cited.In electromagnetic waves in vacuum (an ideal medium) the basic laws from Maxwell give a constant ratio as you stated, the characteristic impedance of free space.If you replace vacuum with alineardielectric, this value changes to a different constant.However, with a material rather than vacuum, things get tricky due to quantum effects in the medium’s atoms, and you get “anomalous dispersion” at higher frequencies (including optical).Many physical and engineering problems require only linear equations, such as Ohm’s empirical equation, but one needs to know when they are applicable to the required accuracy.

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 10:46:33 pm
content: In Ohm's Law, it is permissible to consider any of the three components - voltage (U), current (I), or resistance (R) - as the constant for the relationship described by the equation \${I} = \frac{U}{R}\$. This means that, depending on the context of the analysis, one can express the law in various forms:\${I} = \frac{U}{R}\$.\${U} = {I}{R}\$.\${R} = \frac{U}{I}\$​.Thus, the flexibility of Ohm's Law allows for a comprehensive analysis of electrical circuits by treating any of these components as constant, depending on the parameters of interest in the specific scenario.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 10:50:30 pm
content: No.Ohm found thatRis constant when varying either the voltage or current for a class of materials.One can find a ratio of any variables, but it may not be constant, just a definition of another variable without meaning.

author: radiolistener
author_rank: Super Contributor
timestamp: September 28, 2024, 10:52:26 pm
content: Quote from: TimFox on September 28, 2024, 10:37:10 pmI don't think that topic starter can register the presence of quantum effects on his installationI understand what you are referring to, but it is unlikely that quantum effects could be observed in such a simple setup, even if a more stable power supply is used along with highly precise voltmeters and ammeters.Quote from: TimFox on September 28, 2024, 10:50:30 pmDeriving Ohm's Law can be challenging when dealing with components that exhibit nonlinear characteristics. However, once Ohm's Law is established, its application remains valid, even in the presence of nonlinearity. In many cases, the behavior of nonlinear components can be approximated as linear within certain operating ranges, allowing for the practical application of Ohm's Law for analysis and calculations. I think it is essential just to understand the context in which the law is applied and recognize that its use may be limited for nonlinear components.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 11:28:01 pm
content: Once again, Ohm's Law is a well-defined physical and electronic law that is very useful.Your manipulation of the three termsV,I, andRis elementary algebra, without physical meaning except as a definition for the variableRGeorg Ohm did a careful series of experiments on different media in the 19th century and published his famous empirical law: the physical content is thatRis constant for a given circuit.He tried to explain this important result, but his theoretical explanation is no longer used: solid-state physics in the 20th century began to explain it on a physical basis.This what the world calls "Ohm's Law": you will find this in any reputable published source (you need not believe me).Did you even look at that Wikipedia article?Your discussion of the elementary algebra is not Ohm's Law.Ohm's Law is not relevant to non-linear circuits.Other electrical laws, such as Kirchoff's, do pertain to non-linear circuits; perhaps you are confusing them?My reference to quantum phenomena was not for the OP, but to show how your electromagnetic wave example also breaks down under certain circumstances (non-vacuum).

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2024, 11:52:27 pm
content: Quote from: radiolistener on September 28, 2024, 10:46:33 pmOh dog, notagain.Ohm's Law is valid for - wait for it -ohmicmaterials. Most materials arenotohmic, e.g. diodes. Some components even have ranges where anincreasein current "causes" adecreasein voltage, i.e.negative resistance.FFI, seehttps://en.wikipedia.org/wiki/Ohm%27s_lawespecially the section on "linear approximations". For example, in this component there is 3mA flowing trough it. What is the voltage across it? What is its resistance?I vs. V curve of 10 mA germanium tunnel diode, taken on a Tektronix model 571 curve tracer.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 12:04:12 am
content: Quote from: tggzzz on September 28, 2024, 11:52:27 pmAnd? Since diode is non-linear component, you cannot assume that R will be constant for any voltage.But it don't prevent you to use Ohm's Law to determine it's R for specific voltage.Some components exhibit hysteresis, which complicates the assertion that resistance will always remain constant at a given voltage. However, this does not preclude the use of Ohm's Law to measure the resistance of a component under specific conditions, once the component has been brought to a defined operating state. In such cases, you can apply Ohm's Law to determine the resistance based on the measured voltage and current, acknowledging that this value is contingent upon the specific operating point of the component and may vary under different conditions.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 12:09:15 am
content: Quote from: radiolistener on September 29, 2024, 12:04:12 amSo, whatexactly, is that tunnel diode's resistance when there is 3mA flowing through it?If you are to be believed, that is a trivial question to answer.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 12:14:57 am
content: Quote from: tggzzz on September 29, 2024, 12:09:15 amIn Ohm's Law, three parameters are involved: voltage (U), current (I), and resistance (R). Knowing only one of these parameters makes it impossible to accurately predict the other two.To determine the resistance of the tunnel diode at a specific conditions (operating mode) with specified current of 3 mA, it is also necessary to know the voltage drop across the diode at that moment. Only with both values (voltage and current) Ohm's Law can be applied to calculate resistance of tunnel diode.And given that the tunnel diode is a nonlinear component, you cannot assume that the calculated R will remain constant under varying conditions. The calculated R value will be only relevant for the specific operating mode in which it was determined.Quote from: tggzzz on September 28, 2024, 11:52:27 pmTo accurately apply Ohm's Law in this scenario, you first need to establish the tunnel diode at the specific operating mode point you wish to test. Once the diode is set to this operating mode point, you can measure the voltage across it and the current flowing through it to calculate the resistance using Ohm's Law.However, it's important to note that for the same current (3 mA), there may be multiple voltage values across the diode depending on its operating condition. If the diode is in a different operating mode, achieving the same current and voltage would require adjusting the diode's state through a different method. Therefore, the resistance value calculated will only be applicable for the particular operating mode you've established.This distinguishes non-linear components from linear one, where R is constant, resulting in a unique voltage value for a specific current. However, you can note that Ohm's Law remains applicable in both cases.

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 02:07:55 am
content: In Ohm's Law,VandIare variables, butRis a parameter.Ohm showed that for many useful situations,Ris a constant over a wide range of the two variables.Ohm's Law (when valid) is more than a definition of the ratioR, it is an explicit statement thatRis constant (for a given component).Ohm's Law, and other uses of the word "ohm":Meanwhile, the actual Ohm's Law (for linear ohmic materialsonly) has some interesting implications, only valid for linear ohmic materials.To calculate the resistance of a long wire or similar geometry, with constant cross-section area, one can treat it as a series of short wires in series, or a series of long skinny wires in parallel.This results in the well known formulaR= {rho} xl/A,where {rho} is another constant (the resistivity of the material, usually given in ohm-cm),lis the length of the wire, andAis the cross-sectional area.Again, this only works for a linear ohmic material.The reversal of slope in a tunnel diode is often called a negative resistance, but that is not an ohmic device.It is straightforward to define the "ohm" unit from Ohm's Law as the ratio of volts per ampere, and use that unit for the derivative dV/dI, but that isnotOhm's Law.As an analogy, one defines "efficiency" as the ratio of output power divided by input power.Obviously, efficiency depends on all the variables, such as shaft rpm or power level for a motor.It would be foolish to assume that efficiency of a practical device is constant over the useful range of the device.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 07:59:01 am
content: Quote from: radiolistener on September 29, 2024, 12:14:57 amNonsense. Stop prevaricating and answer the question.For example, in this component there is 3mA flowing trough it. What is the voltage across it? What is its resistance?Use numbers, not words.For another example of a basic component which your incorrect understanding cannot explain, seehttps://www.eevblog.com/forum/beginners/at-nanovolts-do-resistors-still-experience-heat-at-high-amperage/msg4996246/#msg4996246

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 08:38:54 am
content: Quote from: tggzzz on September 29, 2024, 07:59:01 amYour question does not have a straightforward answer as you seem to expect.To clarify, in a nonlinear component like a tunnel diode, for a given current (in this case, 3mA), there can be more than one corresponding voltage depending on the diode's operating region. This is similar to solving an equation like \$y = b x^2 + c x^3\$, where there can bemultiple valid solutionsdepending on the conditions.As I previously mentioned, in order to determine the voltage and resistance accurately, it's necessary to know the exact operating point of the diode, which cannot be inferred from current value alone.The relationship between voltage and current in a tunnel diode is nonlinear, and therefore, it is misleading to expect a single answer without additional context or data.If you'd like to explore this matter further by specifying the exact point on the graph where you're interested in determining the resistance, I'd be happy to assist.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 08:49:35 am
content: Quote from: radiolistener on September 29, 2024, 08:38:54 amPlease give a numerical answer in the context of your previous (false) assertion.Quote from: radiolistener on September 28, 2024, 10:46:33 pmBig hint: Ohm's lawonlyapplies to ohmic devices.Ohm's law does not apply to non-ohmic devices. That is simple and unambiguous, and I would have thought uncontroversial.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 29, 2024, 08:53:40 am
content: When the filament gets hot, the resistance of the filament increases.These are two miniature light bulbs measured with a DP832 power supply:The plots are fromhttps://www.eevblog.com/forum/projects/resistance-of-incandescent-light-bulbs/Notice how the resistance (the blue trace) increases with the applied voltage.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 09:07:28 am
content: Quote from: tggzzz on September 29, 2024, 08:49:35 amMy previous answer conveys the same point. Your question is akin to asking for a single solution to a cubic equation. When informed that it is impossible to provide a unique solution because there may be multiple valid answers, you assert that this is a false claim and request a single numerical value. What you expect?Quote from: tggzzz on September 29, 2024, 08:49:35 amIt is true that a tunnel diode is not an Ohmic device, and thus it would be technically incorrect to describe the relation \${R} = \frac{U}{I}\$ as Ohm's law in the case where \${R}\$ is not a constant Ohmic resistance. However, this equation can still be applied to determine the dynamic or differential resistance of the diode at a specific point on its I-V characteristic curve.Quote from: RoGeorge on September 29, 2024, 08:53:40 amYes, as I mentioned earlier, for a non-linear component, it is essential to set the component to a specific operating point on its I-V curve in order to accurately measure its resistance by considering the values of voltage and current.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 29, 2024, 09:11:36 am
content: Quote from: tggzzz on September 29, 2024, 08:49:35 amWhat is an ohmic device? A device that behaves according to ohm's law.Seems like a circular argument to me.https://yourlogicalfallacyis.com/begging-the-question

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 09:55:07 am
content: Quote from: Andy Chee on September 29, 2024, 09:11:36 amNot anargument, simply astatementof the obvious.The key point is that not all devices/materials are ohmic, and for such devices/materials V is not linearly proportional to R, as implied by V=IR.

author: DimitriP
author_rank: Super Contributor
timestamp: September 29, 2024, 10:02:03 am
content: If you fill two of these buckets half-way, you'll have less water than a single full bucket.Lightbulbs are like these buckets.Resistors on the other hand are more like this bucket below (yeah , it's not perfect, but it illustrates the point :and most importantly, if you fill two six gallon buckets half-way, how many buckets do you have ?

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 10:05:32 am
content: Quote from: tggzzz on September 29, 2024, 09:55:07 amTo clarify, I did not assert that all components are Ohmic. Nonlinear components cannot be classified as such.However, Ohm's law equation can still be applied to determine the dynamic resistance at a specific operating point. It is important to note, though, that this resistance will not necessarily remain the same at other specific operating points. Therefore, the calculated value will be valid only for the specific mode of operation of the nonlinear component.It is also important to consider that nonlinear components may exhibit hysteresis. Therefore, for accurate resistance measurements at a specified operating point, it is crucial to establish the component in the desired mode correctly, as the measured resistance may differ depending on whether the voltage was increased or decreased to reach that point.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 10:08:19 am
content: Quote from: radiolistener on September 29, 2024, 09:07:28 amI don't expect that!Youexpect that from your insistence that V=IR.QuoteYou are on the right track, but the equation is wrong.The relevant equation is Rdynamic=dv/di. Rdynamicis also termed the differential resistance, as implied by the differential equation familiar in calculus.QuoteAlmost. The operating point is required to determine thedynamicresistance.Of course you don't even need a cubic equation to become aware of why dynamic resistance is important; a simple y=mx+c equation is sufficient. It is possible to have negative V/I and positivedV/dI, e.g. a cell/battery.(fromhttps://en.wikipedia.org/wiki/Negative_resistance)

author: MrAl
author_rank: Super Contributor
timestamp: September 29, 2024, 10:11:19 am
content: Quote from: radiolistener on September 29, 2024, 12:14:57 amHello there,This discrepancy about Ohm's Law comes up from time to time on the web. This might be the fifth time or more that I've seen people try to claim that "Ohm's Law" holds for every single situation no matter how obscure.You probably do not realize it yet, but what the equivalent to what you are saying is that:c=a/bis Ohm's Law. That's not a Law, and just because we change the lettering that does not make it what we usually refer to as Ohm's Law either. In fact, Ohm's Law is not a true Law either.Let me list a few here:c=a/bd=g/fa=b*cm=diddly/tweedlyr=v/iWhich of those is Ohm's Law?None of them, not even the last one, because OL involves constant quantities not variable quantities.To be more succinct, we might write in upper case:R=V/Iand maybe we can take that to be Ohm's Law because it helps us SOLVE something without any more information. If you need more information about the current or voltage, then what you are implying is something like this:r=v(t)/i(t)or even:r=v(t)/i(x)and what this tells us is that both 'v' and 'i' are functions that must be defined before we can calculate r.Like some of the other examples posted by others in this thread but I'll add one more:y=r*sin(angle)Now this might look like:y=R*sin(angle)but it's not. It's entirely different. The second means we have a constant radius so 'y' will plot out a half circle if we span the angle from 0 to pi. However, the first is not the same as the second because 'r' is a variable also, and that means it could be a function:y=r(t)*sin(angle)and notice now we have two independent variables not just one (which was the angle previously).Now as to the constraints on the formula:r=v/iif we constrain 'v' and 'i' to stay within certain limits, we still don't have Ohm's Law. I bring this up because you said that sometimes we can calculate Ohm's Law at a point if we limit the span of the variables.That's still not Ohm's Law. That's called "Linearization" about a point. We then go about to 'pretend' that it obeys what we refer to as Ohm's Law.I really like the example of the negative resistance though. That really kicks this thing in the butt. There's no way we can call that Ohm's Law. If we try to say that we can get a negative resistance R with R=V/I we would have to say that one of the variables V or I is negative, as in:R=-2/3or:R=2/(-3)and this makes little sense also because it does not matter what the polarity is, the resistance is always positive. For example, if we apply +5 volts with a current of 5 amps, R=1 Ohm, but if we apply -5 volts with 5 amps, the resistance is still R=1 Ohm.It is also noteworthy to mention that 'resistance' is not the same as 'Ohm's Law' just because we use units of "Ohms" for resistance. Ohm's Law and resistance are two different things. We can have a resistive element that does not follow what we call Ohm's Law. This has been demonstrated many times in the past with the diode equation.In other words, if Ohm's Law holds for diodes just because they exhibit Ohmic behavior about a certain operating point, then why do we need the Diode equation?The diode equation looks something like this:id=is*(e^(qv/nkT)-1)and if we lump some constants and use an approximation based on 'is' always being much smaller than 'id' we can come up with this:v=log(id)*K (where K is just some of the constants lumped)and this shows us that the voltage is proportional to the natural log of the current, not proportional to the current. And this is even after using an approximation that makes the expression even simpler than it was originally.This shows that this:v=log(id)*Kis certainly not the same as:V=I*Rthey are two different relationships. If we could call the first one as the second one then we might as well say we have one formula:v=i*rfor everything in the universe. Why not? If we "just have to know more about the current or voltage" (or whatever else) then we can use Ohm's Law to describe anything we want to use it for.What is the temperature of the sun? It's v=i*r of course.What is the rotation of the earth around the sun? It's also v=i*r, what else could it be.What is the quantum state at the second location when we use teleportation to transmit the state to the that second location? Why, it's v=i*r, naturally!I think this happens because we use formulas regularly without really knowing or remembering the mysteries behind some of them. We all do this at some point. Also, we use the word "Ohms" for resistance and so we can start to believe that everything that has resistance follows Ohm's Law.

author: MrAl
author_rank: Super Contributor
timestamp: September 29, 2024, 10:13:32 am
content: Quote from: Andy Chee on September 29, 2024, 09:11:36 amAn ohmic device is a device that follows Ohm's Law 'almost' everywhere.We have to say 'almost' because nothing holds for Ohm's Law everywhere because it's not a true Law.

author: MrAl
author_rank: Super Contributor
timestamp: September 29, 2024, 10:23:08 am
content: Quote from: Electrump on September 28, 2024, 01:30:38 pmHi,As you are finding out, incandescent light bulbs do not follow the rule that "two identical components in series" must show twice the resistance. That's mostly only at a constant temperature, even for regular resistors, which I'm pretty sure even the man Ohm even knew at the time he did his experiments.Sometimes books tend to generalize a little too much in order to simplify the explanations. This is one of those cases, and it's not a rare thing to see. It's because providing the entire story in one book means using many pages of words which would take up too much room and even partly deride from the main topic which is "basic electricity".The reason light bulbs do not follow this rule is because the filament gets very hot, and that means it draws a lot less current than when it is cold. The ratio when cold to when hot is roughly 8 to 1. That means that a bulb that measures 80 Ohms when hot may only measure around 10 Ohms when cold.When the temperature is somewhere between hot and cold, the resistance can be anything between 10 and 80 Ohms approximately. The relationship is exponential and that means that it is not what we call "linear". There are many, many devices like this like the diode, and we have different formulas for all those devices. We can't use what we call Ohm's Law because as soon as we change the voltage the filament changes temperature, and that changes the relationship between current and voltage.I can post one of the formulas for the light bulb but it might not help much. It would show the approximate relationship between current and voltage for a filament bulb, and it's not Ohm's Law by any stretch of the imaginationOne thing you may find interesting is if you measure the resistance of one bulb with an Ohm Meter (no applied voltage yet) and then connect two of them in series and measure them both with the Ohm Meter, you will see twice the resistance. Why is that working now. It's because the temperature of the filament did not change in this experiment like it did with the original experiment.Likewise, with the filaments hot, you should see twice the resistance of one hot bulb although you cannot measure it with an Ohm Meter unless you had a way to very quickly remove the voltage and quickly make a resistance measurement, which is kind of hard to do.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 10:25:28 am
content: Quote from: tggzzz on September 29, 2024, 10:08:19 amI don't see anything wrong with the concept of negative resistance. This phenomenon occurs in certain non-linear components. Negative resistance is a behavior in devices like tunnel diodes and certain oscillators, where it can facilitate signal amplification or self-oscillation. While it may seem counterintuitive compared to conventional resistive behavior, it is a well-established concept in electronics.

author: MrAl
author_rank: Super Contributor
timestamp: September 29, 2024, 10:28:17 am
content: Quote from: radiolistener on September 29, 2024, 10:25:28 amThat's not the point. Read some of the other replies just before yours. There's a full explanation of what Ohm's Law is all about, and what it is not.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 10:39:21 am
content: Quote from: radiolistener on September 29, 2024, 10:25:28 amGood to see you acknowledge the existence and properties of tunnel diodesThere is no such thing as "conventional resistive behaviour". There are measured V-I curves, andseveralmathematical models are used to explainaspectsof the measurements.Different mathematical models are useful in different situations, as is typical with any kind of model of physical behaviour. Many people have a naive mis-understanding of the models' applicability.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 29, 2024, 10:59:15 am
content: Unlike a diode which conducts when the forward bias voltage reaches a certain value the resistor (yes the bulb filament of an incandescent bulb is a resistor too) changes its value depending on temperature. This happens with all resistors but for most applications the value only changes slightly. For example a heater when it's hot the resistance is higher, not by much but enough that I can tell if it's hot by measuring the resistance. For the light bulb the resistance changes signficantly because its temperture changes a lot in off to on condition. At half voltage the filament isn't as hot and the resistance is lower.

author: chris_leyson
author_rank: Super Contributor
timestamp: September 29, 2024, 11:12:42 am
content: On the subject of negative resistance you could try building a Lambda diode using p-channel and j-channel jfets. Works at a higher voltage than tunnel diodes but cheaper.https://en.m.wikipedia.org/wiki/Lambda_diode

author: m k
author_rank: Super Contributor
timestamp: September 29, 2024, 11:28:55 am
content: Quote from: TimFox on September 28, 2024, 02:55:24 pmI think I've always treated R=U/I sort of fundamentally abstract thing.When that is then connected to reality the real R is not really present, it's a combination of other two definitions.Seems that my real R is always unknown.Where's that resistor story by Edwin Pettis when you need it.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 11:59:33 am
content: Quote from: tggzzz on September 29, 2024, 10:39:21 amI already explained this nonlinear properties before you provided example with tunnel diode.Quote from: tggzzz on September 29, 2024, 10:39:21 amI apologize for the confusing terminology, when I referred to 'conventional resistive behavior' I actually mean 'Ohmic resistance behavior'.Quote from: tggzzz on September 29, 2024, 10:39:21 amIndeed, this aligns with my earlier points and emphasizes the importance of understanding the distinction between linear and nonlinear components, particularly regarding the applicability of Ohm's Law. I explained it before your objection.While you may find the use of the term "Ohm's Law" in the context of nonlinear components somewhat objectionable, it's important to clarify that I talk not about predicting the behavior of the component across varying voltages using Ohm's Law. Instead, I talk about the relationship between voltage and current through resistance as it experessed in Ohm's law equation. This relationship can be used at a specific operating point to find the dynamic resistance, even in nonlinear systems.You previously mentioned that it would be more appropriate to use different notations for resistance, current, and voltage of nonlinear component:Rdynamic​,dv, anddiinstead ofR,U, andI. However, I don't see much value in distinguishing the notation for resistance, voltage, and current between nonlinear and linear components.In my point of view, what is more important - is not the specific symbols used to denote current and voltage, but rather the understanding of the distinction between linear and nonlinear component properties.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 29, 2024, 12:53:48 pm
content: I have a suggestion. In experiment #3 instead of 2.5V use 5V and you see everything is right.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 01:05:06 pm
content: Quote from: m k on September 29, 2024, 11:28:55 amOhm's law is amodelof howsomematerials behave, no more, no less. "All models are wrong, but some are useful". In addition, all models have their limits.All non-superconducting materials have more or less resistance. That doesn't mean there is a linear V-I relationship in which assigning a constant resistance makes sense.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 01:12:49 pm
content: Quote from: radiolistener on September 29, 2024, 11:59:33 amHave you ever taken a course in differential calculus and passed?If you had, you would understand there is a fundamental difference between a curve and theslopeof the curve. In this case, that's V/I and dV/dI. If you can't see the difference between the two, presumably you also think distance and speed are the same as each other.https://electronics.stackexchange.com/questions/498402/what-is-the-difference-between-differential-resistance-and-resistance#498404https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Static_and_differential_resistanceI was taught that in the first two weeks at university.

author: coppice
author_rank: Super Contributor
timestamp: September 29, 2024, 01:21:18 pm
content: Incandescent bulbs are interesting when powered by AC. The temperature of the filament ripples a lot through each half cycle of the power source. So, the resistance is changing a lot. So, the device is not linear. It is not a true "resistive load". How much does that affect things? It depends on details of the exact bulb, but when you have a clean sine wave power source, and look at the current waveform you usually find 10%-25% THD. To someone who understands what is going on, but has never tried measuring it, the size of that THD figure is often surprising.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 01:52:48 pm
content: Quote from: tggzzz on September 29, 2024, 01:05:06 pmAsm kpreviously mentioned, resistance, current, and voltage are abstract concepts used as models to describe electrical behavior rather than physical entities. What fundamentally matters here is the relationship between voltage and current, or more broadly, the relationship between electric field strength and magnetic field strength in the context of electromagnetic waves.This ratio, which we conventionally call resistance and measure in Ohms, remains significant for both linear and nonlinear components.To clarify, I am not referring to the Ohmic properties typical of a linear component like a resistor, but rather to the relationship between voltage and current as an important characteristic of the component at a specific operating point.Although a nonlinear component is not an Ohmic component, the ratio of voltage to current across it still plays the same important role as it does for a linear component, despite the fact that this ratio may vary differently with voltage/current compared to a linear component.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 02:01:44 pm
content: Quote from: coppice on September 29, 2024, 01:21:18 pmAs I'm sure you know, the amount of temperature variation (and hence distortion) will depend on the frequency and the thermal mass. Bill Hewlett famously took that into account in the HP200A oscillator

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 02:13:38 pm
content: Quote from: radiolistener on September 29, 2024, 01:52:48 pmYou're getting there, but while it can be said that a non-linear material/device hasresistance, it is confusing to the point of being meaningless to measure it in Ohms. Ohm's has a meaning in the context of Ohm's Law, and that requires a linear material/device.If you have difficulty with that, then using your concept of resistance, is that resistance of the this component positive, negative, zero, or infinite? (Hint: for your concept,it is all of them)Nowdo you see why your concept of R=V/I is meaningless?

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 02:24:19 pm
content: Negative resistance devices and circuits, where dV/dI< 0, are both interesting and useful.These include two-terminal devices, such as tunnel diodes, and more complicated circuits such as vacuum-pentode transitrons and op-amp circuits with positive feedback.It is important to note that to exhibit negative conductance across the two terminals, all of these circuits require external power: the bias voltage for a tunnel diode oscillator, the plate and screen voltages for the transitron, etc.A passive non-linear device, such as a PN diode, has an incremental resistance (slope or AC resistance) dV/dI> 0 without external power.

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 02:28:25 pm
content: Quote from: tggzzz on September 29, 2024, 09:55:07 amOhm's Law is applicable to a huge set of practical devices, such as wires, which are then defined as "ohmic".This is not circular, merely a definition of "ohmic".Other devices are non-ohmic and Ohm's Law itself is not applicable.The SI unit "ohm" is defined as volts/amperes, in the same way that the volt is defined as joules/coulombs, but those definitions are not "laws".

author: coppice
author_rank: Super Contributor
timestamp: September 29, 2024, 02:39:45 pm
content: Quote from: tggzzz on September 29, 2024, 02:01:44 pmYou might expect that after a century of high volume production the design of incandescent bulbs would have converged to essentially a single design. However, I found (while doing work on power quality instrumentation) that if you try different makes of bulb of the same power rating the THD can vary quite a bit on the same supply. Whether that is due to variation in the thermal mass, the thermal conduction paths, or the IR transparency of the glass I never investigated.

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 02:49:33 pm
content: Quote from: tggzzz on September 29, 2024, 02:13:38 pmYou view the Ohm unit in a narrow context, considering it solely as a concept of resistance that applies meaningfully only to linear components with Ohmic resistance. In contrast, I see the Ohm unit in a broader context - as the ratio of voltage to current or the ratio of electric field strength to magnetic field strength. This ratio reflects the constant relationship for linear components with Ohmic resistance (which we know as classic Ohm's law), but it also applies to both linear and nonlinear components at selected voltage and current levels. This perspective, while extending beyond the classical interpretation of Ohm's Law, continues to address the same fundamental essence expressed as the relationship between voltage and current. It emphasizes the intrinsic physical nature of this relationship, which significantly impacts the efficiency of energy transfer from the source to the receiver.For example, the characteristic impedance of the environment surrounding an antenna is also expressed in Ohms. However, this impedance is varying within the near-field region and, even in the far-field region it depends on the properties of the surrounding environment. Does this mean that we cannot express it in Ohms?

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2024, 03:36:53 pm
content: Quote from: radiolistener on September 29, 2024, 02:49:33 pmYou forget a fundamental aspect of physics that Ohm's Law is a physicalLaw. A law in physics is a model that holds over a wide range of experimental conditions, and can be used to make predictions. Ohm'sLawsays that for many materials, voltage is proportional to current, all other variables remaining constant. Thus, if you know the constant of proportionality, you can predict the voltage given a measurement of current, or you can predict the current given a measurement of voltage.If the relation between voltage and current is not a straight line graph passing through the origin, then the material does not follow Ohm's Law, rather it follows some other, more complex Law.

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 03:45:11 pm
content: Yes, the "Ohm" as a unit in electricity is more general than in circuits that obey Ohm's Law.The unit is merely the quotient of two other units: 1 Ohm = 1 Volt/Ampere, and can be used also for the derivative dV/dR.Standards labs maintain carefully built two-terminal objects calibrated in Ohms to international standards.Similarly, the Hertz was defined as one cycle/second, the Volt as one Joule/Coulomb, etc. to define the terms.The laws of electromagnetism give relations between different properties such as potential, current, charge, etc. and different fields such asE,D,B, andH(vectors).Relationships between the latter four fields will be written differently depending on the choice of units: SI (rationalized MKS), "Gaussian" (unrationalized cgs), and several others that are rarely chosen now.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 04:54:57 pm
content: Quote from: radiolistener on September 29, 2024, 02:49:33 pmThat reads like Post-Modern Literature CriticismWhat you "see" is your own set of rationales and definitions. While that might be OK when conversing with yourself, it doesn't work if you want to converse with other people. For a conversation it is necessary to have common definitions of words and concepts.Now, again, how do you "see" the resistance of the battery? Positive, negative, zero, infinite. Your "vision" of R=V/I requires that it isall of those- which is useless for any practical purpose.Hint: better people than us worked through chains of reasoning such as yours, 1 to 2centuriesago. Stand on their shoulders, don't trip over their discarded toenail clippings

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 05:08:28 pm
content: There is an anecdote circulating on-line about a new employee who was told not to spell "hamster" as "hampster" but refused to change because that's the way she spelled it.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2024, 05:13:23 pm
content: Quote from: TimFox on September 29, 2024, 05:08:28 pmExcellentMaybe the rodents were always found in hampers?

author: themadhippy
author_rank: Super Contributor
timestamp: September 29, 2024, 05:26:58 pm
content: Quotemaybe it was a siberian hampster

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 05:32:33 pm
content: with a paedigree

author: djsb
author_rank: Super Contributor
timestamp: September 29, 2024, 06:17:26 pm
content: From Hempstead

author: Xena E
author_rank: Frequent Contributor
timestamp: September 29, 2024, 06:52:45 pm
content: It's not a hampster, it's a rhat!

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 06:59:32 pm
content: That's why we put basil in the ratatouille ...

author: radiolistener
author_rank: Super Contributor
timestamp: September 29, 2024, 07:53:28 pm
content: Quote from: TimFox on September 29, 2024, 06:59:32 pmIs it rational to start with ratatouille and finish with ratsberry pie?

author: TimFox
author_rank: Super Contributor
timestamp: September 29, 2024, 07:57:41 pm
content: Actually, this joke series is a reference to a BBC comedy, "Fawlty Towers", series 2, episode 6.

author: tooki
author_rank: Super Contributor
timestamp: September 29, 2024, 08:13:29 pm
content: Quote from: TimFox on September 29, 2024, 05:08:28 pm

author: MrAl
author_rank: Super Contributor
timestamp: September 30, 2024, 11:10:10 am
content: Quote from: tggzzz on September 29, 2024, 02:13:38 pmAre you sure about that?Resistance is stated in units of Ohms, it does not matter where it comes from. Negative resistance is a special case.It does not have to follow Ohm's Law, but it can still be stated in units of Ohms.Resistance dissipates energy while something that stores energy like a capacitor or inductor does not have resistance it only stores energy.This means we can say a diode has resistance at a certain point and therefore dissipates energy, even though it does not follow Ohm's Law.This we can have a statement about the unit Ohms without the device necessarily following the Ohm's Law proper.I'm pretty sure you know this but you stated it a little differently.We might also talk about the modernization of Ohm's Law, but that would probably just cause more confusion.

author: MrAl
author_rank: Super Contributor
timestamp: September 30, 2024, 11:14:21 am
content: Quote from: radiolistener on September 29, 2024, 02:49:33 pmI believe he stated that incorrectly. Ohms is a unit while Ohm's Law is a relationship between measurements that applies in some cases.Ohm's Law is not a true law it is an empirical law, but the unit of Ohms is always the unit of Ohms whether it follow what we call Ohm's Law or not. When we have resistance we see energy being absorbed. That's a fundamental physical concept.

author: tggzzz
author_rank: Super Contributor
timestamp: September 30, 2024, 11:27:47 am
content: Quote from: MrAl on September 30, 2024, 11:10:10 amYou make some valid points, but...Negative resistance isn't mathematically a different case.Resistance is a susbset of impedance, where the reactive component is zero. Both are quantified in Ohms.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2024, 02:12:17 pm
content: One case where the incremental or AC resistance (derivative dV/dIat a given operating point) is useful is in the calculation of Johnson (thermal) noise.Since noise is a small variation about the equilibrium condition, any real (non-reactive) component of the impedance (R+jX) will generate the same noise as an ohmic resistor in the equivalent circuit.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 30, 2024, 03:25:41 pm
content: Dude asks a question about lightbulbs.We're now on thermal noise and complex calculusOh, and the OP haslongsince left the building...

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2024, 03:30:19 pm
content: Physics is hard: live with it.Note that the OP's question title includes "Physics doesn't add up".Some of us were explaining the physics.

author: radiolistener
author_rank: Super Contributor
timestamp: September 30, 2024, 04:37:06 pm
content: Quote from: Ice-Tea on September 30, 2024, 03:25:41 pmHold on, we haven't even gotten to relativistic corrections and quantum effects yet - the real fun is still ahead!

author: m k
author_rank: Super Contributor
timestamp: September 30, 2024, 05:10:17 pm
content: Is it time again, seems that my R doesn't include time, it's a point like thing and can't have variations.If it changes to Z it's still sort of a moment, it doesn't include any waveform.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 30, 2024, 07:21:52 pm
content: If you say the light bulb is non linear then just about any resistance is non linear. I don't know of a conductor that doesn't change its resistance with temperature. Most of the time it's insignificant because the temperature changes are too small but there always is a change in resistance with a change in temperature.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2024, 07:28:11 pm
content: Yes, conductive media usually have a measurable temperature co-efficient.However, it is possible to externally control the temperature of the resistive element, and at low power the self-heating should be negligible.As I pointed out near the beginning of this thread, Ohm's Law is an approximation that is usually useful, but it is not strictly exact: many resistors exhibit a voltage co-efficient of resistance (unrelatedto temperature), but it is rarely specified on the data sheet.I have seen physically small (0603 and 0805 SMT) high-value resistors where the resistance changes by > 0.1% at constant temperature for only 10 V applied to 50 megohms.Lower-value resistors in reasonable packages (e.g., RN60 metal film TH resistors) are more linear.Light bulbs areextremelynon-linear, treated as components, over their usual operating voltage and current range, as shown in measurements reported above.

author: tggzzz
author_rank: Super Contributor
timestamp: September 30, 2024, 07:47:39 pm
content: Quote from: m k on September 30, 2024, 05:10:17 pmResistors do have a time dependence: a standard real pain in the backside are carbon composition resistors that drift higher over the decadesResistors also have a voltage dependence. I have some high quality "resistance transfer standards" resistors that were measured in a calibration lab; for one the label indicates:100V 1.025200V 1.027500V 1.001TOhms, that isNo, those connectors aren't BNC, and no, I haven't opened the case.

author: MrAl
author_rank: Super Contributor
timestamp: October 01, 2024, 09:35:38 am
content: Quote from: TimFox on September 29, 2024, 07:57:41 pmThat was a good one.Of the foreign tv shows, I also liked Monte Python of course, and Poirot, but my favorite by far was Coupling which was outstanding.

author: MrAl
author_rank: Super Contributor
timestamp: October 01, 2024, 09:40:35 am
content: Quote from: BeBuLamar on September 30, 2024, 07:21:52 pmI think there is a difference between a light bulb filament and a regular resistor or even copper wire. That's because we do not normally use just a regular resistor or copper wire until they get red or white hot. The filament and it's enclosure is made to keep it hot and stay hot, while with the others we only use them up to a certain temperature. Of course, there is the fusing temperature which we take into account for fuses, but once the fuse blows that's about as big as a change in resistance as we can get. If you want to call that a nonlinear resistance that's up to you

author: MrAl
author_rank: Super Contributor
timestamp: October 01, 2024, 09:47:34 am
content: Quote from: Ice-Tea on September 30, 2024, 03:25:41 pmWell, imagine we are all sitting in a cafe' or dining room or board room and someone walks in and asks a question. We answer that question, then he leaves. Do we suddenly forget all about that and go on to sip coffee or tea and just stay completely silent? I don't think so. If the question is interesting enough, it may lead to a longer discussion among us. That's just what being social is all about. If you would like to stay silent however, that is always your choice, but there's no good reason to try to stop others from continuing the discussion.This topic happens to be one of the most controversial I've seen on the web even though it shouldn't be. It's just about the way humans deal with physics. We are constantly coming up with new ways to abbreviate the complex to make it faster to deal with, and in the process making it a little more obscure for someone who is not in the know. It's both a blessing and a curse.

author: m k
author_rank: Super Contributor
timestamp: October 01, 2024, 10:26:03 am
content: Quote from: tggzzz on September 30, 2024, 07:47:39 pmBut shouldn't there be an indicator that the value includes time.Maybe tOhms, but would yours then be tTOhms or TtOhms, and should there then be more than one value.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 01, 2024, 03:06:43 pm
content: Quote from: MrAl on October 01, 2024, 09:47:34 amThis is the beginner section of the forum. For simple questions and simple answers.To further your analogy: imagine the guy asking the question still sitting at the table, wondering what the hell just happened, feeling like all the fun is being sucked out of his simple experiment, feeling unwelcome at the table, considering never to ask a question with these clowns again and contemplating if maybe electronics isn't for him/her after all.No offense and all that, but as an educational endeavour you guys are kinda missing the mark.

author: IanB
author_rank: Super Contributor
timestamp: October 01, 2024, 03:38:21 pm
content: Quote from: Ice-Tea on October 01, 2024, 03:06:43 pmI thought the same thing. Threads in the beginner section should not get derailed and taken off into the weeds. The goal should be to help the person posting and not conduct private discussions.

author: TimFox
author_rank: Super Contributor
timestamp: October 01, 2024, 05:42:20 pm
content: The balance here is not to leave incorrect information to confuse the OP.(I know: somebody's wrong on the internet.)

author: tggzzz
author_rank: Super Contributor
timestamp: October 01, 2024, 05:50:07 pm
content: Quote from: TimFox on October 01, 2024, 05:42:20 pm... and not to leave incorrect information to confuse other people that randomly bumble into the conversation later on... and to (attempt to) prevent yet another outbreak of misinformation

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 01, 2024, 07:01:47 pm
content: Quote from: Electrump on September 28, 2024, 01:30:38 pmCan you do the experiment 3 again but this time with 5V instead of 2.5V? I bet it would come out right.

author: MrAl
author_rank: Super Contributor
timestamp: October 01, 2024, 07:53:59 pm
content: Quote from: Ice-Tea on October 01, 2024, 03:06:43 pmIt seems he left the board room, or should I say instead he left the bored roomWhile people may agree with that stand and I know people that would, I cannot agree fully because this is not just an electronics site or an educational site it's partly social media. When people start talking, it's often good to read because lots of information comes out of it.I do have to agree that we don't want to go too far with this though, but then usually we don't. We don't usually get into quantum physics and partial differential equation solutions and stuff like that.Maybe we should let the original poster decide, and if he does not come back then no harm done anyway. It's true though that he may get overwhelmed, but then where do we really draw the line. It's hard to say because some 'beginners' are not really beginners, although (and this is a little funny) some experts are not really experts either (ha ha).If you like you can try to come up with a criterion for how much information is too much. You'll have to describe this in detail though. It could make a difference to newcomers one day.

author: Xena E
author_rank: Frequent Contributor
timestamp: October 01, 2024, 09:32:28 pm
content: I downloaded the book and it puts itself forward as being elementary but then uses filament lamps in series and parallel to illustrate a badly made point, which is a bit crappy, unless the author didn't understand that filament lamps are not ohmic devices either. Very misleading to the OP from the very start.It also suggested the reader uses nail clippers to crop component leads.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 02, 2024, 01:06:00 am
content: When you do experiment 1 you can see already that your measurement doesn't make sense.At 2.5V and you have 178mA then the resistance must be 14Ohms but when you measured the resistance when it was cold you got 1.9 Ohms. The resistance increased by almost 10 fold when it's hot.

author: m k
author_rank: Super Contributor
timestamp: October 02, 2024, 12:11:36 pm
content: Only thing that went wrong is that the burst is not reduced.Fundamentally there are three kind of resistances, and many kind of resistors.First there is a DC resistance, R=V/I that doesn't change.Then there is an AC resistance, R=dV/dI that doesn't change either.And finally a real resistance in time that is what ever.Is tangents of AC resistance still a beginner level.That AC thing starts from a unit circle, its radius is 1.That radius is a hypotenuse and its contrary is a tangent of dV/dI, but that's unorthodox and can't remain in a unit circle without an extra dimension.So V/t and I/t have their own unit circles.Next fundamental, power P is voltage times current.Then next,only resistance has radius pointer of its unit circle clock equal for V and I.Other two, inductance and capacitance have them separated, means that they have different phases.By the definition a voltage must be there first or current can't happen.But pure resistance is an ideal and unreal thing.There is always also those other two present, more or less, sometimes very much less, down to a forget it less.Since resistance is also a real mechanical entity it has also many other properties, that can change for what ever reasons.Same goes with those other two types of components.Then we have mathematics and its analyzes.There's no such real thing as negative resistance, but mathematics is purely abstract thing and its analyzes must survive that situation.I'm not mathematically thinking enough that I can explain how that connects to real.For real resistances and their values.Regular SMD ones have only their resistance values, through hole ones have also tolerances, others have few other definitions if there is space.None of those include time.So IMO, presence of time must be specifically added to the conversation, mathematical abstractions also.If nothing is added it's a DC value.

author: MrAl
author_rank: Super Contributor
timestamp: October 02, 2024, 12:55:15 pm
content: Hello again,We usually take what we call "AC resistance" to be the resistance that comes from the skin effect. It's considered an AC quantity because this is the extra resistance that comes from using a sinusoidal source instead of a DC source. The total resistance would then be the DC resistance plus the AC resistance.As to resistors having a time effect, nothing that has length and width has zero reactance. It's a property of the universe. Distance leads to inductance, and width leads to capacitance. Resistance occurs because of both distance and width. This means that any object that has any dimension has DC resistance, AC resistance, and reactance.We don't always think this way because we try to simplify things when possible. We often use the "Lumped Element Model" which is considered valid when the wavelength is significantly shorter than the component dimensions.We usually think of a light bulb as being nonlinear because over it's operating range the resistance changes drastically. If we had a resistor that could take the same voltage as a light bulb, its resistance would not change nearly as much. A light bulb that operates within its range of peration say from 0 to 120v will have a resistance that changes something like 8 to 1, while a regular resistor operated within that same span of 0 to 120v, as long as it is within its operating range, the resistance will change much, much less.A string of around 170 rectifier diodes will have a very wide variation in resistance as we run that from 0 to 120v. Some things are nonlinear that's all.Next, let's connect some diodes in series with some light bulbs and calculate the resistance at 120vA light bulb makes a good heat source when building your own component testing oven. Heat cones even better.

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2024, 02:07:59 pm
content: SMD resistors almost always have a manufacturer's tolerance value: it's just that there is no room on their tiny bodies for many characters.DC resistance, when Ohm's Law is valid, is a straightforward constant ratio of voltage divided by current.For AC purposes, a given two-terminal component at a given frequency is often characterized by a resistanceRin series with a reactancejX, which is useful for current values small enough for a linear approximation where the two terms are constant (at that frequency).Another variable measured in the unit ohms may be called incremental resistance or AC resistance, which is the derivative dV/dIfor small excursions around a given operating point.All "AC" values should be regarded as functions of frequency.Certain interesting devices and circuits, including tunnel diodes and transitron circuits, show a negative value for that derivative, while "normal" parts (sold as "resistors") have positive resistance values.Negative-resistance circuits can be useful, but require external power. "Resistors" do not use external power.

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2024, 03:55:21 pm
content: Quote from: m k on October 02, 2024, 12:11:36 pmThere may be a language issue here: your "AC and DC resistance" are completely mis-named.AC implies alternating which implies a frequency or "/dt" term.Conversely DC implies zero frequency and constant.The standard terminology is static resistance is R=V/I, whereas r=dv/di is called "dynamic resistance", or "differential resistance" or "small signal resistance", as appropriate.The "tangent of static resistance" is the dynamic/differential/small-signal resistance. In the case of an ohmic component, the two are the same.Whether that is a "beginner topic" depends on viewpoint. Certainly I was taught all of the above in the first two weeks at university, but had come across them informally a few years earlier.This diagram is one illustration of the terms

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2024, 04:05:39 pm
content: Quote from: TimFox on October 02, 2024, 02:07:59 pmYour second sentence conflicts with your first: that meaning of "AC resistance" is a complete misnomer, to the point that I've only seen it in this thread. "AC resistance" can be used for resistance as a function of frequency, including skin effects and/or inductive/capacitive components.While the term "incremental resistance" can be justified, I prefer "differential resistance" to emphasise it is the slope of the V-I curve.

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2024, 04:17:21 pm
content: Terminology may differ. I have seen the derivative referred to as "AC resistance" to mean that it is the apparent resistance shown for variations around a quiescent or operating point, to distinguish it from "DC" or "static" resistance (when that is defined).[I am an older person, and I believe that I saw that term used for small-signal parameters in vacuum tubes, such as "plate resistance" for dVpk/dIp.]Ignoring reactance (when possible), AC resistance can also mean the resistance for alternating current flow that has been increased by the skin effect in the conductor at that AC frequency.Note that the skin depth at 60 Hz in copper is only 8.5 mm, so the effect becomes important in bus bars and other reasonably large solid conductors: an old rule of thumb is that copper over 0.75 inch = 19 mm thick is wasted at 60 Hz, so other geometries should be used.My preaching started from an old concern: Ohm's Law (which states thatRis constant for a given component) is a very useful empirical approximation, especially for DC circuits, for many materials. It is not a fundamental law of physics in the sense of, say, Coulomb's Law for electrostatic force.A major triumph of 20th century solid-state physics is an explanation of Ohm's Law for solid conductors, which is a complicated matter best read about in textbooks.

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2024, 06:01:49 pm
content: Quote from: TimFox on October 02, 2024, 04:17:21 pmThe association with AC-coupled valve circuits seems plausible, since associated "magnetic components" were often biassed into a small-signal linear part of their curve. "Magnetics" were still used as part of each single-transistor logic gate in the first computer I used, the Elliot 803. (See one working at TMNoC, and discuss the schematics with the helpers there)I still think the terminology is misleading to the point of being wrong, and ought to have been dropped when units of "µµF" and "Mc/s" went out of fashionQuoteJust so.I seems too many people don't understand that.

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2024, 06:26:24 pm
content: When "c/s" was replaced by "Hz", another suggestion was to rename the liter/min the "Falstaff".Linear vacuum-tube circuits normally biased the plate current to a substantial positive value with zero input voltage, so the plate current could swing higher (voltage closer to the cathode) or lower (voltage closer to the Vbb supply).

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2024, 01:25:19 am
content: @TimFoxDo you remember when the capacitance unit "cm" was replaced by "pF"?

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2024, 01:36:02 am
content: Quote from: radiolistener on October 03, 2024, 01:25:19 amI remember when 10-12F was called "uuF" or "mmF" , or "mickey mikes", before pF and nF became popular."cm" is still the capacitance unit in "Gaussian" unrationalized cgs units, often used by physicists.In my physics education, I was taught electromagnetism in both sets of units: rationalized MKS (as used in SI) and Gaussian cgs.Engineering is almost always done in rationalized MKS (SI).Some of the equations are simpler to write in one system, but others are simpler in the other.Besides Gaussian units, "emu" units measure inductance in cm, but Gaussian uses the "esu" units for electrostatics and capacitance.Typically, esu unit names add "stat" as a prefix, as in "statcoulomb", but emu unit names add "ab", as in abampere.https://en.wikipedia.org/wiki/Gaussian_unitsandhttps://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units#Alternate_derivations_of_CGS_units_in_electromagnetismTo further the confusion, physical field theory often uses other unit definitions, to allow physical constants to be set to 1 and simplify the equations yet further.In Gaussian units, Coulomb's Law for electrostatic forceFin dynes between two chargesq1andq1(in statcoulombs) a distancer12apart in cm simplifies toF=q1xq2/r122The well-known value for the "permittivity of free space" ({epsilon}o) of 8.85 pF/m will give you the relationship between pF (MKS) and cm (Gaussian) capacitor values.

author: m k
author_rank: Super Contributor
timestamp: October 03, 2024, 11:09:11 am
content: Quote from: tggzzz on October 02, 2024, 03:55:21 pmNo, they are very exactly as they should, directly taken from those equations.But as we can see that's not very widely used way to use those names, or at least other users keep low profile.And actually those are not names, both have two separated parts.It's said many times around here that English only, for obvious reasons.Correct terms only is not bad either, but how to do it without silencing the other side.Is linear material correct term.V/I-linear doesn't include other properties, so keep the prefix or use other unambiguous term.Something personal,seems that I can't visualize pure resistors resistance values at all, I can see curves of course, but that's different.Then prefixed stuff, temp sensors, LDRs, VDRs, no problem, but that's not visualizing, it's more like understanding.Maybe seeing curves is actually not so different.E,pretty difficult expression that resistance value of a resistor.

author: tggzzz
author_rank: Super Contributor
timestamp: October 03, 2024, 11:26:05 am
content: Quote from: m k on October 03, 2024, 11:09:11 amWell, that's agloryfor you."Silencing" and "other side" are complete red herrings; neither are appropriate. What is important is the shared understanding between people, both receiving and giving. That requires terminology which helps indicate meaning, and a common agreement on meaning. if that reference isn't sufficient, tryhttps://www.thoughtco.com/humpty-dumpty-philosopher-of-language-2670315

author: m k
author_rank: Super Contributor
timestamp: October 03, 2024, 11:37:45 am
content: From shared understanding to silencing can be just order of words.Generally how message is received.I'm sure my text can be blunt and stuff like that, simply because my native language doesn't include those extra words for being polite, at least naturally.

author: tggzzz
author_rank: Super Contributor
timestamp: October 03, 2024, 12:14:11 pm
content: Quote from: m k on October 03, 2024, 11:37:45 amI'm not sure why theorderof words is so key.I can be blunt too.I'm amazed Finnish (I presume) doesn't include words for being polite.But then Finnish is notoriously unlike other European languages

author: m k
author_rank: Super Contributor
timestamp: October 03, 2024, 04:01:31 pm
content: Order of words has nothing to do with your words, it's just smallest difference that came to my mind.Dynamic language can express things by "bending" words.Couldn't you finally do the effing thing now, please.Could you kindly do the effing thing now, please.We don't have that "please" word, but other words make the message clear.Here it's also completely possible that curse word replaces that "please" word, but that needs a face to face situation.Curse word is not exactly polite, but it can still lighten the message, or not.Then something personal again,I can't really visualize 3D integrations, but I can understand a technical problem of final direction of a set of 3D thrusters.

author: Electrump
author_rank: Contributor
timestamp: October 04, 2024, 09:07:39 pm
content: I will reply with regards to my initial question if you please.Following suggestions above I repeated experiment with resistors. One 10Ω resistor at 2V takes 209mA in my case (becomes very hot as it is a small one), two in series take 102mA and two in parallel take 409mA. So “ohmic” elements obey the laws!I noticed though, that the current increases a bit after a few seconds, so I took a can of “air duster”, which is basically compressed air, and blew cold air onto two parallel resistors. As long as the resistors were cooled the current went down to 406mA, but once stopped cooling, they went back to 409mA. So, there is a slight dependance on the temperature, at least in my case. The interesting bit here is that the resistance seems to decrease with the raise of temperature. This kind of contradicts whatRJSVsaid on the first page of this topic:“The explanation, in physics and molecular terms, is that increased temperature activity consists of faster and likely more widespread movements, of each particular particle, and that creates a kind of opposition to other particle (electrons) travel or out of the more active region.”IanB, the graph for one 2.5V rated bulb will look like thisNo surprise that same two 10Ω resistors in parallel show much straighter line.Ice-Tea, provided that the bulb has 1.9Ω at 2V it should allow (2/1.9) 1.05A current, and not 170 mA, right? And you are eagle eyed! Thanks for spotting the error, the power supply was 2V indeed!radiolistener,additional experiment proved you right regarding the expected result.BeBuLamar, “In experiment #3 instead of 2.5V use 5V and you see everything is right.” Because I initially used 2V and not 2.5V as I stated I have followed your advice corrected to true value. Two bulbs in series, with 4V produce 0.184 current which is ~50% higher. It makes sense provided that the device is non-ohmic and that the resistance must have increased as well.I am not saying I compleeeeetly understand now how it works with the influence of the waves and the vacuum and whatnot, but this is more than enough for me to move on. Thanks

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 09:15:35 pm
content: Quote from: Electrump on October 04, 2024, 09:07:39 pmCool. A couple of things to mention:While most materials increase their resistance when they get hotter, this is not universally true. Some materials do have a negative temperature coefficient, and conduct better when they get hotter. Graphite is one such material, which may be used in the older style of carbon composition resistors.If you run one bulb at 2.5 V and measure the current, then run two of the same bulb in series at 5.0 V, then you should observe the same brightness and the same current.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 09:15:45 pm
content: Were the ohmic resistors in these tests carbon or metal film?Even if a wirewound resistor is made from low temperature co-efficient alloy, mechanical thermal effects of the substrate or core will affect the resistance.Practical metal film and wirewound resistors are usually quoted over a bipolar range, e.g. +/-100 ppm/KCarbon has a strong negative temperature co-efficient of resistivity (really a semiconductor).

author: Electrump
author_rank: Contributor
timestamp: October 04, 2024, 09:27:27 pm
content: I guess it's carbon.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 09:59:44 pm
content: Those are probably carbon film, but not obvious from the photo. Different manufacturers use different colors, but the "extreme dogbone" shape suggests carbon film.Since cheap metal film resistors became available, the carbon ones are less popular (due to bad tempco), but still used when high values or low inductance is necessary.50-odd years ago, in a strenuous physics class with long lab sessions, we measured the resistance of a bunch of different solid samples from 78 K (liquid nitrogen) to maybe 500 K (heated) with traditional equipment ("Kelvin bridge" at room temperature with four-wire connections) to see how different types of materials had quite different temperature dependence.

author: Xena E
author_rank: Frequent Contributor
timestamp: October 05, 2024, 03:32:16 pm
content: Quote from: Electrump on October 04, 2024, 09:07:39 pmBy noticing the aberration you have discovered more that was ever intended in that first exercise.