Timeline for Why do LEDs not obey Ohm's law?
Current License: CC BY-SA 3.0
19 events
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| Dec 28, 2014 at 18:44 | review | Suggested edits | |||
| Dec 28, 2014 at 18:51 | |||||
| May 6, 2013 at 8:10 | vote | accept | Antoine_935 | ||
| Apr 17, 2013 at 5:28 | comment | added | Anindo Ghosh | @Brad Perhaps you need to re-read Ohms law, and see why it doesn't apply to non-Ohmic (yes, google that term) devices. That might provide some clearer fundamental understanding of the law, rather than a debate based on applying specific state-variants. | |
| Apr 17, 2013 at 2:54 | comment | added | Kaz | So it's a matter of word semantics. To me, resistance is an important quantity; it's more than just some number that pops out when you divide voltage by current. It has meaning. | |
| Apr 17, 2013 at 0:52 | comment | added | Kaz | But, say, Hooke's law can be violated. Any elastic thing whose elongation/compression is not proportional to the displacing force does not obey Hooke's spring law. | |
| Apr 17, 2013 at 0:44 | comment | added | Brad | It's a "law" - you can't violate it! | |
| Apr 16, 2013 at 23:26 | comment | added | Kaz | Then again, maybe Alfred has a point. For instance, do you still call it Hooke's Law, if you have a spring that has a non-linear force with respect to elongation? | |
| Apr 16, 2013 at 23:23 | comment | added | Brad | "Proportional" - yes. "R is a constant" - nope, it doesn't state that. Why does it "state this" for R and not for V and I. "Proportionality" is akin to "linearity", which I do agree. Wikipedia may state that - but ohms law does not. It's call ohms "law" for a reason. It is universal, not just for purely resistive, nonlinear devices. | |
| Apr 16, 2013 at 23:12 | comment | added | Alfred Centauri | @Brad, (cont.). From Wiki: Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship... More specifically, Ohm's law states that the R in this relation is constant, independent of the current. | |
| Apr 16, 2013 at 23:01 | comment | added | Alfred Centauri | @Brad, don't you see that you're generalizing the concept of Ohm's law into simply "if you take the voltage and divide it by the current, you get a number"? Of course you do. But there's more to Ohm's law than that! Further, by introducing explicit time variation, you're muddying the waters. The crucial point here is this: if a circuit element obeys Ohm's law, the ratio of voltage to current does not depend on the particular voltage or current value. (cont.) | |
| Apr 16, 2013 at 22:46 | comment | added | Brad | You are describing the fact the the response is nonlinear - and I agree it is not. However that was not the question. By this logic a "variable resistor" would also disobey ohms law. Ohms law defines the relationship between resistance, current and voltage as an equation - proportions to each other. It merely states a change in one will require at least a change on one other to remain valid. You are insisting R must maintain constant while only V and I change for a device. This would describe a device which is linear and purely resistive - but not the only one which would apply. | |
| Apr 16, 2013 at 22:16 | comment | added | Alfred Centauri | @Brad, it doesn't obey Ohm's law - period. For Ohm's law, V and I are proportional, i.e., the ratio is constant. | |
| Apr 16, 2013 at 20:31 | comment | added | placeholder | @Brad I won't reply any further after this. Just because you CAN map the device to be a variable resistor doesn't mean you should. It doesn't reveal any inherent behaviour and is misleading. You could as easily model the device as a variable capacitor with even less explanatory power. The I vs. V characteristics are THE defining aspect and are fundamental to what the device is. And even in this aspect you're wrong because you should define a device in it's small signal resistance NOT large signal as you have done. You must deal with partial differentials to reconstruct the I/V curve. | |
| Apr 16, 2013 at 20:18 | comment | added | Brad | Exactly - like I said: A diode does not have a "fixed resistance". I could however argue that it does have a known resistance for a given current. The question was about if it obeys ohms law, and it does. It just doesn't have a constant resistance. | |
| Apr 16, 2013 at 20:15 | comment | added | placeholder | @Brad exactly, and then how would you use that single data point to then tell you what would happen at say 2X the current? WIth Ohms law and a resistor you'd be able to say 2X the voltage. You are talking about an equivalent resistance at a single operating point. that does not describe a device. | |
| Apr 16, 2013 at 19:55 | comment | added | Brad | If I had a black box, and ran a current through it - then measured the voltage across it - I could calculate it's resistance at that point in time. It' doesn't matter what is in the black box. | |
| Apr 16, 2013 at 19:42 | comment | added | Christoph B | I think your anwswer needs to be highlighted more as it is the only correct one (at the moment I am writing this). Ohm's law is empirical and was originally derived from obserwing the behaviour of wires of different length. Water doesn't obey Ohm's Law, air doesn't - only conductive materials do, and even then not always. | |
| Apr 16, 2013 at 19:41 | history | edited | placeholder | CC BY-SA 3.0 |
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| Apr 16, 2013 at 19:29 | history | answered | placeholder | CC BY-SA 3.0 |