Talk:Ohm's law

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The above rating is completely absurd. Look at the Wiki entry for Electrical Resistance and its rating. Compare content, style and organization of ideas. Actually a critical reader doesn't have to read past the intros. Whoever is making these evaluations simply doesn't understand content, education, audience, accuracy and organization, to say nothing about the importance of the topic itself. blackcloak 07:00, 18 May 2007 (UTC) Now that it has been changed to reflect a more considered appraisal, I accept the current rating. blackcloak (talk) 20:08, 26 February 2008 (UTC)

So much BS here that no one put anything about OHM's LAW in here! If you EE type jerks look at a dictionary, that's what should be here and not y'all's rhetoric.

the Law is U=I*R and the Units are 1V=1A*1Ohm as an example

Whoever you are you need to learn to stop reading, or to skip over, material you're not interested in absorbing. Further, this is not a dictionary. Look at entries in several encyclopedias it you need examples of scope and style. blackcloak 07:25, 22 May 2007 (UTC)

The page says "Ohm's law (named after George Ohm, who discovered it) states that when the resistance of a device is independent of the voltage applied across it, and therefore of the current through it as well."

What is that "when" doing there? Can it come out? Vicki Rosenzweig, Saturday, June 22, 2002

UNIXCOFFEE928Hi! Can someone state the history of the variable 'I' in the context of Ohm's law? What was currrent originally called, for it to be labeled as 'I'? why does resistance increase with heat? fromlisa

International Ampere

```Nevermind, I found it, I'll add it..

There are more constraints to Ohms law. 1. The voltage/current/resistance has to be constant - as ohm defined. This law will be challenged if any of them fluctuates. 2. This is for conductors - not insulators or semi-conductors or super conductors.

[edit] explanation of present ohm's law

Could someone explain how Ohm's law went from J=pE to V=IR?

its $\mathbf{j} = \sigma \cdot \mathbf{E}$ not $\mathbf{j} = \rho \cdot \mathbf{E}$

basically $\sigma=\frac{1}{\rho}$

$\rho=\frac{RA}{L}$

$\mathbf{E}=\frac{V}{L}$

$\mathbf{j}=\frac{I}{A}$

then just substiture into $\mathbf{j} = \sigma \cdot \mathbf{E}$ cancel and rearrange.

[edit] Power law

Energy (watts) = Volts * Amperes ought to be on here or linked from here.

That's Joule's Law. I don't see why it belongs here. --Heron 18:34, 27 September 2005 (UTC)

[edit] V=I·R

According to Halliday/Resnick, the expression V=I*R is NOT a statement of Ohm's law. The formula applies to all devices, while Ohm's law only applies to resistors where current is proportional to the potential difference applied to it.

That's right. Ohm's law is an experimental discovery - "V=I*R where R is a constant" - not just the equation "V=I*R". I tried to make this clearer in the article, by defining 'differential resistance' for non-ohmic devices. --Heron 22:27, 25 February 2006 (UTC)

[edit] Is all this necessary

Having spent several hours dumbing down the intro section I've some further impressions

As written, about half the content herein is so far over the head of the typical reader that I have to question the wisdom it's inclusion at all, but that's Wiki!
While it does no harm (except being a 'turn off'), if left, I suggest the article presentation order be reworked to push material that would be of more general interest higher (e.g. the historical stuff) in the presentation, and move things like phasors and current densities, et al downward. Gotta go!

FrankB 22:21, 15 March 2006 (UTC)

OK, you may have a point. Provide an outline you would find more suitable for the average wikipedia reader, and I'll consider reworking the article accordingly. Please be specific, esp. when it comes to dividing up current sections into general interest and narrow interest portions. --Blackcloak 29Mar06

[edit] Oh really?

"Most electrical engineers use Ohm's Law every working day." Is there data to back this up? What is an "electrical engineer"? I know many "electrical engineers" (as defined by a person with an EE degree) that work only on digital circuits, as computer programmers, and as patent attorneys. I would be willing to believe that most EEs, from time to time deal with Ohm's Law, but every working day sounds like hyperbole to me. It should be verified or else come out. --Lenehey 22:51, 26 April 2006 (UTC)

~ i agree. the 'electrical engineer' statement is dodgy. But the article should somehow make it obvious to people who don't study electronics that V=IR is a fundamental rule in electronics, and a formula thats used extremely often.144.136.8.35 07:17, 24 May 2006 (UTC)Ryan

Yes, really. Obviously the phrase refers to individuals working as electrical engineers, and not someone trained as an EE and doing other things. If you don't think the sentence says that, then you can change it. This wording came out of an objection to the phrase that went something like ""EEs must be intimately familiar with Ohm's equation." I think the comment about digital designers may have some merit, so perhaps the statement should be restricted to those working on analog circuits. In today's world of digital design, the computer program won't allow digital designers to do certain things, like load down an output too much, by internally applying (perhaps) Ohm's law. Of course, virtually all digital circuits, within the working subunits, have resistors. One could argue that logic designers are not really working EEs. They just connect the building blocks created by EEs who create actual circuits, and you better believe their using Ohm's law all the time. We used to call those who would use catalogs, solicit vendors and elicit proposals for contracts to do projects (i.e. use pre-engineered resources) "armchair engineers." I think very few of them could have explained Ohm's law. blackcloak 07:24, 25 August 2006 (UTC)

__________________________________

The USNavy presents ohms law in a very memorable stack as follows:

    P    watts
    E    volts
    I    amps
    R    resistance

Utilizing only three adjacent members of the stack per calculation;

-you multiply up the stack,

-and divide down the stack.

[edit] Intro and other ramblings

"the intensity of the current" is not a very useful/understandable phrase. Something should be done to change it.

The entire intro should be rewritten, perhaps in deference to Herron, to include a reference to the original form, but to discuss the modern form. Confusing readers right at the introduction with historical artifacts is not a reasonable thing to do. The History section is the proper place for Herron's recitation.

Regarding the term law, by today's sensibilities, Ohm's law would now be called Ohm's conjecture, or Ohm's approximation, or maybe Ohm's theory. The term law is no longer used because scientists found that they had to continually modify ideas that codified in scientific law (which most people take to be the final word on something).

In effect, the result of Ohm's observation is that Rsub0 in a power series description of resistance is defined. That would be the modern interpretation of what Ohm accomplished, even though he may have no idea that that was what he was doing at the time.

In the intro, we should be making things as simple, and as current (no pun intended), as possible. It doesn't really matter what Ohm thought at the time except when we're trying to establish the historical record. His name is associated with V=IR now. And unfortunately there is this missing link about how you define R before you can use it in Ohm's law.

Dropping any reference to conductor in the into ripples through the article. Wording will have to be changed elsewhere. blackcloak 15:58, 25 August 2006 (UTC)

Your complaints against me are false, Blackcloak. I have looked through the entire history of the article, and it shows that I did none of the things of which you accuse me. Please direct your ill-informed diatribe at somebody who deserves it. This is how the historical information got added to the intro:

Addition of name of Georg Ohm: not me
Addition of a footnote stating the title of Ohm's work: me
Explanation of the label I: not me, but grammar cleaned up by me
Restated as V/I = constant: not me
Year of discovery added: not me
Restriction of the law to metallic conductors and resistors: me
Historical digression about Fourier's Law & quote from Ohm: not me
Cleanup of above: me
Insertion of Ohm's symbol 's': you

>-( --Heron 18:39, 25 August 2006 (UTC)

As far as I can tell, Heron is the only one who is telling everybody what Ohm's law is and what it is not. The comments have to do with insisting on an historically accurate (and therefore a somewhat more obscure) approach to describing Ohm's law (e.g. see 7/1/06 and 7/26/06 summary comments). If the attribution is inaccurate, I apologize. One paragraph where I mention you is a diatribe? Sorry but you're making it hard for me to take you seriously. Someone who got the point would not have bothered to go back in order to create a list of mostly irrelevant material. blackcloak 05:30, 26 August 2006 (UTC)

Apology accepted, and I offer mine in return for flying off the handle in response to what was only a mild criticism. --Heron 21:05, 28 August 2006 (UTC)

[edit] Direction of current

The diagram show current flowing from positive to negative. Current flows from negative to positive, as the negative side of a battery has an excess of electrons and the positive a deficit. At least this is what I learned during my electronics training in the U.S. Air Force. From Bob.

Hi Bob. Conventional current flows from positive to negative. Electron current flows from negative to positive. Most engineers outside the U.S. military use conventional current. If you don't specify which definition of current you are using, it is normally assumed to be conventional current. --Heron 19:18, 29 August 2006 (UTC)

"Electron current" is a simplified concept created during World War II which relies on the fact that the carrier in metals is the electron, and that vacuum tube electronics uses only electrons.

But electric current in general can have both polarities of carrier, or in other words, in many cases the electric current in a conductor is not composed of electrons. For example, in salt water and in acid or alkaline solutions, no electrons flow, and the current is entirely composed of positive and negative ions flowing in opposite directions. In this case which is the "true" direction of current? To solve the problem we add the two currents together and pretend that only one type of carrier is flowing. So whenever you suffer a shock, we say that a single type of "amperes" flowed through your flesh, even though no electrons flowed inside your body.

In plasmas such as neon signs, sparks, the Aurora, etc., the current is made of electrons and positive ions flowing in opposite directions. Same problem, same solution. In other words, scientists and engineers who must deal with electrolysis (batteries or electroplating) or plasma conductors, they use a very old convention: the simplified idea that only one type of carrier is flowing, and that carrier is positive. So why don't they say that the conventional carrier is negative? BECAUSE IT'S A CONVENTION, just like saying that one end of a magnet is called "N" and not called "Q." If a certain group of people tries to re-label the poles of a magnet, or tries to change the current convention, well, they'll have a huge uphill battle which really is a waste of effort since they're guaranteed to lose in the long run. No technician or group of technicians is going to change the standards set by the entire engineering and scientific community. Remember that Maxwell's equations are based on conventional current. (PS If we assume that negative carriers are flowing, then all current measurements should have a negative value unless for some reason we turn our ammeters around and measure backwards flows. Negative charges flowing backwards is a negative times a negative, giving a positive reading. It's simpler to just assume that positive carriers are flowing forwards.) --Wjbeaty 04:12, 4 September 2006 (UTC)

The way I figure it, an electron has a negative charge (convention)and flows, as stated above, from the negative terminal to the positive terminal. But we know that a negatively charged current flowing in one direction is (first approximation, let's say) the equivalent of a positve current that would flow from the positive terminal to the negative terminal. By looking at it this way, there is no discrepancy as suggested above. blackcloak 06:37, 2 November 2006 (UTC)

[edit] EE goes gaga

I'm one of those (semi-retired) EE's who (used to) use the Law every day; indeed I'm using it today, both in engineering itself and as a mental math problem for one of my students. This relationship is graven in my soul, yet after reading this article, there is doubt in my mind.

All of the extensions and elaborations don't belong here at all; stuff like

$Z(d) = Z_0 \frac{Z_L + j Z_0 \tan(\beta d)}{Z_0 + j Z_L \tan(\beta d)}$

is engineering bafflegab, inserted by somebody trying to show how much he knows. It's certainly not Ohm's Law; it's one of the many, many equations that describe circuit variables under circumstances when Ohm's is not enough. This is confusing to the layman and misleading to the novice engineer.

Then there is the huge attempt to state when Ohm's holds. Non-ohmic and active components may actually have negative differential resistance... This is irrelevant to Ohm's -- by definition. The simple expression is sufficient:

Ohm derived his law experimentally by measuring currents through metal wires. No object obeys Ohm's law perfectly but those that do so reasonably well are called ohmic devices. Many things do not, including prominently almost all semiconductor devices. In these, voltage and current are subject to very different relationships.

Perhaps somewhere down the page there's room for a list of things ohmic: essentially, most common metals, plus carbon. I'll bet some smart fellow can add to that list but there's no need to bloat it. And between the simple statement and the list, there's really no need to beat the horse to death or dip into any discussion of semiconductors, negative resistance, or anything that doesn't follow the Law -- since that's what this article is about. Strain effects, temperature drift -- these all belong in their own articles; they have nothing to do directly with Ohm's.

I'm pretty annoyed at the lengthy, equation-heavy physics section. What's the point? Physicists are more important people than EE's? Physicists have no use for Ohm's qua Ohm's; they use these complex field equations instead -- which Ohm never had anything to do with, so far as I know. You're welcome to set me straight.

A quibble with V. How did this get in here? Is this some sort of British convention? V means volts; it's a unit of measure, just like A means amperes and Ω means ohms. But the variables are E = I • R. Yes, you could write V = A • Ω and you could also write S = T • U and tell everyone what you really mean. Otherwise, if we've sucked enough air on this issue, let's use the industry standard, please.

I learned V=IR in 8th grade in public school, Massachusetts. Must have been part of the curriculum (my teacher wouldn't have been knowledgeable enough to depart from the convention). So V it is. I suppose one possibility is that you were not brought up in the American school system. Anyway, E is used most commonly for electric field, which is volts per meter. I seem to recall V (volts)=Integral of (E.dl), so unitwise, things are consistent. I find it curious how teachers can sometimes be so dogmatic and inflexible, insisting that they know the only right way and then expecting conformity from their "students." (They have to be trained after all.) Here we are all peers. No one seems to care where we each got our PhDs. blackcloak 09:11, 27 July 2007 (UTC)

I learned this basic relationship almost in my crib -- I grew up in a family of hams -- and I've used it, on and off, for 40 years. If this article manages to confuse me, what possible good can it do a general reader? John Reid 21:58, 26 October 2006 (UTC)

I agree with you about the math. All of it should be removed. It does not belong here. Nothing close the the math included was ever part of Ohm's original contribution. While I like your simplified description of Ohm's studies, I have no problem with including related material. Putting this stuff under a properly titled section that clearly alerts the reader to the somewhat peripherial nature of the content would be entirely appropriate. The basic rationale is that Ohm's law feeds into so many different areas and that there should be a easy way to navigate to these other related topics. All entries of this nature are sufficiently brief to suit the purposes intended here.

Having said that, there is a basic problem steming from what Ohm actually showed. Before Ohm, the concept of electrical resistance had not been developed. There was no unit of resistance; there wasn't any knowledge about what, if any thing, was actually flowing in wire. The idea of resistance to flow in other fields was understood (tho perhaps not widely). So Ohm had the problem of describing a proportionality factor, later to become known as resistance with a unit bearing his name, when there was no formal definition of anything like electrical resistance. It is a real chicken and egg situation. While Ohm did not actually define resistance, someone (who?) had to have extended Ohm's work to a definition of resistance. Once we have the definition, we can create simple and practical formulas, like the one known today as Ohm's law. In summary, Ohm's developed formula that strongly hinted at a simple definition of resistance, others defined resistance, still others then used the definition to refashion Ohm's original observation into an equation that we now know as Ohm's law. Assuming you agree with the above assessment, we now have the problem of deciding what we're going to refer to as Ohm's law in this wikipedia entry. The way it is now written, it is a strange combination of the original contribution and the later simplifications, and uses a definition that Ohm did not have. This is one reason why the history section should be returned, so some of this can be explained. Alternatively, we could split the article into two or three entries, e.g. Ohm's original form of Ohm's law, The definition of electrical resistance, and The modern form of Ohm's law.

Generally, there is so much that needs to be done to really treat the subject properly that it will take someone with real energy and persistence (there is no consensus on this article by editors, content is controled by those who are relentless in pushing their views on what is right, so watch out) to really fix things. It's actually a good example of how the lack of controls within the wikipedia system leads to an article with content and organization problems like those you have properly and astutely identified. blackcloak 06:37, 2 November 2006 (UTC)

I'm sorry, but I don't go along with the historical issue worry. I'm not really concerned with what Georg Ohm actually did, only in the law named in his honor. The only point in question is the claim made (in lead) that Ohm 1826 stated the law. I don't see any citation for this -- for that matter, I don't see any decent citations at all.

My concern is what Ohm's Law is. History does give us a certain perspective. What did Ohm see? Resistance cannot directly be measured, only inferred; this is what he did. I don't believe there is any way to measure voltage or current directly, either.

Current is measured practically by breaking the circuit, inserting a current divider, and a galvanometer in series with the low-current path. Then you do a lot of calculations. All the galvanometer is measuring directly is magnetic field strength. For commercial purposes, one throws all the above into a box, calibrates it against known currents, and labels it an ammeter.

Voltage is measured practically by putting the galvanometer in series with a constant resistance (or at the output of a voltage divider) and probing between two points. Again, commercially, the approach is to bundle the stuff in a box, calibrate it against known voltages, and call it a voltmeter.

Resistance is measured practically by making current and voltage measurements simultaneously and doing the math. Commercially, one simply throws an ammeter into a box with a constant voltage source, calibrates it against known resistances, and labels it an ohmmeter.

One may build a sufficiently robust galvanometer to be inserted directly in series with the test circuit, without the use of a current divider. A crude galvanometer is easily made by taking a common compass and laying it alongside a wire. This lets one make current measurements without depending on resistance. One may also fabricate a battery and assume it is a constant voltage source. Finally, one may fabricate a bar of any particular material and see if it is ohmic in nature. Under these conditions, one may observe Ohm's Law in action. Double the number of cells in your battery; if the galvanometer deflection doubles, you assume your load is ohmic.

Now that is all there is to the subject. It's just plain wrong to talk about application to AC circuits; it's a considerably more complex matter. Temperature effects deserve no mention here; they are in addition to Ohm's, which is simple and, like all simple abstractions, not entirely accurate for all kinds of reasons. It's even more nonsensical to talk about strain.

The physics is okay, only to tie Ohm's onto the next rung on the ladder. But I see no citation that tells me Physicists often use the continuum form of Ohm's Law and I suspect they don't. A short distance down that section an attempt is made to derive Kirchoff's; this is so convoluted that I half begin to believe it's not true. One or two simple diagrams would explain the matter far better.

I propose to rewrite this article, removing or seriously reducing any discussion of temperature, heat, strain, and sheet resistance. I say this is just all built-up cruft nobody has bothered to scrape off. I shall demote the physics section to a point after the layman reader has been given a clear, adequate explanation suitable for understanding why he can't plug a hair dryer intended for the car into his home wall outlet. I shall move the equation-dense derivation of Kirchoff's to the place it belongs and generalize Ohm's -- as Ohm's, as a practical tool for electronic engineers and technicians -- into Kirchoff's briefly and clearly, with a couple of simple diagrams. I'll also see if I can't put in some good sources, like Ohm's paper itself.

Anybody who wants to contest this, time is now. Drag up your sources. At present, the entire article is topheavy, on no real legs at all. If this were about Pokemons, it would have been on AfD a year ago. John Reid ° 07:13, 18 November 2006 (UTC)

If you go back about six months ago (use the history tab), you will find a history section. This actually does present some detail regarding exactly what Ohm did. I tried to locate on the internet an English version of the 1826 (7?) paper, but could not find one. You can find a pdf file of the original in German, and the equations suggest what Ohm was up against. In hindsight, the problems was that there was no definition of resistance. What he did was to show that measurements of voltage and current (quite a bit more complicated than your explanation suggests)were consistent with an equation (much like what we now know as Ohm's law) that incorporated a constant (equivalent to a length of wire, and what now might be called the source resistance plus and resistance associated with leads and contacts) plus the length of the wire under test. I saw no equation in the paper that actually looked like the present form. You are right when you suggest that the intro contains an attribution error. There is nothing strange about this; it's a very common mistake.

As for the laundry list of changes, I don't agree with much of it, and I've already expressed some of my reservations. But I am wondering if it is mostly an organization problem, and a problem of mutually agreeing to what the audience is and what their tolerance level (for detail) is. As I suggested, I think you'll be endlessly reverted if you make all the changes you mentioned. Eventually you'll give up. So, why don't you start with an outline. Then get other contributors to add to it, and have a stake in the outcome (and be willing to defend you as well). Personally, I would rather see a high level outline that proposes multiple wikipedia articles, all pointed to by one lead article, so that all the current contributions can be incorporated. That way no one can get really upset. Then the problem becomes one of fighting over what limited set of ideas will be presented in the lead article. The initial outline would summarize the ideas to be contained within each of the articles and show how references will link everything together. If other editors can see the logic of the approach, it has a chance of succeeding. blackcloak 03:52, 4 December 2006 (UTC)

[edit] Sketch

I removed this sketch:

Image:Ohmslaw1.jpg

which is of no use without an explanation. --Heron 20:46, 8 February 2007 (UTC)

Even with an explanation, it's of scant use. It simply demonstrates, in a laborious way, the polarity conventions of voltage and current. Robert K S 22:07, 8 February 2007 (UTC)

I'm unhappy that my work is deleted.
In simple circuits one uses Ohm's law in R+ (R+ means Real numbers, zero or positive).
To calculate mesh currents one uses Ohm's law in R (R means Real numbers, negative, zero or positive).
In my work you can see that in Ohm's law in R appears the sign + or -.
Also in Faraday's induction law (also a law in R) appears the sign + or -.
So, e = + N d(flux) / dt or e = - N d (flux) / dt.
Tsi43318 22:01, 9 February 2007 (UTC)

I'm sorry that you're unhappy, but this is Wikipedia, and stuff gets deleted all the time. There are two reasons for deleting your drawings:

They are hand-drawn, which makes them unsuitable for publication. Look at any other article and you will see that all the drawings are machine-drawn, and usually to a high standard.
They appear to be original research, which means that they represent your view of the subject and not the mainstream view. Ohm's Law is simple, and exists in one version, not six.

If you think you have a good idea, then please discuss it on the talk page, with hand-drawn sketches if you like, but you will find that nobody will allow such things in the article itself. --Heron 13:47, 10 February 2007 (UTC)

Thanks, Heron, for taking this task on. As for Tsi43318, he does not appear to have read the section where signs are discussed, nor to have read any of the discussion on sign conventions. Then, referring to Faraday's induction law to show that some (physics/engineering) equations have negative signs in them is just absurd. On another topic, is it time to declare 75.111.128.53 a vandal? Errors fixed many times, if not malicious, has no understanding and repeatedly enters garbage. blackcloak 06:28, 12 February 2007 (UTC)

Please, mention what I should read. Tsi43318 13:10, 12 February 2007 (UTC)

Dear Heron,

Thank you for your answer.

>They are hand-drawn, which makes them unsuitable for publication.

To tell the thruth I cann't machine-drawn and I'm not interested in it. I was 30 years professor and all my courses were hand-written and I was not the only one. Even when I was student in Paris there was a professor with a hand-written course. And in all my discussions with students the drawings were hand made. A candidate publisher can adapt a sketch, drawing to his wishes. So one can eventually change your Vir.png to vir.svg but one cann't ask you to do this job.

>Look at any other article and you will see that all the drawings are machine-drawn, and >usually to a high standard.

For hand-made drawings you need only seconds but for machine-made minutes. To improve the quality after some time handdrawers redrawn faster than machinedrawers. A hand-drawer is like a painter, an artist. I learned my students to draw a Gauss bell curve (statistics) on the blackboard. And Belgium is the land of beer, chocolate and hand-drawn comic books.

Much more important than hand- or machine-made is the content of the drawing. In statistics I made a drawing that I never saw in books. A professor at the California State University wrote me about my hand-made work: "I always enjoy simple modifications of basis concepts that others have not noticed after all this years."

>Ohm's Law is simple, and exists in one version, not six.

There is only one law but it can be expressed in different ways. The drawing in the article represents only Ohm's law in R. I should mention here that V and I belongs to R and R belongs to R+. I propose to add also a drawing that represents Ohm's law in R+ for V>0 and V<0. So the numbers in this drawing are all in R+.

>If you think you have a good idea, then please discuss it on the talk page, with hand-drawn >sketches if you like...

Noticed!

Tsi43318 13:01, 12 February 2007 (UTC)

I can only repeat what I said. Wikipedia's house style is what it is, and neither you nor I can change it. Your point about R and R+ is too complicated for this article. This article is about Ohm's law, and you don't need to know about number fields in order to understand it. --Heron 23:10, 24 February 2007 (UTC)

[edit] Two schemes

I added the first scheme that was used in this publication. Here the European presention was used. Later is was replaced by the USA presentation. It is maybe not bad to give both? Tsi43318 21:14, 9 February 2007 (UTC)

What "scheme" are you talking about? Do you mean schematic diagram? The idea is to make things as simple as possible, without making it so simple that a novice can't figure out what is being communicated in a diagram. (I can tell English is not your native language, and that appears to be part of the problem.) blackcloak 06:28, 12 February 2007 (UTC)

Dear Blackcloak, On 9 Feb I puted Vir.png from Heron back in the article. This intervention survived three users until you came. I propose to put it back for the European readers.Tsi43318 07:22, 12 February 2007 (UTC)

[edit] Some explanation of conventions needed?

Perhaps there should be some material added about the conventions used when actually applying these equations? I was confused for a bit while trying to apply the equation P=I^2*R when calculating the power dissipation rating needed for a resistor. For instance, if you are applying a 50A DC current across a 100 ohm resistor, without knowing the conventions used, you can easily arrive at a needed dissipation of 250,000W! It took almost an hour searching the web before I found that the R value in that equation is conventionally in KILO ohms, not ohms. Applying the correct value of .1Kohms for a 100 ohm resistor gives the correct answer of 250W. I am not entirely sure whether this should go on this page or the Joule's Law page, since they are derivative of each other and the equation I mentioned above is present on both pages.

edited once for spelling

24.32.223.33 23:11, 2 June 2007 (UTC) CharlesN

The correct value is 250,000W, or 250kW, for the values you mention. 50A through a resistance of 100Ω produces a voltage drop of 50×100=5000 volts, or 5kV. Power dissipation from 5kV and 50A is 5000×50, or 250kW. — BillC ^talk 23:18, 2 June 2007 (UTC)

Ah I'm sorry I should have specified. I'm talking about a low voltage, high current DC circuit here, not AC. 24.32.223.33 09:01, 3 June 2007 (UTC) CharlesN

I was talking about DC values too. However, it doesn't make any difference: AC root mean square values have the same heating effect in a resistance. It's still 250kW, whether we are talking about AC or DC. — BillC ^talk 10:19, 3 June 2007 (UTC)

BillC is correct. Watts = amps^2 * ohms, so 250 kW is the correct answer. If that is not the value you expected, then your input data must be wrong. Tell us the problem you were trying to solve, and we might be able to help you. --Heron 11:33, 3 June 2007 (UTC)

Ok I'm feeling a little stupid right now. :) I went back and double checked my worksheet, and it turns out I simply misread (my own) handwriting. I had written a note down about a 100W resistor for some reason and misread that as the ohm value. The actual value was .1 ohm, 250W resistor for current limiting at 50A, 5VDC (I think I have it calculated properly now?). This was from a project I started a couple decades ago when I was in elementary school (a high current power supply), and I had forgotten the original figures (as well as apparently most everything I USED to know about electronics). Sorry for the confusion on my part. I DID, however, find that one reference on the web (I can't seem to find the page again) that said milliamps and kilohms were conventionally used in modern electronic circuits and could be placed into the equations directly, without any further explanation. That confused me. After doing some more research, I found another page that explained it more clearly; you use kilohms with milliamps OR ohms with amps, but never kilohms with amps and vice versa (if I am understanding it correctly now). Again, I apologize for the mistake. Thank you for clearing this up and pointing me in the proper direction. But perhaps this information could be added, to prevent confusion on units used?

24.32.223.33 13:24, 4 June 2007 (UTC) CharlesN

Good, I'm glad that's sorted. We should certainly state the units for all equations, where appropriate, but I would be very wary of using anything but the SI base units. If we start saying that some equations work with kilo-this and milli-that, then we will confuse more people than we help. Forget anything you have read about milliamps and kilohms being 'conventionally' used - maybe some experienced engineers do it in work that is not intended to be read by anybody but themselves, but it is not conventional practice and we should not be recommending it. Best wishes, --Heron 20:04, 4 June 2007 (UTC)

Understood, thanks for sorting this out.

24.32.223.33 23:32, 4 June 2007 (UTC) CharlesN

[edit] problem

using ohms law , how much current will flow in a circuit that contains a (10V)source and a 4 K resistor?

The answer is in the second sentence of the article. I=V/R. --Heron 20:04, 8 July 2007 (UTC)

People think that they can use Wikipedia to calculate questions without having to strain their imbecilic minds, good job on not giving him the answear :) 200.105.223.94 23:14, 6 August 2007 (UTC)

[edit] Cavendish

Perhaps a mention should be made that Cavendish did indeed more or less find out Ohm's Law long beofre Ohm did. (only problem being that Cavendish was slightly insane and hid most of his findings).

A little copy paste from Britannica 2005 Ultimate Reference Suit DVD (article Cavendish, Henry written by Charles Süsskind): (note: Ohm's law was the last of the importants finding that Cavendish discovered independently mentioned in the encyclopedia, so the text is not fragmented.)

Finally, in a series of experiments on various conductors, he discovered that the potential across them was directly proportional to the current through them, thus anticipating the law enunciated by Georg Simon Ohm, a German physicist, in 1827. The last finding was the more remarkable since Cavendish had no means of measuring current and managed only by turning hisown body into a meter, estimating the strength of the current by grasping the ends of the electrodes with his hands and noting whether he could feel the shock in his fingers, up to his wrists, or all the way up to the elbows. All these researches were subsequently repeated, after the discovery of his notebooks and manuscripts overa century later, by the great Scottish mathematical physicist, James Clerk Maxwell, who devoted the last five years of his life to the task and published an annotated version of the electrical papers of Cavendish in 1879. 200.105.223.94 00:56, 29 July 2007 (UTC)

Edit, I don't know when Cavendish discovered this, but he died in 1810, before Ohm discovered his law.200.105.223.94 01:09, 29 July 2007 (UTC)

[edit] This page needs to be cleaned up- crude references have been inserted

Hello,

I'm not a Wikipedia editor, nor a subject matter expert, but I'm pretty sure the references to "your mother" and other crude verbage inserted in the text of this page are not actually related to ohm's law. Perhaps the page can be rolled back to an earlier state? —Preceding unsigned comment added by 207.218.88.215 (talk) 18:47, 4 December 2007 (UTC)

[edit] Use of the word flow, as in a current that flows through a resistor

It is common usage to say that a current flows through a resistor. While it may be a stricltly correct way to say that moving charge in a resistor is current through a resistor, it does not convey, especially to a young reader, the sense of a flow of something through an electrical device like a resistor. I think it would be better to return to the use of the flow. blackcloak (talk) 00:29, 22 March 2008 (UTC)

[edit] Physics section

I am quite unhappy with the physics section. It seems quite perverse, if not downright wrong, to derive Kirchoffs laws from Ohm's law. In any case it does not belong in this article and I think it should be deleted. Are there any other opinions on this?

What does belong in a physics description of Ohm's law is a derivation of the law from the physical (electronic) properties of conductors. I am not proposing inserting that in this article either as it is going to be too advanced, both mathematically and physically. But maybe a seperate article with jsut a mention of the result and a link here? SpinningSpark 19:22, 22 March 2008 (UTC)

[edit] The stuff about Ohm's law not existing before Ohm defined it?

Blackcloak I don't get it. It's a law of nature.

That's like saying Newton's laws of motion didn't exist until Newton defined them?

I've deleted that part for now but could you try to clarify?

--165.21.155.113 (talk) 04:08, 30 March 2008 (UTC)

I think the point that Blackcloak is making is that it was not possible to state Ohm's Law in the form we know it now until the concept of resistance had been established. Of course, the physical phenomena of current proportional to voltage existed before that as Cavendish's work shows. 155.113, you might think you have deleted this, but I can still see it in the article. Some rewording would be in order here. SpinningSpark 09:44, 30 March 2008 (UTC)

Let me try to respond to the two comments above. First the "law of nature" idea. We have the way nature works and then we have the way man describes the way nature works. You're confusing the former with the latter. BTW the term law was used, esp in the 1800's, as we figured out the basics. Later we learned that "nothing" is known for sure, finally and completely, so we now call theories what we would have just called another law. Ohm's law is the traditional way to describe the equation I=V/R. Nature does not create formulas; "laws" are human descriptions of how nature works.

On the second comment, Ohm did not actually write an equation that we now call Ohm's law. Go back a year or two or three and read the earlier discussion. There was once a history section that went into the details a bit. Look through a copy of the original treatise to see how Ohm approached his description of his experiments. I was able to find it online- but not in English translation. While the concept of resistance was probably understood by some (remember Cavendish did not publish his work), the equation I=V/R can not exist until the unit of resistance is defined. Why isn't the equation I=2*V/R or V/(pi*R)? For simplicity, it appears that the units were chosen (in the 1861-1864 timeframe) to make the equation a very simple one. The intro states clearly that the modern simple form of the equation depends upon a well chosen definition of resistance- a definition that Ohm himself did not make. blackcloak (talk) 04:31, 31 March 2008 (UTC)

It would be more clear, then to point out that Ohm's law was originally the statement of proportionality, and that the equation in its simple form depend on defining compatible units. That's not the same as saying that the law couldn't or didn't exist before the ohm was defined. What happened with that history section you mention? Should it be brought back? Dicklyon (talk) 05:08, 31 March 2008 (UTC)

Simple stuff first. Some editor removed it and no one put it back. Wiki allows one person to dictate. You have to constantly revert if someone removes material you're convinced has to be included. I do think it should be brought back. Others will say the article is too long already. blackcloak (talk) 06:06, 31 March 2008 (UTC)

Looks like a simple case of incompetent partial revert of vandalism. Fixed. Dicklyon (talk) 06:38, 31 March 2008 (UTC)

The problem is what is Ohm's law. Is it what Ohm himself said (wrote), even if he did not identify anything he created a law? Is it the modern form V=IR or more correctly I=V/R? In earlier discussion, see above and archived material, the consensus, if there ever is one, is that the modern form of Ohm's law should be the subject of this article. I'm willing to go along with that, provided it is clearly understood that Ohm himself never, as far as I'm been able to tell, wrote the modern form. Ohm himself did not have a strict proportionality. He described the voltage and current characteristics of various electrical experiments involving lengths of wire. His equations had the length of wire under test as one of the independent variables. The equations also had an additional term that we now understand to be proportional to the source resistance. There is a more complete description above. Of course, we now make the observation that resistance is proportional to length, but I'm not even sure he said that explicitly. Perhaps you now see why this is not as clear cut as one would hope. Now for the more difficult issue you raised. Did the law exist before the unit of the ohm exist? First I am assuming you are not talking about nature's "law", but rather our human description of the way nature works in the form of a mathematical equation, to wit I=V/R. My answer is the modern form did not exist before the unit of resistance was defined. If the unit of resistance had been defined in some more complicated way, our present definition would probably still be called Ohm's law, but would look different. Having said that, clearly scientists would have understood proportionality of V/I implies a fixed proportionality factor (along with its associated units), and doubtlessly Ohm understood this as well. And probably scientists thouaght of as a proportionality factor that implied "resistance." The problem was that the international community had not formally adopted a unit of resistance until somewhere around 1861. I say that the modern form of Ohm's law could not exist (as a commonly agreed upon description of the behavior of V and I in resistance circuits) before the unit of resistance, call an ohm, was defined formally. That is not to say that anyone was slowed down by the idea of this law not being formally adopted in performing and describing experiments. On the other hand, Ohm's results were not immediately accepted by the scientific community, even though he took pains to describe his measurements and analysis techniques and did not go beyond his observations and calculations. So one answer to your question is that of course the law, meaning the idea and not its formal statement, existed before 1861. It had to- the scientific community doesn't just decide one day that they have a new law. blackcloak (talk) 06:06, 31 March 2008 (UTC)

Let sources be your guide. If you don't find a source saying the law didn't exist before the ohm, then why would you add it to the article? As to limiting the scope to the modern form, I think that's absurd; historical context should always be a part of an article's scope, assuming historical secondary sources can be found. Dicklyon (talk) 06:38, 31 March 2008 (UTC)

Let's put on our critical thinking cap. How do you prove a negative? Do you really expect to see a reference for something that did not happen? As for the absurd comment, again you have to go back to earlier discussion. It's there. Others didn't give a damn about anything other than the modern version. You've arrive late to the discussion- of course that does not mean you can't change things- it just means you're reversing earlier "decisions." And yes, the little historical stuff that has remained was shoehorned into the intro- mostly by me. blackcloak (talk) 06:57, 31 March 2008 (UTC)

Not sure what you mean about proving a negative, but then why add something that isn't verifiable? As to my comment about absurdity, that's partly to draw out anyone who might disagree. If we don't hear from them, we don't have a problem. Dicklyon (talk) 07:02, 31 March 2008 (UTC)

Hi there, I was 113 above. Blackcloak I'm still not getting your argument. I don't think I was confusing the difference between what humans call a "law" and the way the world works. I think I know the "law" pretty well. And it's cute that Ohm didn't write it down as such or that the word "law" wasn't popular until the 19th c, etc. but at the end of the day is a user of Wikipedia curious about the meaning of Ohm's law gonna care?

If you've been following the discussion elsewhere, then you know that it is indeed the modern form of the equation that we are calling Ohm's law in the article. Your comment about Newton clearly implied that it was absurd to think that Newton discovered or created a "law" since apples had had no difficulty falling to ground before Newton arrived on the scene. My discussion about law was just to let you know that there is no special meaning to the term law in scientific literature. It's just a word that means "description" in the context of science. But thanks for your comment. It should help Dicklyon better understand what readers expect. blackcloak (talk) 06:16, 1 April 2008 (UTC)

So just to continue on a bit more. What we call today Ohm's law (read Ohm's Description)is a mathematical equation, along with a set of constraints that specify how and what must be done (read measured) in order to apply the mathematical equation. That specific equation has an R for resistance in it and that R has units of ohms. The term ohm was given to the ratio of one volt over one ampere in about 1861/4. That means that the modern form of Ohm's Description was formally adopted in about 1861. The specific equation and its definition, in all its glory, that we use today did not exist before 1861. Earlier versions, involving lengths of wire, of it did, going back to Ohm in 1826 with his treatise. And even earlier versions were created, but not published at the time, by Cavendish. I really don't know how to be clearer about this. So if you still don't understand, please be very specific in describing what you don't get. Thanks. blackcloak (talk) 06:16, 1 April 2008 (UTC)

I've seen a lot of stuff on Wikipedia from what appears to be either well-meaning or mean-spirited postmodern revisionists where someone is trying to prove a rather banal point. Ala the scientific revolution and whether it really existed or Occam's razor and whether he really said it or of course Galileo and his ball-drop.

A user curious about the historiography of science might care, but not your average reader who wants a statement of the law, how it's applied, a little bit of history of how it was understood, and some further areas for consideration.

Thanks! --M a s (talk) 09:34, 31 March 2008 (UTC)

So let us know how you think things should be changed. At what point in the article did you stop reading because it was providing too much, or too difficult, material? blackcloak (talk) 06:16, 1 April 2008 (UTC)

Nothing is too difficult that stopped me from reading it, my friend. I stopped reading when I read and deleted the comment about how the law didn't exist prior to the 19th c. when it was defined.

To me it looks like you just misread the comment, as the above sentence seems to indicate (since you don't identify what actually was defined in 1861/4). The general idea (how I think you're using the term law) was not defined in 1861/4, the unit of resistance (as one volt divided by one ampere), called the ohm, was defined in 1861/4. Cavandish, Ohm and Faraday all understood the (at least approximate) proportionality of V to I. Interestingly, they all used conductance, or similar terms, to describe the capacity of a wire to conduct electricity. That's why the wording had been carefully crafted to refer to the modern form of Ohm's law. blackcloak (talk) 07:52, 2 April 2008 (UTC)

If people really believe or want to debate that science is invented and not discovered, then there might be some good discussions on the historiorygraphy pages as mentioned.

The argument that the law didn't exist before the Ohm was defined because people didn't have the concept of an Ohm is pretty specious, pretty much a tautology to me.

Cheers!

--M a s (talk) 11:49, 1 April 2008 (UTC)

[edit] AC section

I do not agree with many of the changes made by Blackcloak to the ac section.

Ohm's law only applies to circuit elements that do not have reactive properties

The above statement is correct. Ohm's law describes what happens in purely resistive devices. Ohm's law, in the context of this article, means I=V/R (or R=V/I). Some of the confusion may be over what is meant by circuit elements. Perhaps you'd like to provide a counter example. To what circuit element having reactive properties can Ohm's law be applied in order to predict the actual resistance of the circuit element at any instant when the voltage and current are measured? blackcloak (talk) 04:31, 31 March 2008 (UTC)

Ohm's law applies to inductors, capacitors, and everything else in the DC case, doesn't it? Dicklyon (talk) 05:18, 31 March 2008 (UTC)

How can you say this? Ohm's law does not have a C in it for capacitance. Ohm's law does not have a symbol for inductance in the equation. Ohm's law (the non-extended version), the subject of this article, only has an R, a V and an I, so it can only describe the interrelationship of current, voltage and resistance. blackcloak (talk) 06:32, 31 March 2008 (UTC)

It doesn't need a C, because the DC current through a capacitor doesn't depend on C. But inductors do have a resistance, and their DC current does depend on their R; a leaky capacitor can have an R, too. Dicklyon (talk) 06:49, 31 March 2008 (UTC)

this originally read;

Ohm's law holds for linear circuits where the current and voltage are steady

The idea of linear circuits is too vague and inappropriate for this article. Something has to be added to make this kind of characterization more accessible to young readers. The word steady is not sufficiently descriptive, esp when terms (perhaps also somewhat confusing) like DC and AC are also used. blackcloak (talk) 04:31, 31 March 2008 (UTC)

It's no less clear, and more correct, than your limitation of Ohm's law to "elements that do not have reactive properties", is it not? Dicklyon (talk) 05:18, 31 March 2008 (UTC)

My comment above should make it clear that I believe Ohm's law only applies to resistance, or perhaps more generally to electrical elements that do not have reactance. And no, non linear circuit elements are ok, and the currents and voltages need not be steady, provided you understand that the imputed resistance must be computed based on simultaneous and substantially instantaneous (if things are change rapidly) voltages and currents. blackcloak (talk) 06:32, 31 March 2008 (UTC)

Reactance doesn't make the law invalid in the DC case. One can still do a DC or Ohm's law solution of arbitrary RLC circuits in the DC condition. It still applies, in spite of the presence of reactive components. And I think your interpretation in the case of nonlinear circuits is pretty non-standard; in Ohm's law, R is a constant, not a time-varying ratio of V/I as in a nonlinear circuit. Otherwise, what would the law say? Just that there exists a ratio V/I that can vary? That would say nothing. Dicklyon (talk) 06:49, 31 March 2008 (UTC)

I don't know where you got the time-varying part- I haven't used that particular term. I've simply said you can make the measurements simultaneously, and if you've done that you can use Ohm's law- provided there are no reactive components. Indeed, resistance is a function of all kinds of variables, in general, and, surprise surprise, Ohm's law remains valid even when you apply a slightly different voltage, or increase the temperature by some small amount, etc. And now that you've been a little more complete in your description of making measurements of RLC circuits in the DC condition, you are correct there. But a caution is necessary. If you describe the characteristics of your measuring device and tell me how long you are willing to wait to make your measurement, I (think I) can always select values of RLC that will result in you being unable to properly use Ohm's law to accurately determine the resistance. The reason you are correct in saying that it still applies in spite of the presence of reactive components under DC conditions is clear. The capacitors are charged to a fixed level and the inductors have a constant current flowing through them. The amount of stored energy is not changing. And this is exactly the state that would occur if the capacitors were removed and the inductors replaced with a shorting wire. And that means they are effectively not in the circuit. So how is that different from what I am saying- no reactive components? All you're saying, which is rather trivial, is that when measurements are made under DC conditions, any reactive components in an RLC circuit don't influence the measurement. As for the V/I comment, volts divided by amps has units of resistance, namely ohms. And oh by the way, the statement of Ohm's law that is presented in the article does say (despite all our discussion about other ways Ohm's law can be properly used) that the device under test is a conductor, not some general RLC circuit. Our discussion is for the most part academic. BTW I would think a thermistor meets the conditions of the definition. blackcloak (talk) 05:46, 1 April 2008 (UTC)

I remain baffled; what is it that you think Ohm's law says, if not that current is proportional to voltage? How can taking the ratio of voltage to current in a non-ohmic device lead to the notion that "Ohm's law remains valid". At least you have correctly got my point that, as you summarize it "when measurements are made under DC conditions, any reactive components in an RLC circuit don't influence the measurement." And yes, a thermistor obeys Ohm's law as long as you hold its temperature constant. But V/I for something like a diode is not a resistance, and the diode does not obey Ohm's law. Dicklyon (talk) 06:05, 1 April 2008 (UTC)

If I make the measurement of the thermister at 40deg (by applying a voltage, reading a current and calculating a resistance) and get a value of 100 ohms, I have used Ohm's law in valid way. If I then raise the temperature to 50deg and similarly get a value of resistance of 110ohms, I have used Ohm's law in a valid way. In the second measurement the current will be different (smaller) from the first. Both measurements were made under DC conditions, so I imagine you would consider both computed values for R to be valid and both measurements and calculations were made in a manner consistent with Ohm's law. Now at what point between the two temperature was Ohm's law invalid? Or say I change the temperature slowly, from 40 to 50 degrees over a period of an hour, making measurements along the way, when in this slowing changing scenario did Ohm's law become invalid? blackcloak (talk) 06:56, 1 April 2008 (UTC)

In all cases where you held the temperature constant, the thermistor was an ordinary resistor, and Ohm's law applied as you said. If, on the other hand, you were to put that thermistor in air and run different currents through it, you would not see a proportional voltage. Hence Ohm's law would not be applicable to that situation; that is, the device can not be characterized by an R. Dicklyon (talk) 07:11, 1 April 2008 (UTC)

Okay, I think I have the clincher on the diode example you're having difficulty with. Ideal capacitors do not dissipate heat as they are charged and discharged. Ideal inductors do not dissipate heat as they are energized by current, or when they are de-energized by dropping the current to zero. Any stored energy from ideal reactive components re-enter the circuit and are converted to heat in resistors, in particular, but also in any other component that has a V/I characteristic, such as a diode. Specifically, if you establish a current passing through a forward biased diode by applying a voltage, establishing a V and an I per Ohm's law, then I^2*V/I watts will be dissipated in the diode, ahem resistor of value V/I. Don't believe me? Try it- just be careful, you could burn yourself. Oh, and BTW, if you put a resistor in series with the diode, compute voltage drops and power dissipation, how can you obey the law of conservation of energy if the diode isn't effectively functioning as a resistor (as predicted by Ohm's law)? blackcloak (talk) 06:56, 1 April 2008 (UTC)

No argument that the device would dissipate the power, and would satisfy Kirchkoff's laws in a circuit, etc. But it's not the case that the device can be described by Ohm's law; it's not a resistor, doesn't have an R that describes a proportionality between voltage and current. It's not ohmic. Again, I have to ask, what do you think Ohm's law is? I would think more than just that one can divide V by I and call the result R, which is what you seem to be saying. Dicklyon (talk) 07:11, 1 April 2008 (UTC)

Elsewhere in this discussion I've made it very clear what I think Ohm's law says. We got into this discussion because there is/was an AC section where the generalization to Ohm's law was summarized. Our discussion has moved well away from what the article says about Ohm's law outside of the AC section, and into areas not intended to be included in the perview of Ohm's law. We started by trying to be clearer about what constraints must apply to the unit of circuitry under test. In this side discussion I've pushed for the idea of interpreting Ohm's law a little more generally, namely that anytime you have a circuit element through which you measure a current simultaneously with a measurement of the voltage across it (and measure fast enough that the sampling time is short compared with the period of the highest frequencies present), you have met all the necessary conditions to apply Ohm's equation for the purpose of determining an (effective, if you prefer) resistance of the circuit element (note, this does not mean the circuit element IS a resistor) under the particular voltage and current conditions of the measurements. Further, I am saying that you can use the value of the imputed resistance to calculate the power being dissipated whenever the measured current and voltage conditions are met. You (Dicklyon) on the otherhand are insisting that Ohm's law only permits one value for an R, the ohmic R, the one value that represents the ratio of V to I under all none-too-severe (goes nonlinear) conditions, as long as you can also specify the temperature of the unit of circuitry under test. blackcloak (talk) 07:52, 2 April 2008 (UTC)

Thanks for being clear on that. In response, I'll clearly state that your position is absurd and not supported by anything in sources. You don't need to use a generalization of Ohm's law to get the power, which is I*V. And your made-up definition of "effective resistance" is not Ohm's law; Ohm's law is about proportionality, or linearity, of resistive elements. Does anyone disagree with me or support Blackcloak on this point? Dicklyon (talk) 14:45, 2 April 2008 (UTC)

I guess I've avoided most opportunities to say that your position is absurd, but I think I now have to. You are taking measurements of I and V (across a circuit element such as a diode) and multiplying them to get a computation of power. This is exactly correct. But then when I take the same measurements of I and V and divide one by the other, to get something that has ohms as its units, and then I say that if some computed value has ohms as units then it may be seen as an effective resistance, you say that that is incorrect. Both ways lead to the same dissipated thermal power. I say if it quacks like a duck and walks like a duck, we might as well call it a duck. You can multiply, I can't divide? Sorry, but you'll only get support from those who see the issues in the very narrowest of terms (which, I remind you, IS the way the article is now written). blackcloak (talk) 19:34, 2 April 2008 (UTC)

I didn't say it isn't "correct"; I said it's "made-up" and "not Ohm's law". If you make up that definition for "effective resistance," then the computation is "correct," but I'm not aware of any sources that do that, or that would call it Ohm's law. As to the power dissipation, yes we agree on what it is; that is also nothing to do with Ohm's law; instantaneous power is current times voltage, always and forever, for resistors, capacitors, diodes, or anything else (although it can turn negative sometimes for reactive devices). So, you may have a duck there, but don't try to force-fit it to Ohm's law. Dicklyon (talk) 01:00, 3 April 2008 (UTC)

No matter what you say, students, engineers and professors are going to use the equation V=IR under circumstances that you would regard as failing to meet the rigors of Ohm's law. Since V=IR is a more general concept than Ohm's law, it must be defined first. So, obviously, we need a name for the equation V=IR which does not incorporate the name Ohm. For now I'll call it the omega equation because it arises from a definition of resistance. Then the omega equation under certain constraints of "linearity" (being intentionally vague) becomes the definition of Ohm's law. This may have all been worked out by someone (we need a reference to a discussion on the equation independent of Ohm's law), if we're to understand the comment way above by Heron. blackcloak (talk) 06:40, 1 May 2008 (UTC)

This section has gotton too deep to edit. Please start a new one to re-open this discussion if you think we need to discuss generalizations of Ohm's law. Dicklyon (talk) 15:09, 1 May 2008 (UTC)

On checking books, I find that there is indeed a defined concept of effective resistance, and that it's defined in terms of power (ratio of power to mean-square current) for AC circuits. For DC current, that would give the answer as V/I; however, none of these books make that connection, nor do they connect it with Ohm's law; they do note that when there's iron the circuit (i.e. it's nonlinear due to hysteresis), the effective R is not a constant of the circuit. The implication, to me at least, is that Ohm's law, the proportionality of V to I, does not apply in that case (obviously, it does not apply in that case, and that's what they seem to be saying by noting that R is not fixed). A few books even use the term effective resistance, without definition, when talking about diodes; but they never connect it with Ohm's law. And I find lots of other uses of "effective resistance", but none defined as you have, and none connected to Ohm's law. Dicklyon (talk) 05:37, 3 April 2008 (UTC)

Here's what I found using the same technique you used, varying the search words.

Search of: effective-resistance diode Building Scientific Apparatus: A Practical Guide to Design and Construction - Page 442 by John H. Moore, Christopher C. Davis, Michael A. Coplan, Sandra C. Greer - Science - 2002 - 704 pages Because the current through a diode is a nonlinear function of the voltage across it, the effective resistance changes with diode voltage. ...

Power Supplies, Switching Regulators, Inverters, and Converters - Page 212 by Irving M. Gottlieb ... neither a filter choke nor a free-wheeling diode is necessary with inverters. ... timing resistance /?, and the effective resistance of transistor Q2. ...

Electronic Components and Materials: Principles, Manufacture and Maintenance - Page 289 by S M Dhir - 2000 ... the case for asymmetric VDRs; the obvious example is a solid state diode. ... This is equivalent to saying the effective resistance has fallen steeply. ...

Hands-on Electronics: A One-semester Course for Class Instruction Or Self-study - Page 197 by Daniel Moshe Kaplan, Christopher Gerard White - Electronics - 2003 - 204 pages daraf Unit of inverse capacitance. decibel Unit for specifying a voltage or power ratio on a logarithmic scale. dynamic resistance Effective resistance of a ...

Searching for resistance-of-a-diode, over 60 hits

Search of resistance-of-a-diode ohms-law

http://www.neng.usu.edu/classes/ece/1010/Diode%20Lab%20-%20Revised.doc

Search resistance-of-a-diode ohms

Textbook of Applied Electronics - Google Books Result by R.S. Sedha - 2008 - 992 pages Mathematically, the static forward resistance (in ohms) RF = VF/IF The static forward ... The ac resistance of a diode, at a particular dc voltage, ...

and over 2000 more hits.

I liked the discussion at http://en.allexperts.com/q/Physics-1358/multimeter.htm

Read the section called Dynamic Resistance here http://www.innovatia.com/Design_Center/Transistors.htm It states by Ohm's law, V/I is a resistance, and computes a diode's resistance. Ohm's law is not applied directly, but the equation of Ohm's law is.

Obviously there is a bunch of stuff to sort through if we're going the route of accepting internet sources as havins some level of authority. If we decide to limit Ohm's law to describing ohmic circuits/elements, there should be a change in the definition in the intro to include the idea that the resistance remains substantially constant over a range of v (and i). blackcloak (talk) 10:24, 3 April 2008 (UTC)

Yes, I had done that search (in books) and looked at those, too. As I said, they don't define it as you do, and they don't connect it with Ohm's law. Dynamic resistance, the slope dV/dI is what is more often used to characterize things like diodes, and that does not obey Ohm's law. And some define static resistance for what you're calling effective resistance, but that doesn't obey Ohm's law, either. I certainly do not recommend accepting Internet sources as authorities, but if you're going to assert something, you need at least some source, and books are usually more reliable than web junk like the page that asserts the informality "By Ohm’s Law, v/i is a resistance." Just because v/i has units of resistance doesn't make this an application of Ohm's law. And the discussion you like concludes "it does not follow Ohm's Law!" Dicklyon (talk) 14:55, 3 April 2008 (UTC)

I have to say Dicklyon, you deserve a barnstar for your stamina in fending off this ludicrous point of view. If Ohm's law is not V is proportional to I then it is not a law at all, merely the definition of resistance. I am sure that is not what Ohm intended or what is generally understood now.

Dynamic resistance is a bit of a red herring to this discussion. It is most useful in developing small signal equivalent circuits. Here the signal is assumed to be small enough in amplitude that the small portion of the curve of the device transfer function appears to be effectively linear as far as the ac signal is concerned. A small signal equivalent circuit can then be developed where resistors of the value dV/dI are substituted plus a generator to take account of the non-origin crossing of the tangent. These dynamic resistors are treated as constants (ie regular resistors) in the analysis, or at least near enough constant for the purposes of the designer of the amplifier or whatever. However, I don't see that it helps one side or the other in this debate as small signal equivalent circuits are entirely notional circuits used to predict the effect on signals. SpinningSpark 20:47, 3 April 2008 (UTC)

I think you missed the major point, Dicklyon: Many of those references, books or otherwise, compute a resistance of a diode. Obviously that was the point of the internet search, which you began. Equally obviously, the equation used is V=IR for that computation, in the form R=V/I. The computed value of the resistance has units of ohms, again obviously. Now the units of ohms come from the approx. 1861 (English) effort to standardize on a suitable name and value for the proportionality factor of V/I. For the most part, the references only use the facts just outlined. Ohm's law has not come into the picture so far. Finding a value for a resistance for a diode under a set of measurements conditions can be done with the information available to this point. Further those measurement conditions involve establishing a voltage across the device (diode), measure that voltage, while simultaneousely measuring the current going through the device. Also note that the diode (presumably forward biased) is a conductor because current can be made to flow through it readily. So far I haven't claimed to use Ohm's law, but I have what I need, i.e. an effective resistance for the diode. Now the problem is, according to the current version of Ohm's law, as presented in the current intro, I have indeed met all the conditions. Nothing in the current statement of the law prevents me from finding the resistance of a diode, and then claiming that the computed resistance is found in accordance with (the current statement of) Ohm's law. blackcloak (talk) 07:41, 4 April 2008 (UTC)

So the obvious conclusion is that I=V/R is not actually Ohm's law; it is merely an equation that shows the relationship amoung the quantities I, V and R under the agreed upon measurement conditions, the characteristics of the device/circuit under test being left unconstrained. While it is true that Cavendish, Ohm and Faraday and others had concluded that the equation described what they observed, what we now call Ohm's law, in the simplest and narrowest (ohmic device) sense (Dicklyon's preference), requires that we know more (or assume certain things) about the device/circuit under test. In Ohm's world, and in the telegraph world of transmission wires (1840s and 1850s), what was known was that the device under test was a length of wire. (Indeed our current definition of the ohm is based, in part, on the need for standardizing specifications of telegraph wires.) So what do we need to know or assume to meet the requirements of the narrowest form of Ohm's law, permitting us to describe an ohmic device? We need to know that the ratio of V/I is constant over a significant range of values of V (and I). And that has to be written into the definition of the simplest/narrowest form of Ohm's law. So far I have not seen any reference that actually does that. Since there are at least four ways the equation I=V/R is used, three of which actually incorporate the use of the words "Ohm's law," I think we have no choice but to outline these various uses in a summary manner so that readers of the article can appreciate the range of application associated with the equation. blackcloak (talk) 07:41, 4 April 2008 (UTC)

Firstly, the wording has been changed from what Ohm's Law is, to what Ohm's Law is not. Ohm's Law is not a lot of things and it is futile to list everything something is not. This article suffers badly enough already from this syndrome and it has been mentioned before on this talk page.

You are not sufficiently clear in the above comment for me to even try to figure out what you mean. Please be more specific. blackcloak (talk) 04:31, 31 March 2008 (UTC)

I maintain the original is a more accurate description for two reasons. Ohm's Law does apply to circuits containing reactive components if the current and voltage are steady. Ohm's Law does not apply to circuits which have non-linear components even if the circuit has no reactive properties. The relationship of current to voltage in an ideal diode, for instance, is exponential.

Your use of the term non-linear is not sufficiently clear. If you are thinking temperature coefficient as the source of the non-linearity, then you are wrong, in my opinion. No matter what temperature a filament, for example, is at, the current, voltage, resistance relationship still holds. Only the exact current I can pass when a voltage V is applied across the filament of resistance R, any measurements being made are done at the same "instant." This point is made several times throughout the article.blackcloak (talk) 04:31, 31 March 2008 (UTC)

Here nonlinear means essentially that Ohm's law doesn't apply; sort of a tautology, really. It's for things like diodes, where I is not proportional to V; it's not a reactive element, but a nonlinear element. If it's not sufficiently clear, it should be clarified, not removed. A light-bulb filament is another nonlinear element, because, as you seem to be alluding, its resistance changes with its power dissipation, via its temperature; its current will not be proportional to voltage, whether varied quickly or slowly, unless so quickly that you can approximate the temperature as not changing. Ohm's law just doesn't apply to this situation; period. Dicklyon (talk) 05:18, 31 March 2008 (UTC)

I think you should go back and read what is said about ohmic devices. An ohmic device is one where a single number for the resistance suffices to adequately describe the performance of the device in an electrical circuit. In other words you are not going beyond its linear range. On your diode example, clearly it is not an ohmic device. That does not mean that you can not take simultaneous measurements of current and voltage and then compute a "resistance". Indeed the number for the resistance actually represents the ratio of V to I and therefore under those specific conditions follows Ohm's law. At every point along a I vs V curve, Ohm's law is followed. It is just that the resistance is a function of voltage. BTW, if you look at log and exp op amp circuits you will see diodes used in place of resistors to function as voltage dependent resistors. On the light bulb example, it appears as though you are thinking that the filament's resistance is one value, presumably the limiting value as the current through it goes to zero. I think of a filament as a current dependent resistor (with a time lag- which you seem to agree we should ignore). The point is at any instant in time when you make simultaneous measurements, you will measure a voltage, a current, and, if you could do it independently, a resistance, together obeying Ohm's law. blackcloak (talk) 06:32, 31 March 2008 (UTC)

No, that's absurd. The ratio V/I for a nonlinear device is not a resistance, and not related to Ohm's law. You can make an approximation, e.g. using the slope of the V/I curve to get a relation between changes in V and change in I, or a short-time R for the temperature-dependent case, but neither of these is Ohm's law. Dicklyon (talk) 06:49, 31 March 2008 (UTC)

I'm not sure what you think is absurd. Let's go back to basics, as in the intro. When you have a conductor (the filament), and when you put a voltage across two points along that conductor (the two leads that come off of it is a good enough place to measure the applied voltage), a current will flow. Are we together to this point? We can measure the current that flows through the filament with a current meter placed in one of the line from the power source to the filament. Can I assume were on the same page up to this point? Ok, we seem to have met all the conditions declared in the statement of Ohm's law. So we can calculate R by dividing the voltage by the current. The R we compute is the resistance of the filament under the conditions of the measurement, and Ohm's law allowed us to make this observation. A resistor that has a significant temperature dependence is still a resistor. blackcloak (talk) 05:46, 1 April 2008 (UTC)

A generalization to AC circuits and reactive components is possible

The concepts of impedance and reactance are not stricly speaking a generalisation of Ohm's Law. Rather, they are putting the solutions to the differential equations, for the special case of steady ac, into a form that looks analogous to Ohm's Law and is therefore easy to understand by those familiar with Ohm's. They still represent entirely different mathematical equations. I would be happy for mention of differential equations to be taken out as this might be too advanced for most readers of this article, but the statement as it stands still needs to be changed.

I guess you have a more developed idea of the concept of generalization than I do. Perhaps you prefer the word "extension" or whatever. That is what editors are supposed to zero in on. Please feel free to edit to improve, not edit to remove. I can't do everything. blackcloak (talk) 04:31, 31 March 2008 (UTC)

This statement,

This is because inductors and capacitors store energy during part of the ac cycle

has been changed to;

Purely inductive and capacitive circuit elements only store energy during part of the ac cycle

which has tbe implication (I am sure not intended) that only purely reactive circuits store energy. All ac circuits will store at least some energy in at least stray components. The original statement is both simpler and more accurate.

There is more, but I do not want to make this post unreadably long. I hesitate the revert Blackcloak's edit in its entirety, but nevertheless, a great deal of it needs to be unpicked. SpinningSpark 10:21, 30 March 2008 (UTC)

It may be simpler and more accurate but its nearly illiterate. "This is because?" Give me a break. I've just tried to get it to a more readable form, and tighten it up a bit. My edit is correct. We're not talking about all ac circuits- that's far too complicated a subject to even raise in this article. Maybe the word ideal should have been used instead of purely. blackcloak (talk)

"This is because" may not exist in whatever language you speak, but in English it is a perfectly acceptable construction. Please be a little less offensive in your comments. SpinningSpark 09:23, 31 March 2008 (UTC)

Thanks for your candid reply. It helps me greatly in calibrating the quality of your contributions. blackcloak (talk) 05:46, 1 April 2008 (UTC)

There was a lot I didn't like in it, too, and I reverted it before noticing your comments here. My edit summary was probably incorrect in attributing the "could not exist before" comment to him, but anyway he put back in a silly statement that had just been removed. I disagree with the bloated wording in the disambig link at top, and the opening sentence. So there's a lot to discuss before we let the article have such a major rewrite. Dicklyon (talk) 16:22, 30 March 2008 (UTC)

Rest assured it was put back for a reason. The disambig link at top was put in earlier by some other editor. I modified it to improve the wording- was awful before. There is no bloated wording. The best way to present material, IMO, is to use a top down approach. Give the broadest view possible at the top (i.e. set the context), and then slowly bring it down to the specific. Using this approach, the physics section should be moved to the very bottom- after the AC section. That way young, or non-technical types, will never get to the material that is way over their heads. blackcloak (talk) 04:31, 31 March 2008 (UTC)

The the AC section was unreferenced, and since the sinusoidal approach that it was attempting to use has a rather strained relationship to the complex impedance notation and is not very general, I found a reference that develops the complex exponential approach, and relates it to Ohm's law, and based on a rewrite on that source. Please comment on this approach, and provide an alternate source if you'd prefer to see it done a different way. Dicklyon (talk) 03:55, 31 March 2008 (UTC)

I think I need to be very direct with you (Dicklyon) here. IMO, you do not have a well developed sense for how to present ideas such that they are accessible to young and inexperienced readers. I think to are trying to be helpful, and I applaud your efforts. You clearly have a good understanding of what is going on technically. You are just not placing yourself in the mind of someone who comes to this page with little to no understanding of the basics and who wants to be led through the material as directly as possible. You are writing for a college student who already understands what a differential equation is- which means you've already lost 90% of your potential audience. Because you are trying to say too much with too few words, your terse presentation assumes that a knowledgeable reader is inserting the missing material, that your reader is able to see through the liberties you taken in describing difficult material. blackcloak (talk) 05:11, 31 March 2008 (UTC)

True, sometimes my style is too terse for the inexperienced reader. We can work on that. On the other hand, I don't oversimplification to the point of being incorrect is an acceptable alternative. I'll be happy to work on it with you. Dicklyon (talk) 05:22, 31 March 2008 (UTC)

Thanks for your willingness to see my comments in a positive light. I would like you to respond to the paragraph above where I talk about a top/down approach. Do you agree in principle that the development should be like a guided tour, starting with setting the scene/context and then slowly adding the appropriate detail? blackcloak (talk) 06:40, 31 March 2008 (UTC)

My proposal is to remove the physics section and the ac section to another article and put short one or two sentence intros to these other articles, similar to the way sheet resistance is handled. The physics section is particularly inappropriate because it deals with resistivity and not resistance, and resistivity has its own article. Also note that there already is an article on electrical resistance. blackcloak (talk) 05:11, 31 March 2008 (UTC)

That might be OK. I certainly agree that the physics section shouldn't be so prominent in the article. So give us a description of the relevant set of articles and what you think should go in each, so we can talk about it and get consensus. Dicklyon (talk) 05:22, 31 March 2008 (UTC)

Dicklyon, you are doing a wonderful job with this article. A few of points though;

[edit] diagram

You have lost the diagram from the 'how engineers use . . . ' section. Was this deliberate (I never liked that diagram). If it was, the text is still referring to a diagram that is no longer there.

That was an accident, but I agree it's an awful image, and the essay about it is unsourced and not very helpful, so out it goes. If someone wants to do a better job of showing a typical nonlinearity to which Ohm's law does not apply, that might be OK. Dicklyon (talk) 15:16, 31 March 2008 (UTC)

[edit] s operator

What you have done to the ac section is correct but there is a slight difficulty. The s operator represents the Laplace transform operator used in the general case of transient circuits. We are no longer describing (only) ac circuits so the heading is invalidated. Under steady ac conditions (or a linear supersition of sinusoids) the s operator can be replaced with the jω operator. oops just noticed you said just that at the end of the section - still the heading does not quite gel with the content.

It is not necessary to introduce Laplace transforms to use complex exponentials parameterized by s. That's why I found a ref that does it that way. Dicklyon (talk) 15:12, 31 March 2008 (UTC)

Oh no, I was not suggesting introducing a discussion of Laplace transforms into the article - that would be going in entirely the wrong direction. I was only suggesting that by introducing the s operator the section heading is now wrong. While only the jω operator was being discussed the heading "AC circuits" was appropriate. Now the section has become more general I think it should be something like "Transients and ac circuits" perhaps. SpinningSpark 19:41, 3 April 2008 (UTC)

[edit] Physics section

Wonderful - you have got that down to the essentials. I don't think that there is any longer a need to remove this to another article. I still think ,though, an article on the physics of Ohms Law is needed which I might write one day when I have finished the dozen or so other articles I currently have in progress.

SpinningSpark 08:34, 31 March 2008 (UTC)

I just took out the long proof that was irrelevant, wrong, and unsourced; I left the physics as it was. Dicklyon (talk) 15:13, 31 March 2008 (UTC)

[edit] "Generalized" Ohm's Law

In my graduate course in plasma physics, reference has been made to a generalized Ohm's law, of which I found no mention in this article (or anywhere else in Wiki). The equation takes the form

$\vec{E}+\vec{v}\times{\vec{B}}=\eta\vec{j}+\frac{1}{ne}\left(\vec{j}\times\vec{B}\right)- \frac{1}{ne}\vec{\nabla}\underline{\underline{P}}+\frac{m_e}{ne^2}\frac{\partial\vec{j}}{\partial t}$

as it appears in my notes. (The double-underlined P is the electron pressure tensor.) It is equation 3.245 (or slightly more generally 3.211) in these PDF notes at UTexas.

I've verified it elsewhere on the web, but it might be worth including here. I'm not confident about the relevant place to include this and render it seamless with the rest of the article. Warrickball (talk) 10:57, 13 May 2008 (UTC)

First of all, I don't see how any equation that includes terms for the mass and charge of an electron can be generalised anything. In fact, your ref calls the equation a modified Ohm's law. The authors are extending the equation to include effects beyond Ohm's law. In my opinion, including equations of plasma physics in the Ohm's law article would be unhelpful. That is not to say that articles about plasma physics should not be written, and they could be referenced here as a "see also" if there is a connection.

SpinningSpark 19:25, 14 May 2008 (UTC)

Fair enough. I last saw the original Ohm's law a long time ago now, so I wasn't sure where this new equation fits in. I'm also very skeptical about it being "generalized", but that's the name under which it appears here and there. I'll reserve it as content for an article on the equations of plasma dynamics or something.Warrickball (talk) 17:52, 29 May 2008 (UTC)

Talk:Ohm's law

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Contents

[edit] explanation of present ohm's law

[edit] Power law

[edit] V=I·R

[edit] Is all this necessary

[edit] Oh really?

[edit] Intro and other ramblings

[edit] Direction of current

[edit] EE goes gaga

[edit] Sketch

[edit] Two schemes

[edit] Some explanation of conventions needed?

[edit] problem

[edit] Cavendish

[edit] This page needs to be cleaned up- crude references have been inserted

[edit] Use of the word flow, as in a current that flows through a resistor

[edit] Physics section

[edit] The stuff about Ohm's law not existing before Ohm defined it?

[edit] AC section

[edit] diagram

[edit] s operator

[edit] Physics section

[edit] "Generalized" Ohm's Law

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