Talk:Ohm's law

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

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.

Contents

[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 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:

  1. 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.
  2. 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)
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