Questions about History of Ohm's Law and "Watt's Law" - Page 1
Forum section: electronics / general

author: Electro Fan
author_rank: Super Contributor
timestamp: March 04, 2018, 11:46:19 pm
content: I posted something similar to this in one of the equipment related forums. Not too surprisingly it didn't get a lot of traction over there.It was driven by some Q&A regarding V=IR and P=VI, etc. but it was off-topic so I thought I'd move it to this forum in case we have any history buffs here. Thx----Does anyone know when V=IR was first referred to as "Ohm's Law"? Who coined the term and when?Also, does anyone know if Georg Ohm knew that P=VI? Maybe he was aware of it but didn't publish it? Sort of hard to imagine why he would have held it back if he knew it. And if Georg Ohm didn't name V=IR after himself and someone else bestowed the term "Ohm's Law" on V=IR as a tribute to Georg Ohm, why wouldn't P=VI be included in what we now call "Ohm's Law?". Perhaps P=VI was determined after Ohm's Law was attributed strictly to V=IR? It does seem that the recognition of P=VI must have happened sometime after Georg Ohm was feated with the naming of "Ohm's Law."And if Georg Ohm didn't publish P=VI or even know it, who was the first to put forward the notion of P=VI, and when? From what I've read, some people refer to P=VI as "Watt's Law" but others seem to say Watt's Law is more a commonly applied term than an official term. According to history Watt was feated by having the unit of a Watt named after him, but Watt didn't invent/discover/promote P=VI as he worked on steam engines rather than electricity.Net, net: If P=VI wasn't a concept put forward by Georg Ohm, and given that the relationship of P=VI doesn't seem to be attributed to him, who was first to figure out P=VI, when did the discovery of P=IV occur, who was first to start calling P=VI "Watt's Law", and when did the naming convention of P=VI as "Watt's Law" first appear?Thanks for any good history lessons anyone can provide.

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2018, 12:04:08 am
content: Firstly, people don't name laws after themselves. That would be uncool. Laws or discoveries are attributed to people after the fact, sometimes inaccurately.Secondly, Ohm's law is a property of ideal materials. An ideal "Ohmic" material obeys Ohm's law; most or all real materials do not.Thirdly, P = VI has nothing to do with Ohm's law. It is a fundamental mathematical relationship that occurs throughout the whole of physics. It expresses the thermodynamic principle that it takes work to move a quantity of something through a field that interacts with that something through a force. The equation P=VI is identical in physics to the equation that says power = force times velocity, or power = pressure difference times volumetric flow.

author: dmills
author_rank: Super Contributor
timestamp: March 05, 2018, 12:11:48 am
content: My recollection was that the actual statement was more along the lines of:"The current thru a metallic conductor is proportional the the electric potential across it", but I cannot now find the quote.The watt as a unit being 1 joule of one second is an SI thing and came along later, (Watt was a steam engine guy who massively improved early steam plants, I think we actually owe him the Horsepower as a unit).Regards, Dan.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 05, 2018, 01:14:33 am
content: Quote from: IanB on March 05, 2018, 12:04:08 amRoger all that, although I'm not sure it's completely fair to say that P=VI has "nothing to do with Ohm's law". As shown below in the image from Wikipedia, representations of Ohm's law in the form of the Ohm's law wheel are pretty common place, so maybe P=VI has "something" to do with Ohm's law. I'm guessing P=VI as it relates to electricity didn't show up in literature until after V=IR; just curious to to know who first made the connection or promoted the use of P=VI in the field of electricity?https://en.wikipedia.org/wiki/Ohm%27s_lawIn any event (and to your point that Ohm and Watt didn't name these "laws" after themselves), the questions remain, who initially coined the terms "Ohm's Law" and "Watt's Law", and when (what years)? Just curious to know if these "laws" were initially noted as such by any particular scientists, scholars, or as part of any notable scholarly awards, or did these terms just show up through mainstream journalism and/or popular use? Thx

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 01:41:15 am
content: Quote from: Electro Fan on March 05, 2018, 01:14:33 amOhm's Law defines the relationship between Voltage and Current when applied to a material with a particular property. That property has been assigned the term "resistance". The numeric value of this "resistance" is determined by the measurement of the applied voltage and current and then solving the equation.The power calculation P=VI does not come from any such derivation process.If you are looking at the "shape" of the two equations, then that's a pretty flawed view.If you are looking at the fact that "V" and "I" appear in both equations, then you need to look closer at the relationship of these two quantities. It's V/I for one and V*I for the other. These are two completely different quantities.And ... I am curious ... what is "Watt's Law"? I've never heard the term before - but when I Googled it, I came up with the calculation of power. The first reference came from a source called spazztech.net which seemed unconvincing:What is Ohm's Law and Watt's Law?:Ohm's Law defines one of most fundamental relationships in electronics. That is the relationship between Voltage, Current, and Resistance. Watt's Law defines another of the most fundamental relationships in electronics. That is the relationship between power and the quantities defined by Ohm's law. We can not go very much farther into electronics until these concepts are understood."Watt's Law defines another of the most fundamental relationships in electronics. That is the relationship between powerand the quantities defined by Ohm's law." Wrong. Resistance is the only thingdefinedby Ohm's Law. Voltage and current are simply measured quantities - and they are all you need to calculate Power.

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2018, 04:21:20 am
content: Quote from: Electro Fan on March 05, 2018, 01:14:33 amYou can't always trust Wikipedia, and this is one of those times. P = VI is not a "law" in the sense you are thinking of it. It is a fundamental axiom of physics, something that is always true, whether or not resistance is involved. It is the logical outcome of the fact that work must be done on a charge to move it against a potential gradient. If a charge moves "uphill" in an electric field it gains potential energy and if it moves "downhill" it loses potential energy.I am not sure which worker originally extended these concepts of thermodynamics to electricity, but it would have been around the time that thermodynamics was being put on a rigorous footing in the realm of physics and physical phenomena.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 05, 2018, 05:44:42 am
content: I hear you about Wikipedia, and I’m not hung up on whether something is referred to as a “law” or axiom or just a concept. I’m more interested in this case in learning about the history of how knowledge was accumulated regarding these fundamental principles (another word) in the harnessing and technological development of electricity.As for P=IV certainly someone who started using V=IR and thought they were on a new fast track for circuit making in the 1800s must have been surprised when despite the fact that they got V to equal IR they nonetheless let some magic smoke out because they hadn’t been aware of the need to keep an eye on V multiplying I and they over-ran the power carrying capacity of their resistors or whatever resistors were called at the time. I think it would have been cool (literally and figuratively) to be present when someone in a mid 1800s lab, fresh from reading about “Ohm’s law” or whatever it was called early on, realized that the same V and I values from V=IR were the values needed to deal with VI=P. I would think it must have been a tremendous epiphany to go from “we have not been very sure about what causes what since the beginning of humanity” to “look how elegantly all this stuff fits together.”I’m not debating what gets classified as a law or an axiom, or a separate concept, or what branch of science things get classified under (although those things would also be interesting to know). What I’m mostly trying to understand is the sequence and intervals of time (and who was involved) to get a better sense of how these fairly recent developments occurred, including how the naming conventions came about - and how the knowledge spread. I find the math, physics, and overall science to be fascinating but I think it’s also interesting to understand how humans acquired the knowledge. It’s sometimes a fine line between discovering and inventing and I think there might be useful lessons to learn and preserve about the intersection of electricity and the development of humanity.To your point, anyone can write an article and have it propagated (although I’m more thankful for Wikipedia than skeptical of it) - so here’s a chance for people who are knowledgeable and passionate about electricity to discuss some of the key historical developments. There are probably lots of EEVers who studied EE history who could help put down some dependable, well documented facts on the chronology of fundamental investigations, discoveries and inventions. Maybe some day the discussions on EEVblog will be useful bits of anthropology for future generations. Kind of like if you don’t get your family tree from your older generation it might be harder for your younger generation to understand and appreciate what precipitated what. Peace out.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 05, 2018, 06:32:17 am
content: Quote from: Brumby on March 05, 2018, 01:41:15 amI hear you about some of the references to "Watt's Law" maybe not being super top notch but fwiw, attached is an image from a Stanford University research center that uses the term "Watt's Law". Personally, I think "Watts Law" is more likely to be a term adopted by laymen rather than the overall population of EEs but as mentioned in my reply to IanB my primary question is about how people began to connect the dots between V=IR and P=IV, and how the naming conventions came about.As for this:""Watt's Law defines another of the most fundamental relationships in electronics. That is the relationship between powerand the quantities defined by Ohm's law." Wrong. Resistance is the only thingdefinedby Ohm's Law. Voltage and current are simply measured quantities - and they are all you need to calculate Power."I'd say that while V and I can be simply measured quantities and R can be defined (calculated) by Ohm's Law, it is also very possible that V and R can be simply measured quantities and I can be defined (calculated) by Ohm's Law, and likewise I and R can simply be measured quantities and V can be calculated by Ohm's Law. So all three values have a relationship to each such that any two can enable the determination of the third. Further, taken as a whole, the three not only have relationships among the three but if you know any 2 of the values you can not only determine the third but you can also determine P. So while "Watt's Law" might not be an orthodox or even official name for P=IV, and while P=IV is not part of Ohm's Law per se, I don't think it's unreasonable to say that P=IV has a relationship to the quantities defined by Ohm's Law. Just saying "wrong" seems a bit rigid, if not "wrong" itself.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 08:34:37 am
content: Quote from: Electro Fan on March 05, 2018, 06:32:17 amOf course there are relationships between the quantities - but, to me, you have changed what you have been saying.If you don't believe you have changed what you thought you were saying, then either I have the wrong end of the bull - or you haven't expressed yourself clearly.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 08:41:40 am
content: Quote from: Brumby on March 05, 2018, 01:41:15 amRead that very carefully.The power calculation can be performed WITHOUT ANY KNOWLEDGE of the resistance. Voltage is a quantity in it's own right and can be measured without knowledge of any resistance. Same for current.So saying the power calculation"is the relationship between power and the quantities defined by Ohm's law."is just plain wrong.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2018, 08:47:17 am
content: Where would 'Watt's Law' feature in KVA ?Watt's Law.....never heard of such a thing !Wish I could find Dad's Electricity & Magnetism theory book from the '40's, that had all this stuff in it and in black and white !

author: Electro Fan
author_rank: Super Contributor
timestamp: March 05, 2018, 09:17:13 am
content: Quote from: Brumby on March 05, 2018, 08:41:40 amJust for the record, I wrote at the start (in the original post above):"From what I've read, some people refer to P=VI as "Watt's Law" but others seem to say Watt's Law is more a commonly applied term than an official term."You replied saying that you found a definition somewhere of "Watt's Law" that you found to be wrong. It was definition you found and you posted into your reply. My only points (paraphrasing) have been that P=IV uses 2 values that are also contained in V=IR and that with a knowledge of 2 of the four (V,I,R,P) you can solve for the other 2. You can debate this further but I don't know why.To put the thread back on track (as opposed to debating like lawyers who insert their own words within each new draft while arguing about t crossing and i dotting), the original questions in the very first post at the top of this thread were and still are:Net, net: If P=VI wasn't a concept put forward by Georg Ohm, and given that the relationship of P=VI doesn't seem to be attributed to him, who was first to figure out P=VI, when did the discovery of P=IV occur, who was first to start calling P=VI "Watt's Law", and when did the naming convention of P=VI as "Watt's Law" first appear?

author: bitwelder
author_rank: Super Contributor
timestamp: March 05, 2018, 10:25:20 am
content: Quote from: tautech on March 05, 2018, 08:47:17 amI guess by now there are several old books about theory of electricity that have been scanned and put online (Google Scholar, perhaps?)Probably from there is possible to build a first guess at when such 'laws' have been first named (if they are, I'm a also doubtful about Watt's law)

author: dmills
author_rank: Super Contributor
timestamp: March 05, 2018, 10:51:22 am
content: Been in this game for 30 years, and "Watts law" is a new one for me.Regards, Dan.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 11:51:07 am
content: Quote from: Electro Fan on March 05, 2018, 09:17:13 amThere is and never has been any such thing as "Watt's Law". The only mention of this phrase that I have seen have been found in sources that do not demonstrate any level of engineering expertise. It would seem somebody made up the name because of the visual similarity.QuoteIf that's what you meant, it was not clear in the words you used.QuoteI wouldn't debate this. If you thought I would, then you really interpreted my comments incorrectly.QuoteOhm did his work 1825 and 1826 and published his results in 1827.Joule published his work that the heat generated from and electrical system wasproportionalto the square of the current times the resistance: I2.R around 1840. So, if you wanted to put a name against a "Law" relating to power, then you'd be less wrong by referring to Joule. But he's already got a unit name after him for energy.Who "worked out" P=VI is going to be the first person who did the substitution of Ohm's Law into Joule's equation. That's like asking who was the first person to write Ohm's Law as R=V/I.And, repeating: There is and never has been any such thing as "Watt's Law". The only mention of this phrase that I have seen have been found in sources that do not demonstrate any level of engineering expertise. It would seem somebody made up the name because of the visual similarity.In other words, I see that it was created out of ignorance - something I would expect from the half-baked "information" that is spewed out on the internet.Besides, James Watt died in 1819 and being more of a mechanical engineer, putting his name against this is absurd.So, please, let's drop this term.

author: Nusa
author_rank: Super Contributor
timestamp: March 05, 2018, 01:27:46 pm
content: Quote from: Brumby on March 05, 2018, 11:51:07 amYou can't blame the internet for this one, although the rest of what you way might be true. The term has been in use for at least 40 years that I'm aware of (in the US anyway, and I am old enough to remember). Maybe not in the hard sciences where it's the power equation, but it's common in practical math books and trade school curriculums.I don't see a "name" catching on as any reason to get upset, so long as the formulas being taught are correct.Quote from: Brumby on March 05, 2018, 11:51:07 amNot absurd at all. James Watt had a great deal to do with defining horsepower, and the term "watt" was named after him and defined as joules per second, which when adopted in 1882 was probably used more often with mechanical energy than electrical. 1 imperial horsepower is slightly less than 746 watts of mechanical energy. 1 metric horsepower is a bit less...I don't recall that number. These days we often measure engines in kW...it's still mechanical energy.

author: dmills
author_rank: Super Contributor
timestamp: March 05, 2018, 02:28:32 pm
content: And the horsepower turns out to be somewhat greater then a typical heavy horse can actually manage on a sustained basis!This was deliberate on Watts part as it meant that someone purchasing one of his 2 horsepower engines would be pleasantly surprised, and customer satisfaction mattered.Steam engine performance was usually compared with a centrifugal fan as the load, the torque going as third power of rotational speed meant they could be used over a usefully wide range of power levels (Power at a given speed was thus a 4th power law as long as the rotor stays subsonic).Watts law, would, if it was anything be better as something from thermodynamics or fluid flow, these things would at least have had some bearing on what he was known for.Regards, Dan.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: March 05, 2018, 03:14:20 pm
content: A Watt is a unit of power. It is not specifically related to electricity. Basically one Joule per second. A Joule being the energy required to accelerate 1kg of mass by 1m/s speed. James Watt, who it is named after, was not an electrical researcher but a designer of stationary steam engines.

author: Vtile
author_rank: Super Contributor
timestamp: March 05, 2018, 03:54:56 pm
content: Quote from: tautech on March 05, 2018, 08:47:17 amPlease also note that some of these might come from different languages or different parts of the disciplines and be only coarse translations originally.I have seen some old (1950 or older) source to mention P=UI as Joule's law to make distinction for Ohm's Law. (P=R*I^2)Watt is naturally just SI-system litter as a general power unit and not related directly to the electrical power, since we are not racists and separate different powers.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 05, 2018, 05:08:38 pm
content: Thanks for all the posts - now we are getting somewhere, and I think we are in forward gear, while looking retrospectively

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 05, 2018, 06:11:19 pm
content: Quote from: bitwelder on March 05, 2018, 10:25:20 amIt looks like that was done in the Wikipedia article, searching Google books. It sites a 1855 book that mentions "Ohm's Law":Quotehttps://books.google.com/books?id=TDEOAAAAYAAJ&pg=RA1-PA43&dq=ohm%27s-law+date:0-1860#v=onepage&q&f=falseQuoteNote that the Ohm's law referred to is not V=IR, but in terms of the resistivity of materials: that the resistance is proportional to the length and inversely proportional to the cross sectional area.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 10:40:44 pm
content: Quote from: Nusa on March 05, 2018, 01:27:46 pmQuite so - but Ohm's Law was named because it was Ohm who did the work that produced it. Same with Boyle's law, Hooke's law and Coulomb's law. Watt never got within cooee of such a formula. Further, I can only see the name "Watt's Law" coming about because the answer is in watts.That's the two part absurdity of it ... to my mind, at least.As for my attribution to the internet age for the origin, I must admit my error. Ignorance and misnomer have been around for far, far longer. All the internet age has done is enable instant global propagation.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2018, 11:33:19 pm
content: Quote from: Nusa on March 05, 2018, 01:27:46 pmYes, the teaching of the formula is the important thing.However, the EEVblog forum seems very well indexed on search engines, so I am happy for this discussion to be explored, so that anyone curious can see what we think of it.QuoteI don't argue against any of this. Thenaming of the quantityafter him is well deserved ... but attributing a "Law" to him is something quite different. He may have worked in a closely related field, but he did not produce the specific math - ie. P=VI - that Ohm, Hooke, Boyle and Coulomb did for their laws. If anything, Joule was a lot closer ... and I would accept an argument that it was he that really cracked that nut.If you really want to hang on the the name - I'd be a lot less perturbed if it was called the "watt law". No proper noun. No possessive "'s". (and the term "law" here being given a bit of leeway.)

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2018, 11:50:23 pm
content: Nah Brumby, you're going up the wrong path.Once we start attributing just what's a unit of 'measure' to a law it's destined to everlasting complexity and confusion !Just as KVA is a unit of measure and uses the same formula P = VI, when calcs are done for KVA do we describe it as derived from the 'KVA law' ?

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 12:02:43 am
content: I said I would be a lot less perturbed. I didn't say I would be happy. I would be happy if it was just called the power formula.You could call it "Joule's Law" with the "Ohm's Law" substitution. It may be cumbersome, but it's accurate.I'd be interested in the feeling of defining "Dave's Law" as the power required for the temporal shift of a DeLorean - being 1.21GW. How say you?

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 12:08:01 am
content: Quote from: Brumby on March 06, 2018, 12:02:43 amBut you just can't do that either !Power, what sort of power ?Hydraulic ?Wind ?Volcanic ?

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 12:16:31 am
content: The context of P=VI kinda gives the answer away.But the alternatives you have given are all mechanical energy quantities - and they will have an answer in watts as well.

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 01:10:57 am
content: Quote from: Brumby on March 06, 2018, 12:16:31 amMaybe, but good luck using P=VI to calculate hydraulic power in watts !

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 01:12:34 am
content: Now you are being sillier than you were.The context for P=VI isn't hydraulics - but youcanhave an electrical to mechanical conversion unit to make them comparable.It's called a motor.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 06, 2018, 01:49:32 am
content: Found this:A book published in 1891 that looks to be a translation into English of Georg Ohm's 1827 book entitled:"The Galvanic Circuit Investigated Mathematically"https://www.princeton.edu/ssp/joseph-henry-project/ohm/The_Galvanic_Circuit_Investigated_Mathem.pdfOhm's book was translated by William Francis, a student in Philosophy, the University of Berlin, with a preface by the Editor Thomas D. Lockwood, M.I.E.E, Vice-President of the Am. Inst. of Electrical Engineers.While the 1891 book is mostly a translation of Ohm's 1827 book it contains some observations and explanations regarding Ohm's book, including some comments that reflect the work and thoughts of other scientists including Cavendish and Maxwell.Apparently there was another English translation of Ohm's 1827 book known as Taylor's Scientific Memoirs but Lockwood thought it was difficult to procure. Lockwood said"the intrinsic value of [Ohm's] book is so great that it should be read by all electricians who care for more than superficial knowledge."He goes on to say that prior to Ohm "quantitative circumstances of the electric current had been indicated, in a vague way, by the use of terms "intensity" and "quantity," to which no accurately defined meaning was attached. Ohm's service consisted in introducing and defining the accurate notions - electromotive force, current strength, and resistance. He indicated the connection of these with experiment, and stated his famous law that the electromotive force divided by the resistance is equal to the strength of the current."Lockwood says that"It is perhaps worth recalling, that Henry Cavendish, in his secret and solitary researches, made experiments in 1781, the results of of which practically anticipated Ohm's considerations; but Cavendish having satisfied himself, did not apparently consider it worth while to take any one else into account."Lockwood goes on to talk about Maxwell's comments about Cavendish's work.Lockwood says"This result, obtained by Cavendish in January, 1781, is an anticipation of the law of resistance discovered independently by Ohm, and published in 1827. It was not until long after the latter date that the importance of Ohm's law was fully appreciated, and the management of electric resistance became a recognized branch of research."Lockwood says"Some doubts having been suggested with respect to the mathematical accuracy of Ohm's law, the subject was taken up by the British Association in 1874.... with results contained in the report of the British Association for 1876."It is clear that Lockwood wanted to recognize Ohm's brilliant work and to also give some solid recognition to Cavendish. With all this in mind, Lockwood goes on to say"The above is not referred to as any depreciation of the work of Ohm, but simply as an interesting point in connection therewith."Lockwood concludes his Editor's Preface with:"Ohm's work stands alone, and, reading it at the present time, one is filled with wonder at his prescience, respect for his patience and prophetic soul, and admiration at the immensity and variety of ground covered by his little book, which is indeed his best monument.It is my experience that many quote Ohm's law and talk of Ohm's law, who know little or nothing of Ohm himself, or of his book; and it seems to me that a large portion of these, will welcome the opportunity to read, in their own language, what Ohm himself has to say about his own law, which so long has been practically sealed up, and has been accessible only at second hand.With the hope that this opportunity will be utilized fully, and that this classic may become a familiar friend, the "Mathematical Consideration of the Galvanic Circuit" is once more launched."T.D.L.Melrose, Mass., 1891.There is much to find in the translated book including lots of math and early terminology. Happy perusing to all EEVers. Hope some of you will read and share your insights on how Ohm's law was developed and expressed by Ohm and how his work relates to that of other scientists.

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 02:31:08 am
content: While the history is no doubt fascinating, not everyone will be inclined to go into such detail. What will matter to most - EE's and enthusiasts alike - is that these laws are known and are useful.Deriving everything from first principles or following the history of who did what and when does not really affect our usage of the "Laws" and formulae in designs and projects of today. As long as we understand the mechanics of these things enough for us to manipulate them, that is all that matters. You could spend a lifetime examining the development of so many technical advances, but you don't need that to make a flip-flop.While I appreciate the work done by so many - and I am thankful for all of it - I would much rather spend my time going forward by using it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 06, 2018, 02:45:19 am
content: Some of us like restoring old gear. Some of us try to solve new problems. And some of us like documenting how we got here. There is plenty to keep all of us happy, and we all don't have to pursue every rivulet.I for one have discovered that I am not a full fledged volt nut, but I do enjoy following the stories of those who are. Same thing applies for the history of ohms law.

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 02:59:47 am
content: Quote from: Brumby on March 06, 2018, 01:12:34 amYep, and it produces power in Watts !I ask you in the context of this thread how could you use the so called "Watts Law"to calculate the power in watts from hydraulic pressure and flow rate ?You can't ! Period !So this "Watts Law" can only be applied to electrical power right ? What about any other type of power where the unit calculated is also in watts ?How many "Watts Law"'s must we have ?Answer = NONE

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2018, 03:24:18 am
content: Quote from: tautech on March 06, 2018, 02:59:47 amWell, since you ask, if we have a pump moving Q m3/s of water with a pressure rise of DP N/m2, then the power of the pump in Watts is Q * DP. We could call this "the Watts law for pumps".Unfortunately for hydraulic engineers in the USA, they tend to measure flow in gallons per minute and pressure rise in psi*, which means they don't get Watts at all when they multiply them together, they get some crazy unit of power called "pound force gallons per square inch per minute", which they then multiply by some particular conversion factor to get a result in horsepower. This would be comical if it were not so true*Actually they might measure pressure rise in feet of head, but this doesn't really improve things

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 03:36:50 am
content: First, there is no such things as "Watt's Law".There is a power calculation for electrical units:P=VI where:P=wattsV=VoltsI=Ampsand from what I can gather, there is this for hydraulics:P=p.Q/600 where:P=kilowattsp=bar (inlet/outlet pressure difference - analogous to voltage)Q=l/min (flow rate - analogous to current)For completeness, you should probably add an efficiency factor, but the basic equation is there.

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 03:40:48 am
content: Quote from: IanB on March 06, 2018, 03:24:18 amYes, I noticed this in my brief travels - but this is just a (messy) unit conversion exercise.

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 03:41:58 am
content: Quote from: Brumby on March 06, 2018, 03:36:50 amThere, you said it !I wholeheartedly agree !

author: IanB
author_rank: Super Contributor
timestamp: March 06, 2018, 03:43:30 am
content: Quote from: Brumby on March 06, 2018, 03:36:50 amThe "/600" only happens because you are not using SI units. Let pressure be in Pa (N/m2) and Q be in m3/s and when you multiply them you get N/m2x m3/s = Nm/s = J/s = watts.QuoteTrue, but electric motors have an efficiency too.

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 03:43:49 am
content: Quote from: tautech on March 06, 2018, 03:41:58 amI've been saying that from the very beginning.

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 03:47:00 am
content: Quote from: IanB on March 06, 2018, 03:43:30 amI realised the difference - but I'd only just started my research into hydraulic equations 15 minutes ago - and I was just citing two sources with the same equation. Thank you for the "tidying up".

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2018, 03:50:36 am
content: Quote from: Brumby on March 06, 2018, 03:43:49 amNearly, well close, if you call your fourth post the beginning of when you were convinced this was all

author: Brumby
author_rank: Unknown
timestamp: March 06, 2018, 03:58:05 am
content: The only thing I was really calling "BS" on was the use ofthatterm. (I don't even like using it, it's that bogus.) I was also a bit unhappy about some of the overly casual content. But all of this coming from a site called "spazztech", I can't say I was surprised.