Isn't voltage a potential difference? ----- Original Message ----- From: "Gerhard Fiedler" To: Sent: Saturday, June 17, 2006 3:44 PM Subject: Re: [EE] Soldering Thermocouple Eire? > Peter wrote: > >>>> Potential and voltage are one and the same in theory but in practice it >>>> is more complicated. >>> >>> I'm not sure this is on firm ground. I'm no ace in physics terminology, >>> but >>> I think "voltage" is a term for the value of an electric field >>> potential, >> >> The definition of potential is 'the energy required to move a punctiform >> charge unit from the point being measured to infinity'. All normally >> measured voltages are *relative* differences of potential, not absolute. > > I still don't see how you can make a difference between 'voltage' and > 'potential' -- at least not any difference that would be relevant to the > Seebeck effect. (At the very least, you should be more careful with the > papers you cite in a given context: the one you cited uses 'potential > difference' and 'voltage difference' as synonyms :) > > Every voltage is relative to something. Every potential is relative to > something. No difference here. (Like in a normal circuit: voltages are > usually given relative to ground. They represent potentials relative to > the > generally not known -- in absolute terms -- ground potential.) > > Absolute potential is relative to infinity, and has a voltage that is > given > relative to infinity. Difficult to measure, as you state, but still > existent as concept. No difference either. (You could say, for example, > that a certain potential has 1 kV -- absolute, i.e. relative to infinity. > That 1 kV would then be its voltage, no?) > >> In this sense a voltage is a potetnial *difference*, > > A voltage is a difference just as a potential is a difference. Either must > be referenced to something (this 'something' includes infinity) to make > any > sense. > >> but if only a potential exists then a voltage must not also exist > > Describe a situation where a potential exists but no voltage... For every > point that has a known potential, I can give you its voltage (relative to > infinity); just tell me its potential :) You seem to not take into > account > that the reference that is implicit in the concept of an absolute > potential > (infinity) necessarily always exists, and is used as reference point for > expressing the value of this absolute potential (in volt). > > >> But the definition of the t. voltage is absolute, not relative. > > Oops... Didn't you just base your argument regarding the difference > between > the concepts of 'voltage' and 'potential' on the statement that potential > is absolute whereas voltage is relative? And here you introduce the > concept > of an 'absolute voltage'? > > This inconsistency left aside (as it is not really relevant for the > thermoelectric voltage)... is it really absolute? I thought it was > relative. The Seebeck effect seems to talk about the /difference/ of > potential that is caused by a /difference/ in temperature along a > (semi)conductor. I don't see anything absolute here. > > Again citing from the paper you mentioned: "Seebeck effect: A temperature > difference between two points in a conductor or semiconductor results in a > voltage difference between these two points." And later: "A voltage is > therefore developed between the hot and cold ends with the hot end at > positive potential." Just in case my explanation is not scientific enough > :) > > This is confirmed by the practical ways to measure that potential > difference (in a typical thermocouple setup): you measure a difference of > two Seebeck voltages, both a function of the temperature /difference/ > between two points. Every thermocouple measurement needs to reference the > temperature difference you gain from the measured voltage to some absolute > reference (the so called cold junction temperature) -- and this can't be > measured with a thermocouple (alone), never ever. This is not because we > measure the difference of two Seebeck voltages; this is because the > Seebeck > voltages themselves are voltage differences based on the temperature > difference. > > >> So while the t. potential should be there, > > Where is it, as an absolute potential? As I see it, it is always, > necessarily, relative to another point -- at least what they call the > Seebeck effect. Of course, you can always reference both the temperature > and the potential of both ends of the rod under test to infinity (absolute > zero of both temperature and potential). But that doesn't seem to add much > value -- compared to looking at it as one point (with a temperature and a > potential) relative to another point (with a temperature and a potential). > > I also don't think you can speak of a Seebeck effect if you have only one > single point (with its temperature and its potential). You need two > points, > with a temperature difference between them, connected by a conductor, for > the Seebeck effect -- which causes a potential (or voltage) difference > between them. I don't think that the electric potential of a single point > (with uniform temperature) changes while changing its temperature. The > Seebeck effect (aka thermoelectric voltage) needs a temperature > /difference/ between two points on the conductor. Nothing absolute in > this. > > >> ... it must be measured with reference to something else. This implies >> the need for at least *one* contact or quasi-contact. Even the pin/bar >> proposed by Olin has one contact. > > No. The "contact" could be (and should be) of the same material as the rod > under test. This may put some restrictions on the range of suitable > materials, but not on the principle itself. > > I think you may be mixing up the fact that in order to measure the Seebeck > effect in a closed current loop we need necessarily two different > materials > (and therefore measure the difference of the Seebeck effect between the > two > materials) with some other things. > > A few affirmations (all IMO, of course :) > > 1- In order to create a closed loop where current can flow (which is the > case of all "normal" ways to measure voltages), it is necessary to use two > different materials in order to see a Seebeck effect. If the loop > consisted > of the same material, the Seebeck effects on both sides of the hot spot > would cancel themselves out. That's the thermocouple: the resulting > voltage > is a function of the difference of the Seebeck effects in both materials > and the difference in temperature between the hot and the cold end. (This > implies a certain configuration of what is hot and cold, but can be > reformulated for any other configuration.) > > 2- The Seebeck effect itself does not need a second material; it happens > within one material. It creates an electric field, which implies a > potential difference (aka voltage difference) between points of different > temperature along the conductor. These different potentials can be > measured > (without introducing a second, different material) in a number of ways -- > but all these methods have one thing in common: they don't rely on a > continuous current flow, therefore they don't need a closed loop, which > makes them conceptually different from the way we commonly measure > thermocouple voltages. However, measuring electric potentials in such ways > is nothing particularly strange (in laboratories that deal with electric > fields, at least). > > 3- While it may be that a common use of 'voltage' and 'potential' exists > that implies that 'potential' is absolute and 'voltage' is relative, the > author of this distinction doesn't adhere to this use (he earlier > suggested > an 'absolute voltage'). I agree with the author's practical sense (here > and > in general ) and say that both voltage and potential can be relative > (and in most applications are) and absolute. (For example, mechanic > potential energy is pretty much always used in a relative form, and it's > conceptually pretty similar to an electric potential.) I also think that > the value of an absolute electric potential is given in volt, and probably > can therefore be called a 'voltage' (an absolute one, maybe, but still). > > If there is a difference between the terms, I'd say the voltage is the > value of the potential (in the sense of "we have here an electric > potential > with an unknown voltage" or so)... but then, as long as we talk about > 'bigger' and 'smaller' (as in the Seebeck effect), we always talk about > the > (relative) values. > > > Not sure this is of interest to many, but for me it is surprising that we > need to discuss this (and important that we do, at least for me -- I could > be wrong on any or all questions :). > > Gerhard > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist