What keeps electrons spinning around their nucleus?
in [Relativity]
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From: mmeron on 29 Mar 2005 19:52 In article <1112125397.698536.112020(a)g14g2000cwa.googlegroups.com>, "PD" <pdraper(a)yahoo.com> writes: > >mmeron(a)cars3.uchicago.edu wrote: >> In article <d2abun$65m$3(a)rainier.uits.indiana.edu>, >glhansen(a)steel.ucs.indiana.edu (Gregory L. Hansen) writes: >> >In article <Rj12e.16030$C7.2138(a)news-server.bigpond.net.au>, >> >Bill Hobba <bhobba(a)rubbish.net.au> wrote: >> >> >> >><mmeron(a)cars3.uchicago.edu> wrote in message >> >>news:yr02e.13$45.3299(a)news.uchicago.edu... >> > >> >>> >Which is just another reason force should be considered as a >secondary >> >>> >concept - the PLA is the primary concept. End of rant. >> >>> > >> >>> Oh, the PLA most certainly is ***the*** primary concept. So >primary, >> >>> in fact, that it serves as foundation not only for Newtonian >mechanics >> >>> but (with appropriate generalizations) for most of physics. No >> >>> argument about it. >> >>> >> >>> This said, the fact remains that, for applying the PLA, a level >of >> >>> mathematical knowledge and sofistication is required which is way >> >>> beyond the capabilities of a beginning (high school) physics >student >> >>> and, in fact, way beyond this that most people ever reach. So, >we >> >>> maintain forces as a crutch to be used till more is learned. >That's >> >>> really their remaining role. >> >> >> >>Most certainly. But if we are to use forces then I think we need a >> >>discussion of what they really mean along the lines of what Feynman >did in >> >>the lectures. In fact I consider that to be compulsory reading >even for >> >>grade 8 students (at least the chapters they have the mathematics >to >> >>understand - other chapters can be added as their mathematical >knowledge >> >>grows). Having understood what Feynman wrote I think a lot of >confusion can >> > >> >The Feynman lectures? For 8th graders? Aren't the Feynman lectures >the >> >ones based on the college courses he taught that the professors >attended >> >and the freshmen dropped out of? >> > >> >>be avoided. In my case I always wondered why a definition could be >a law. >> >>It confused me for many years until I read a rather nifty old book >on >> >>classical mechanics. It clearly explained the real import of >Newton's Laws >> >>was in his third law. That was the start of actually understanding >what was >> >>happening. The full resolution came with Landua - Mechanics. I >think it >> >>would be great if students did not need to go through this process >and were >> >>taught what was happening right form the start - at least as much >of it as >> >>they can initially handle. >> > >> >I think a lot of this will be resolved for the person who can accept >that >> >there can be several completely valid ways to approach a subject. >The >> >importance of the PLA (Principle of Least Action, right?) doesn't >make >> >Newton's approach bad, just different. >> > >> >One objection I would have in trying too hard to teach it "the >right" way, >> >besides confusing the students, is that too often it denies the >things >> >they know. Everyone knows what centrifugal forces are, nobody is >confused >> >on the fact that you feel it in something that's spinning and you >don't >> >feel it in something that's moving uniformly. And then these >eggheads >> >come along and say centrifugal forces don't actually exist, although >the >> >layman knows those nonexistent forces seem to work pretty well when >the >> >laundry machine hits the spin cycle. And the layman isn't wrong. >> >Insufficient centripetal force to retain the water and sufficient >> >centrifugal force to expel the water are separated by a simple >> >transformation. The layman doesn't think of it in that way, but he >knows >> >that things happen when you spin. Does the egghead actually >accomplish >> >anything by trying to excise the word "centrifugal" from the >language? >> > >> There is this bit of fun to get from telling people >> "what you thought to be true is not so, the truth is quite different" > >> (with the implied "I'm smarterr than you're, nah nah nananah":-)). >> And while, in an honest moment, I'll admit that there is some (albeit > >> low) enjoynment to be derived from this, basing the teaching of a >> discipline on this is not advised. > >Yeah, it's fun, but it also serves a purpose. It teaches them to be >*clear-thinking* and careful. When the puzzle is unraveled and they >find that it is indeed completely consistent with the simple principles >they just learned, this actually restores their faith that they can >understand it and make it all fit together. > That assumes that thre was a puzzle to be unraveled. Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: Tom Capizzi on 29 Mar 2005 20:09 "Puppet_Sock" <puppet_sock(a)hotmail.com> wrote in message news:1112140306.287685.147910(a)z14g2000cwz.googlegroups.com... > Tom Capizzi wrote: >> <puppet_sock(a)hotmail.com> wrote in message >> news:c7976c46.0503291409.5e027614(a)posting.google.com... >> > dubious(a)radioactivex.lebesque-al.net (Bilge) wrote in message >> > news:<slrnd4dc08.n0h.dubious(a)radioactivex.lebesque-al.net>... >> >> Nick: >> >> >What is the velocity of an electron in a shell? >> >> >> >> Velocity isn't a quantum mechanical observable. >> > >> > Of course it is. What ever do you think you get when you take >> > the time derivative of position? >> >> Electrons don't really have position. They have complex probability >> amplitudes. >> What do you get when you take the derivative of that? > > Come on. Review your QM. Position is an observable. > Velocity is the time deriv of position. "Don't really > have" isn't a sensible statement. What they have is > expectation values, Eigen values, etc. > And Heisenberg says you can measure one or the other, but not both simultaneously with certainty. In any case, tell me how you can tell the position of an electron in an orbital. > So it is perfectly sensible to ask such questions as > "if I measure the position of an electron, what is the > expected value, and what is the distribution?" > > Or "if I measure the value now, or in ten minutes, > will I get the same result?" > Socks >
From: Creighton Hogg on 29 Mar 2005 20:05 On 29 Mar 2005, Puppet_Sock wrote: > Tom Capizzi wrote: > > <puppet_sock(a)hotmail.com> wrote in message > > news:c7976c46.0503291409.5e027614(a)posting.google.com... > > > dubious(a)radioactivex.lebesque-al.net (Bilge) wrote in message > > > news:<slrnd4dc08.n0h.dubious(a)radioactivex.lebesque-al.net>... > > >> Nick: > > >> >What is the velocity of an electron in a shell? > > >> > > >> Velocity isn't a quantum mechanical observable. > > > > > > Of course it is. What ever do you think you get when you take > > > the time derivative of position? > > > > Electrons don't really have position. They have complex probability > > amplitudes. > > What do you get when you take the derivative of that? > > Come on. Review your QM. Position is an observable. > Velocity is the time deriv of position. "Don't really > have" isn't a sensible statement. What they have is > expectation values, Eigen values, etc. > > So it is perfectly sensible to ask such questions as > "if I measure the position of an electron, what is the > expected value, and what is the distribution?" > > Or "if I measure the value now, or in ten minutes, > will I get the same result?" Ah, I think the difference here is that others are probably using the term observable the way I would use it: an operator that commutes with the Hamiltonian.
From: mmeron on 29 Mar 2005 20:17 In article <GJk2e.16991$C7.9156(a)news-server.bigpond.net.au>, "Bill Hobba" <bhobba(a)rubbish.net.au> writes: > ><mmeron(a)cars3.uchicago.edu> wrote in message >news:ox12e.17$45.3415(a)news.uchicago.edu... >> In article <Rj12e.16030$C7.2138(a)news-server.bigpond.net.au>, "Bill Hobba" ><bhobba(a)rubbish.net.au> writes: >> > >> ><mmeron(a)cars3.uchicago.edu> wrote in message >> >news:yr02e.13$45.3299(a)news.uchicago.edu... >> >> In article <hT%1e.15978$C7.12491(a)news-server.bigpond.net.au>, "Bill >Hobba" >> ><bhobba(a)rubbish.net.au> writes: >> >> > >> >> ><mmeron(a)cars3.uchicago.edu> wrote in message >> >> >news:4q_1e.11$45.3008(a)news.uchicago.edu... >> >> >> In article <1112038910.784307.230090(a)o13g2000cwo.googlegroups.com>, >> >"PD" >> >> ><pdraper(a)yahoo.com> writes: >> >> >> > >> >> >> >Gregory L. Hansen wrote: >> >> >> >> In article ><1112035436.549412.84900(a)z14g2000cwz.googlegroups.com>, >> >> >> >> PD <pdraper(a)yahoo.com> wrote: >> >> >> >> > >> >> >> >> >TomGee wrote: >> >> >> >> >> Wormy, Bilge, PD, and all you other lemmings, >> >> >> >> >> >> >> >> >> >> you cannot understand that it is the _measurement_ of the >force >> >> >> >which >> >> >> >> >> is fictional and not the feeling of being pulled out as a >> >carousel >> >> >> >> >> spins. >> >> >> >> > >> >> >> >> >Nope. You feel the force pulling you *in*, which is a force you >are >> >> >> >> ... >> >> >> >> >Let me ask you another question. As an elevator suddenly starts >to >> >> >> >> >descend, you feel lighter. Are you in fact lighter? Why should >> >> >> >Earth's >> >> >> >> ... >> >> >> >> >Let me ask you another question. You are on a road making a >> >circular >> >> >> >> >bend left in your car. What force acting on the car enables you >to >> >> >> >> >> >> >> >> Reference frames, Paul. TomGee feels, TomGee is in, TomGee >turns... >> >> >> > >> >> >> >> Who's making the measurement? A hypothetical 2nd observer >> >stationary >> >> >> >with >> >> >> >> respect to the Earth, or TomGee? >> >> >> >> >> >> >> >> In those three situations, TomGee is in an accelerated frame. If >> >> >> >TomGee >> >> >> >> is pushed to the left against a wall, there's an inertial force >> >> >> >pushing >> >> >> >> him to the left. A 2nd, inertial observer might say no, TomGee >is >> >> >> >REALLY >> >> >> >> being pushed to the right, but so what? He's not in the car. >He's >> >> >> >just >> >> >> >> asserting that there's something special about his reference >frame >> >> >> >such >> >> >> >> that he can make valid observations but TomGee can't, and >pretending >> >> >> >> there's no valid way to transform between them. >> >> >> >> >> >> >> >> Centrifugal force is called a force because it acts like a force. >> >It >> >> >> >will >> >> >> >> cause something to accelerate relative to the stationary observer >> >> >> >who's >> >> >> >> already pinned against the wall. The stationary observer is, of >> >> >> >course, >> >> >> >> spinning madly about if viewed by the hypothetical 2nd observer >> >who's >> >> >> > >> >> >> >> at rest relative to the Earth, but so what? There's nothing >special >> >> >> >about >> >> >> >> the Earth frame, nothing wrong with the accelerated frame. If >> >TomGee >> >> >> >is >> >> >> >> pinned to the wall of a centrifuge, then TomGee is still at rest >> >with >> >> >> > >> >> >> >> respect to himself and he can define a reference frame from his >> >> >> >> perspective. >> >> >> >> -- >> >> >> > >> >> >> >Yeah, but I'm opposed to this kind of presentation, especially to >the >> >> >> >uneducated. I really believe that forces should be taught as being >> >> >> >characterized by interactions between two objects, and those >> >> >> >interactions should be traceable to one of the four (or less) >> >> >> >fundamental interactions, and that forces are the cause and >> >> >> >accelerations are the effect. >> >> >> > >> >> >> I don't see anything in the formulation of newton's laws saying that >> >> >> forces must be traceable to fundamental interactions. What should >be >> >> >> taught is that there are "physical forces" which are, indeed, a >> >> >> measure of interactions between objects, and there may be additional >> >> >> "inertial forces" which are an artifact of the choice of reference >> >> >> frame. But being an artifact, does not mean that they've no >> >> >> observable effects on motion relative to said reference frame. I >see >> >> >> no reason to treat it as some sort of a dangerous knowledge that >> >> >> should be kept away from the uninitiated, else their mind may >> >> >> explode:-) >> >> > >> >> >Which is just another reason force should be considered as a secondary >> >> >concept - the PLA is the primary concept. End of rant. >> >> > >> >> Oh, the PLA most certainly is ***the*** primary concept. So primary, >> >> in fact, that it serves as foundation not only for Newtonian mechanics >> >> but (with appropriate generalizations) for most of physics. No >> >> argument about it. >> >> >> >> This said, the fact remains that, for applying the PLA, a level of >> >> mathematical knowledge and sofistication is required which is way >> >> beyond the capabilities of a beginning (high school) physics student >> >> and, in fact, way beyond this that most people ever reach. So, we >> >> maintain forces as a crutch to be used till more is learned. That's >> >> really their remaining role. >> > >> >Most certainly. But if we are to use forces then I think we need a >> >discussion of what they really mean along the lines of what Feynman did >in >> >the lectures. In fact I consider that to be compulsory reading even for >> >grade 8 students (at least the chapters they have the mathematics to >> >understand - other chapters can be added as their mathematical knowledge >> >grows). Having understood what Feynman wrote I think a lot of confusion >can >> >be avoided. In my case I always wondered why a definition could be a >law. >> >It confused me for many years until I read a rather nifty old book on >> >classical mechanics. It clearly explained the real import of Newton's >Laws >> >was in his third law. That was the start of actually understanding what >was >> >happening. The full resolution came with Landua - Mechanics. I think it >> >would be great if students did not need to go through this process and >were >> >taught what was happening right form the start - at least as much of it >as >> >they can initially handle. >> > >> Well, here is the rub, how much they can initially handle. For most >> of them the answer is "not too much". Keep in mind that education is, >> by and large, a processes involving quite average students being >> taught by quite average teachers. > >True. > >> I've no doubt that if you'll pick >> the cream of the crop of high school students, top 1-2%, and hand them >> to a teacher of Feynman's caliber, you'll see some truly amazing >> results. Unfortunately, you cannot use this as a general purpose >> model:-( > >Hmmmmm. I think this is something we need to address in education. Perhaps >we need a model more aligned to each individual students requirements rather >than a class where the same information is given to every student. Well, this could work better, no doubt. The problem is, this requires yet more qualified teachers. An issue of, first, availability and, second, money to pay for it. > The current system encourages it to be dumbed down to even lower than what the >average student can understand - after all only 50% of students will be >average or above. Indeed. And when you get to the mode (common in the US) where a primary figure of merit for school is percentage of students graduating, then the tendency is simply to set the bar so low that everybody can pass. > I always rather liked what was done in the later years of >uni. We had class material worth a certain percentage of the exam and >assignments and projects worth so much. Perhaps we could encourage >interested students to see teachers with questions they could investigate as >part of optional assignments for extra credit? And yes my experience is >most students simply do not ask the type of questions I do so basing >circulum on the problems I had is not a good choice. First thing to realize is that most of us present here (this ng, I mean) are not a representative sample of the student population as a whole. Thus, our ideas of how the system should work have little to do with reality. By and large students are in school because they have to, not because they want to. >> >> That said, there is certainly room to analyze the way things are being >> taught. The natural tendency is to teach things in the historic order >> in which they were developed (an educational equivalent of "ontogeny >> recapitulates philogeny"). That may not necessarily be the best >> course of action. > >Having read the replies by yourself, Gregory and PD I relise in suggesting >that I had forgetten one of the things I emphasize in education. It must >actually be tested rather than be based on what I or others may think >belongs in a course. > Aye, exactly. Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: Ken S. Tucker on 29 Mar 2005 20:25
Tom Capizzi wrote: > I have heard that the evidence for glass flowing is not reliable. It doesn't > happen very quickly, so it originally came from examination of old glass. > The old glass was found to be thicker at one end. The assumption was > that it flowed. However, it was later shown to be an artifact of the way > glass used to made, by spinning large sheets of it. Has anyone else heard > similar reports? Yes, we dug up some old 1920-30 bottles that had surfaced in a bluff dump for the antique value, many where quite distorted but not broken. I also use *used* glass to repair windows and found it difficult to cut. The pro-glass cutter tells me it developes a variable surface over time, making the score difficult to do, as I've learned. I spoke to a pro-chemist and he confirmed glass to be a liquid. I guess that means it has viscosity. OTOH old telescope mirrors and lens seem to sustain good optical characteristics, but the glass may be of differing composition. Ken |