What keeps electrons spinning around their nucleus?
in [Relativity]
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From: Gregory L. Hansen on 29 Mar 2005 21:22 In article <bJl2e.50463$u76.37118(a)trndny08>, Tom Capizzi <etianshrdlu(a)verizon.net> wrote: > >"Gregory L. Hansen" <glhansen(a)steel.ucs.indiana.edu> wrote in message >news:d2bom8$ldk$2(a)rainier.uits.indiana.edu... >> In article <RD52e.20$45.3912(a)news.uchicago.edu>, >> <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: >> >>>>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":-)). >> >> I think exactly that every time I see "glass is a liquid" come floating >> around. Glasses tend to be harder, stiffer, and more creep resistant >> than most crystalline solids, and without the redeeming quality of >> (eventually) dripping through a funnel. But long after the words "solid" >> and "liquid" were defined based on bulk properties, atoms were discovered >> and then it was noticed that glass isn't crystalline, and ever since, >> weenie egghead wanna-be's have been spouting "Glass is a liquid, what you >> thought was true is wrong, I'm smarter than you are, nah nah nananah!" >> Thereby sharing with people the factoid that glassy materials lack >> long-range order, but otherwise mystifying the process of science. >> > >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? I've heard of it, I don't know the history of it, but the float glass process is not yet 50 years old. Float glass can be made remarkably smooth and uniform. With previous methods involving beating or spinning, I'd imagine uniformity was hard to achieve. Glass can creep or sag. That's not the same as flow, lots of things can creep or sag. Glass could have internal stresses. On the other hand, Uncle Al has mentioned glass shards in Egyptian tombs are still sharp. If it's going to flow, those edges would have blunted in a few thousand years! "Solid" and "liquid" are practical terms-- liquids flow and solids retain their shape. And glass does a remarkably good job at retaining its shape. Liquids don't have a meaningful rating on the Moh's hardness scale. For all that, "glass" isn't a particular material, it's a microstructure of a material. Metals can be glassy-- and they're harder and stiffer than the crystalline counterpart (Google on "liquid metal"). Plastics can have varying degrees of crystallinity or amorphousness. I'm sure I've read about specialty glasses that do flow. Silicon dioxide can be quartz or flint, or just plain window glass. -- "When the fool walks through the street, in his lack of understanding he calls everything foolish." -- Ecclesiastes 10:3, New American Bible
From: Gregory L. Hansen on 29 Mar 2005 21:32 In article <Ndl2e.17030$C7.11444(a)news-server.bigpond.net.au>, Bill Hobba <bhobba(a)rubbish.net.au> wrote: > >"PD" <pdraper(a)yahoo.com> wrote in message >news:1112120245.503240.33900(a)g14g2000cwa.googlegroups.com... >> IMHO, the number-one problem students have in physics is making a >> connection between physics and reality. You can appeal to them about >> what they've all seen about the flight of a baseball, and then you ask >> them to draw the trajectory of a fly ball in a physics problem and you >> get straight lines, you get trajectories with corners, you get crazy >> stuff. And I've *asked* them, "You've been to baseball games, right? >> Does this look like what you see?" And they'll look at me blankly and >> say, "But this doesn't have to look real. It's physics." That's a heck of an observation. >> >> The number-two problem is ill-formed and overlapping concepts, like >> distinguishing displacement, acceleration, and velocity in the >> catch-all "to go", or the distinction between force, momentum, kinetic >> energy, and power. Teaching students to be careful and precise in their >> definitions is asking a lot of them. >> >> The number-three problem is pre-existing misconceptions, like thinking >> that a bullet falls to the ground because it is slowing in flight. >> >> None of these would be addressed by pulling out a Power Tool, making a >> Tim-Allen guttural grunt, and saying, "Now THIS is REAL physics!" IMHO, >> we do our students a service by teaching them to >> * check against their intuition >> * recast their intuitive ideas with carefully defined terms >> * recognize places where the edges of their intuition are a little >> fuzzy >> * be rigorous in making explicit predictions they can check with >> observation, and thereby incorporate those observations into an >> expanded intuition. >> To me, this is teaching them to think like a physicist. > >PD you have experience in teaching students physics - I do not (limited to >teaching some math). So I must say your observations hold much greater >weight than mine. But perhaps as I suggested in another post there may be >some value in tailoring material to the requirements of each student? After >all when a student does have questions like I did and they get no >satisfactory answer are we not doing a disservice to that student? Of >course the problem is how can we accommodate these students with limited >teaching resources. Perhaps we need more teachers like Jaime Escalante >http://www.govtech.net/magazine/visions/feb98vision/escalante.php >He does not believe in separating students out - he believes in challenging >them. But I am biased - along with Feynman he is one of my heroes. They might be directed to sci.physics if they have the psychological fortitude to face accusations of crackpot and troll, and web pictures of people with their heads up their butts. -- "In any case, don't stress too much--cortisol inhibits muscular hypertrophy. " -- Eric Dodd
From: Bill Hobba on 29 Mar 2005 22:45 "David Cross" <spamdenied(a)nospam.net> wrote in message news:co0j41dgr2ekm7opfiej4qqt9h6b5nhv4k(a)4ax.com... > On Tue, 29 Mar 05 14:12:53 GMT, jmfbahciv(a)aol.com wrote: > >I always have problems drawing that force diagram. The arrows > >never "matched" my common sense. And torque always confused me > >and, as a result, I always reverted to "memorized" formulas > >whenever I did those problems. > > I think I was lucky; I learned about torques after I'd learned how to apply > cross products. :) Therefore all I needed was the radial distance away from > the center and the applied force. :) Having studied physics after completing a math degree I had the same experience. Trouble is I ran into the problem Feynman alluded to - the math can hide the physics. Thanks Bill > > --- > David Cross > dcross1 AT shaw DOT ca
From: George Jones on 29 Mar 2005 23:13 puppet_sock(a)hotmail.com wrote: > 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? > Socks I don't know what you mean by this. In quantum theory, observables are modeled by self-adjoint operators on the Hilbert space of quantum states. The observable X that represents position is defined by (X psi)(x, t) = x*psi(x, t) for all states psi(x, t) in its domain. How (in the Schrodinger picture) is the time derivative of X taken? From your other post, I think you mean something like the following. The oservable that represents momentum is P = -i*hbar*d/dx, and the Hamiltonian is H = P^2/(2*m) + V(x). If A is any operator, the time dependence of its expectation value <A> := <psi|A|psi> is given by Ehrenfest's theorem: d<A>/dt = 1/(i*hbar)*<[A, H]> + <@A/@t>. When A = X, [X, H] = i*hbar*P/m and @X/@t = 0 (since X is not explicitly time dependent). This gives the suggestive (because its similar to the classical case) d<X>/dt = <P>/m. However, neither d<X>/dt nor <P>/m are "observables" in the standard usage of the term. P/m is an observable, but I don't how often it is called the velocity observable. Regards, George
From: mmeron on 30 Mar 2005 00:57
In article <lrm2e.17108$C7.9311(a)news-server.bigpond.net.au>, "Bill Hobba" <bhobba(a)rubbish.net.au> writes: >> Seeing how Landau wasn't above >> mentioning both centrifugal and coriolis forces in his Mechanics, is >> see no reason for purism here. > >He sure did. But notice he makes it clear they are the result of using non >inertial frames to write the lagrangian in. > Did I (or anybody else here) said otherwise? Mati Meron | "When you argue with a fool, meron(a)cars.uchicago.edu | chances are he is doing just the same" |