What keeps electrons spinning around their nucleus?
in [Relativity]
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From: jmfbahciv on 30 Mar 2005 06:10 In article <07n2e.41$45.5359(a)news.uchicago.edu>, mmeron(a)cars3.uchicago.edu wrote: >In article <GJk2e.16991$C7.9156(a)news-server.bigpond.net.au>, "Bill Hobba" <bhobba(a)rubbish.net.au> writes: <snip> >>> 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. The Mass. board of education is now trying to add science as part of the high school graduation requirement. The year proposed was 2008 but news broadcasts are now saying it can't happen until 2010. Listen for the whinging from teachers' unions. One of them is already running ads that they can't teach science because there is only 1 microscope/ 10 students. That's an awfully high ratio even though the ads want you to think that it's too low. When I went to high school, I estimate the ration to be 1/100. /BAH Subtract a hundred and four for e-mail.
From: bz on 30 Mar 2005 07:46 dubious(a)radioactivex.lebesque-al.net (Bilge) wrote in news:slrnd4l3i1.6h5.dubious(a)radioactivex.lebesque-al.net: > Tom Capizzi: > > >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? > > Even if true, I'm not sure that it's relevant. Lots of materials > flow under pressure. For example, the copper gaskets used for > conflat flanges, which are sort the ultimate in vacuum seals, depend > upon the copper to cold flow when you tighten the bolts to acheive a > vacuum seal, but no one treats copper as a liquid. I imagine that > anything would flow under its own weight if the piece of material > had the right dimensions, i.e., long, not very wide, thin and then > stood on its short edge. Many things break, rather than flow, under tension. Many things crumble, rather than flow, under pressure. Some things flow under pressure but not under tension. Some things are much stronger under pressure than tension and do flow. glass may be one of those. It is stronger under pressure than tension. Some things are much stronger under pressure than tension and do not flow. concrete is an example of such a substance. -- bz please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. bz+sp(a)ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
From: PD on 30 Mar 2005 09:02 Bill Hobba wrote: [snip] > > 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. > > Thanks > Bill > Feynman is one of my heroes, as well, and note that he was fond of making sure he could explain things to his grandmother before claiming to understand them. His little book on QED is an example of mastery of this. I absolutely agree with you that, unlike high school and the No-Child-Left-Behind Act, university courses have no mandate to cater to the lowest common denominator. On the other hand, undergraduate courses should not be, IMHO, purely a filter to weed out all but the best, either. The goal of any educational system is to train first, discern second. Ironically, high school classes with a student/teacher ratio of 20:1 or 25:1 are much better suited to individualized teaching than a university class section with a student/teacher ratio of 75:1 or 300:1. The place where real pushing on an individual basis occurs is graduate study -- I recall from my own experience what a different world it was, suddenly, both enormously demanding and also enormously more educational. IMHO, way, way too many students go to college these days. Like a factor of 10 to 20 too many. PD
From: PD on 30 Mar 2005 09:31 David Cross wrote: > On Wed, 30 Mar 2005 03:45:34 GMT, "Bill Hobba" <bhobba(a)rubbish.net.au> wrote: > > >"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. > > What had happened for me was that we were learning about the forces exerted by > magnetic fields, and the professor, although he didn't have to, taught the > cross-product form of the F = qvB sin theta equation in order to get us to > understand the physical significance of the perpendicularity of the force to > the velocity and field vectors. > > As a result, when I came across r x F (that is, rF sin theta), I didn't have > to be sidetracked by rather strained explanations for explaining why the units > of torque are Newton-meters and why the torque was perpendicular to the force > and the radial direction. I just had to skip past all that and realize that > the physical significance is the same as that for the qv x B :) > > Having seen the cross product, it was then child's play to understand the dot > product, since the two just express different kinds of directional character > to the physical quantities we deal with in mechanics and electromagnetism > (although I realize this is lazy speaking when it comes to the dot product > since work is a scalar; nonetheless, appreciating that the directional > character of the force plays a role in how much work is done is crucial to > special cases in mechanics, such as circular motion. :) ). > > As for force/torque diagrams, I find it helps a lot to have a good physical > intuition. That has to be developed; I didn't really start to grasp force > diagrams until I did many problems involving friction, statics, net forces, > what-have-you. But once you grasp them, it's a short step to automatically > using your right hand to work out which way the torque is for conditions where > the net torque needs to be zero, or when you know it's rotational mechanics. > :-) > > --- > David Cross > dcross1 AT shaw DOT ca My own experience with *learning* the value of the cross-product didn't actually hit home until I was learning curl and all those vector identities. That should tell you about the lag in understanding. Up until that point, I thought there was little value to the cross-product other than short-hand for the right-hand rule and sin(theta). However, in *teaching*, I learned the magical illuminating power of precession. I filled a bicycle tire with birdshot and mounted it back on the wheel, and extended the axle so that I could affix attachment points. This way, I could suspend one end of the axle by a rope. We would talk about torque due to gravity on the wheel's center of mass, about the pivot point at one end of the axle, and I would let the motionless wheel drop, and I could see in their eyes "short hand for right-hand rule and sin(theta) -- big deal". And then I would spin up the wheel with a modified shoe polisher, and we would all point in the direction of the wheel's angular momentum, and then point to the board where T = dL/dt was written, and *predict* what would be different in this case before I let go. When I finally released the other end of the axle and let it precess, I could look out into the classroom and see palpable mental victory and sudden, profound appreciation for the cross product. They could *see* the vectors extending from the wheel as it hung there. (We would then slow the wheel to change the rate of precession, or change the spin direction of the wheel, just to pound in the point, but by that time, they were way ahead of me.) PD
From: PD on 30 Mar 2005 09:33
TomGee wrote: > Tom Capizzi wrote: > > "TomGee" <lvlus(a)hotmail.com> wrote in message > > > SNIP > > > > > Is everyone here so full of knowledge that they think the basics no > > > longer apply? > > > > > > > You don't recognize the basics when you see them. Centripetal force > is > > a legitimate force, centrifugal force is not. > > > > > Wrong, Cap, it is you who doesn't recognize the basics staring you in > the face. Both are legitimate forces; it depends on which frame you > happen to be in. > > > > > > By the way, centripetal > > acceleration is not a force, it is an acceleration caused by > centripetal > > force. > > When the string breaks the orbiting mass follows a straight line away > from > > the center of rotation. It doesn't take a force to "make" the mass > flee the > > center. It does take a force to restrain the mass from departing. > > > > > If you were to remove that force, would the mass just stop moving? If > not, why not? Answer: Law 1. The natural tendency of a moving mass > is to continue moving following a straight path at a constant speed > whenever there are no external forces acting upon it to cause it to > accelerate (or words to that effect). The inertia of a mass works > against any external forces working to accelerate it Close enough up to here. > and that inertia > is called an inertial force. Pure basics! And then you blow it. > > Centrifugal force is what causes a mass to flee from a centripetal > force from the viewpoint of a rotating reference frame, which is no > less valid than a non-rotating frame since, under strict definitions > like those you impose on centrifugal force, there is no such thing. > > TomGee > > > TomGee > > > |