What keeps electrons spinning around their nucleus?
in [Relativity]
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From: PD on 29 Mar 2005 14:43 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. PD > > >Should we skip the chapter on Newtonian gravitation in favor of the > >equivalence principle? > > Good point. > > Mati Meron | "When you argue with a fool, > meron(a)cars.uchicago.edu | chances are he is doing just the same"
From: PD on 29 Mar 2005 14:52 Harry wrote: > <mmeron(a)cars3.uchicago.edu> wrote in message > news:7K52e.21$45.3808(a)news.uchicago.edu... > > In article <Xns9627C5AEB62D6WQAHBGMXSZHVspammote(a)130.39.198.139>, bz > <bz+sp(a)ch100-5.chem.lsu.edu> writes: > > >mmeron(a)cars3.uchicago.edu wrote in > > >news:8B12e.18$45.3391(a)news.uchicago.edu: > > > > > >> In article <Mo12e.16031$C7.902(a)news-server.bigpond.net.au>, "Bill > Hobba" > > >> <bhobba(a)rubbish.net.au> writes: > > >>> > > >>><mmeron(a)cars3.uchicago.edu> wrote in message > > >>>news:XQ02e.15$45.3352(a)news.uchicago.edu... > > >>>> >coordinates, but I think what is misleading is to call the > correction > > >>>> >terms "forces". > > >>>> > > >>>> Only if you attach more meaning to the term "force" than it deserves. > > >>> > > >>>Ahhhhhh. Yes. As Feynman says it is half a law. It gains its full > > >>>meaning when combined with other laws and/or concepts such as Coulombs > > >>>law or the introduction of non inertial reference frames. > > >>> > > >> Yes, it is a rather complex issue. I wrote some stuff about it in the > > >> past, here, but I never kept a copy. But it certtainly needs some > > >> sort of broad framework, to make sense. > > > > > >Especially when someone keeps insisting that force is always the result > of > > >acceleration > > > > Cause, not result. > > > > > and that without acceleration (as for example when a gyro > > >precesses at a constant rate, or when a mass moves at a constant velocity > > >because it is overcoming drag or friction) there is no force and no work. > > > > > Where there is net force, there is acceleration. The F in Newton's > > law is the total (i.e.) net force acting. Since forces are vectors, > > it is perfectly possible to have different non-zero forces to sum up > > to a zero net force. > > > > If I put your finger in a vise and squeeze, your finger is being acted > > upon by two forces, equal and opposite. The net force is zero and > > your finger is going nowhere. Which by no means mean that since the > > net is zero, there are no observable (or, for that matter, audible, > > i.e. loud screams) effects present. > > Right. Active forces (or how to call them, I think Newton called them > impressed forces) cause acceleration. But a centrifugal force is a reaction > force Ack! Not in Newton's sense of action and reaction. > to a centripetal force, which is caused by a change in direction. Ack! Confusing cause and effect. The force causes the change in direction, not the other way around. > I > wouldn't describe such as "pseudo force" or "fictional" force, as that gives > the wrong impression that no real force is exerted. Again, if you consider it carefully, it's not that complicated. If you are rear-ended in your car, the reason your neck gets hurt is because your body accelerates forward and your head does not. There is no need to assert a force exerting backwards on your head to account for this (although it would appear that there was one in a frame that accelerates forward in impact). PD > In reality such forces > are real enough to break your neck! Newton also didn't call them "pseudo" or > "fictional", AFAIK; he just called them innate/inertial forces. > > Harald
From: PD on 29 Mar 2005 14:54 TomGee wrote: > Yes, PD, but none have anything to do with our subject specifically nor > do they support your claims. > TomGee Yes, what? I don't know what you're responding to. You snipped all of my questions to you. PD
From: TomGee on 29 Mar 2005 14:58 Well, Greg, even PD admits that the centripetal acceleration is a force, but he does not know the bases of such forces. In the case of the electron, it is the em force and in the case of his examples, it is the gravitational force. Obviously, if we decide to not call centrifugal force a force any longer, the other should not be called one either. The centripetal force is imposed as an inward pull while centrifugal force is a function of inertia, which is that tendency of a body to continue unaccelerated when no net force is acting upon it. The inertia of a body is what keeps it wanting to leave its orbit but the pull of gravity keeps it from doing that. He has denied that em is the force involved in an atom which binds the electron into orbitals. He was taught that both centripetal force and centrifugal force exist, but now he lemmingly goes over the cliff by discarding centrifugal force but retaining centripetal force without knowing why. Wormy is doing the same, of course, as a Chief Lemming. Is everyone here so full of knowledge that they think the basics no longer apply? TomGee
From: Gregory L. Hansen on 29 Mar 2005 15:35
In article <d2c4nm01uvu(a)drn.newsguy.com>, Daryl McCullough <stevendaryl3016(a)yahoo.com> wrote: >Gregory L. Hansen says... > >>Daryl McCullough <stevendaryl3016(a)yahoo.com> wrote: > >>>I guess whether or not the ball accelerates when you let go of it >>>depends on how you define "acceleration". Here's the way I see >>>things, mathematically: Letting V be the velocity vector, and >>>A be the acceleration vector, we have: (where e_k = the kth basis >>>vector) >>> >>> V = V^k e_k >>> A = (d/dt V^k) e_k + V^k (d/dt e_k) >>> = (d/dt V^k) e_k + V^i (d/dt e_i) (changing the dummy index k to i) >>> >>>Now, we can rewrite d/dt e_i as follows: >>> >>> d/dt e_i = dx^j/dt d/dx^j e_i >>> = V^j (d/dx^j e_i) >>> = V^j G^k_ji e_k >>> >>>where the matrix G^k_ji is defined via >>> >>> d/dx^j e_i = G^k_ji e_k >>> >>>Putting this altogether, >>> >>> A = (d/dt V^k) e_k + V^i V^j G^k_ji e_k >>> >>>Or in component form, >>> >>> A^k = (d/dt V^k) + V^i V^j G^k_ji >>> >>>If you release a ball while sitting on a spinning carousel, >>>the first term (d/dt V^k) is nonzero, but the second term >>>V^i V^j G^k_ji exactly cancels it, to give zero acceleration. >> >>The way I see that, you have declared "There shall be no mechanics >>performed in an accelerated frame!" > >Why do you say that? I'm telling you *how* to do mechanics >in an accelerated frame. There is no way to do it without >connection coefficients. The question is whether you call them >"force terms" or "acceleration terms". > >>What makes you think the e_i have a time dependence? > >Well, in order to compare vectors at different times, you >have to have some operational way to do parallel transport. >For spatial vectors, you can use gyroscopes: Set the gyroscope >into motion pointing along a particular vector A at time t1. >At a later time, t2, you compare A at time t2 to the direction >that the gyroscope is pointing. And then declare that it's not forces, but time-dependent basis vectors, that make the gyroscope turn? What would we say about a gyroscope precessing under the influence of gravity? > >>On a spinning carousel, if I define e_1 by a line >>extending from the center to the black horse, and >>e_2 from the black horse to the red one just ahead, >>then they're not going to change. > >The coordinates of the black horse don't >change with time, but the basis vectors do. The basis vectors change with time WITH RESPECT TO THE BASIS VECTORS OF THE INERTIAL FRAME. That's picking the inertial frame out as a special frame, and referring all mechanics done in the accelerated frame to the inertial frame. It requires the accelerated observer to have an inertial frame to reference. Or, if no inertial frame is observable (e.g. an observer on a spinning planet with a cloudy sky), it requires the observer with a Foucalt pendulum or a cannon to construct a hypothetical frame that makes the inertial forces disappear, and then declare the hypothetical frame to be the one that should really be used. And that program has been followed before, in the general theory of relativity. Except in that theory there's no such thing as an inertial frame except as an approximation in a limited region. -- "The result of this experiment was inconclusive, so we had to use statistics." (Overheard at international physics conference) |