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What causes atomic perpetual motion?

  1. May 21, 2006 #1
    Hello,
    Just like in the topic, what causes electrons to move around the nucleus in perpetual motion (they never stop). For now, it's the only question. I'll have some more when I get the answer.

    p.s Actually, let me ask another one. A little bit unrelated. What causes electrons to move in a specific path?

    Thanks,
     
  2. jcsd
  3. May 21, 2006 #2

    mathman

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    For question one, there is no such thing as "friction" inside an atom, so the electrons don't lose any energy. (This is a much simplified answer, but it is the essence of what's happening.)

    For question two - electrons don't really have paths. They have energy levels. Levels can change only in discrete (quantum) jumps. An atom is NOT like the solar system.
     
  4. May 21, 2006 #3
    the electron in an atom has "kinetic" energy and "angular moentum", but it doesnt orbit the nucleus, infact its pretty confusing to call these things this way, because the electron is "smudged" around the nucleus (the pattern depends on the angular momentum and kinetic energy - both are discrete).

    the electron's minimal energy state - called a ground state is greater then zero, and thats why it cant "stop", in this state theres no angular momentum though.
    much like quantum harmonic oscillators (see zero point energy).

    hope i didn't confuse you too much... try looking at quantum models of hydrogen in java applets, it might help. (google it up)
    and have a look here: http://en.wikipedia.org/wiki/Hydrogen_atom
    and in particular the electron's "orbitals": http://en.wikipedia.org/wiki/Image:HAtomOrbitals.png
     
    Last edited: May 21, 2006
  5. May 21, 2006 #4

    Danger

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    Heartless, it might be helpful if you stop thinking of electrons as solid objects. In my case, at least, it works a lot better to consider them to be little clouds of wave functions that surround the nucleus.
     
  6. May 22, 2006 #5
    Both of these questions stem from some fundamental regrettable defects in our educational system that insists that teaching completely wrong things is supposed to have some educational value. The fact is, electrons don't 'move' in the ordinary sense of the word. They are not little negatively charged spheres moving in elliptical orbits around the nucleus.

    At this point, you must abandon all your notions of common sense and what physics 'should' be completely. Any attempt of retaining them will result in disappointment and a feeling that quantum mechanics is 'spooky' (fear not, even then you will be in good company). An electron is represented in quantum mechanics by a 'wavefunction', a function that assigns (at a given instant of time) a complex number to each point in space. This is analogous to representing a system by a point in its phase space in classical mechanics.

    According to the Born statistical interpretation, the relevance of this complex scalar field on space is that at any point of space, the square of the modulus of the wavefunction at that point gives the probability of finding the particle at that position upon performing a 'measurement'. This wavefunction changes with time, and its 'dynamics' is governed by a very famous equation known as the Schrodinger equation. This equation is the fundamental pillar of quantum mechanics in the same sense that Newton's second law is the pillar of classical mechanics.

    This equation can be used to show that an atom is stable, that is the electrons stay inside the atom. They do not move in a specific path, they do not even move. In quantum mechanics they are just wavefunctions undergoing unitary evolution within their separable Hilbert space and if you accept the Copenhagen interpretation that's all you need to know. The realm of the quantum is even further from common sense than relativity, though the mathematics tend to be a bit easier.

    Incidentally, I'm not sure if you understand this, but perpetual motion is the natural state of any particle. The question to ask usually is why is that NOT in perpetual motion. Inertia of motion as derived from Newton's First Law tells us that if no forces are acting on a body then it's possible to find a family of reference frames such that the body undergoes no acceleration (stays in perpetual motion). Hope this helps.

    Molu
     
  7. May 22, 2006 #6

    rcgldr

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    If electrons aren't particles, then how can they collide with solids as in the case of high speed accelerators? I was under the impression that there was significant mass (or at least energy) to electrons at very high velocity, as in the case of linear accelerators.
     
  8. May 22, 2006 #7

    Mk

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    Parteekle-wave duality! An electron is both a particle, and a wave: both a discrete little grain of sand, and an ink smudge or cloud.

    I think loom91 said it best! Good job loom!

    Like mathman said, and Danger (Socraticly :wink:) said, as well as loom91, the electrons don't move in a specific path, they are actually more of a cloud with different energy levels. The different energy levels are the simplification you have seen, as turned into orbits (well, they are called 'orbitals'). In high school chemistry, they grace over truer-visually molecular structure (less than one class period usually), and connect it to why the angles between bonds are how they are. Excess unbonded electrons pile up at the top part and repel the Z and Y atoms away: why they are bent and triangular and dodecahedral instead of just chains.

    ....X
    .../..\
    .Z.....Y
     
    Last edited: May 22, 2006
  9. May 22, 2006 #8
    I like to think of an electron as a place with properties, not a particle. I don't like particles. Little ball bearings, bah. But a place can be nice and fuzzy, with no silly edges or surfaces or pointy bits. So it's got no defined size, or particular point where's it's at. It's very easy to get a rough concept of this. Just try pointing at the USA.
     
  10. May 22, 2006 #9
    I think the best description of quantum mechanics in layman's terms is from the master himself.

    -
    R.P.Feynman

    A part of this appears as a short clip in the documentary No Ordinary Genius.
     
  11. May 23, 2006 #10

    rcgldr

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    They do move in a specific path in accelerators, and as individual instances.

    My assumption is no one truly understands the nature of electrons yet.
     
  12. May 23, 2006 #11
    I don't know your educational level, but high-school physics/chemistry covers topics such as quantization of dynamical variables, electron orbitals, wave-particle duality, time-independent Schroedinger equation, probability density of the electron in a H atom, linear superposition of wavefunctions, 'shapes' of orbitals etc. Your textbooks may be a good place to get some idea about the topics. Free electrons (electrons that are not bound to a nucleus) are considerably less quantum an are sometimes well-approximated by classical concepts.

    But even then, they do not move in a specific path. I'll repeat this in case it is not clear, electrons do NOT move in a specific path UNDER ANY CIRCUMSTANCES, never. This is a misconception you have to correct now. The electron's mass is small enough for the HUP to be non-negligible, and they never ever have a well-defined trajectory in the classical sense. I don't know who teaches you this sort of rubbish. Even in particle accelerators, there's always uncertainty in their position and velocities, though probably less than inside an atom.

    And I'm speculating here, but the relativistic speeds in an accelerator will introduce additional complications and we must examine Dirac's equation for the relativistic electron.

    It is quite true that no one truly understands the nature of electrons yet, but not in the sense you mean it.
     
    Last edited: May 23, 2006
  13. May 23, 2006 #12

    ZapperZ

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    Please note that in particle accelerators, the trajectory and description of particle motion is often classical. You are welcome to look at the popular codes that are used to describe such trajectories such as PIC codes and PARMELA.

    Does this mean we can treat all electrons in all cases as classical particles? Nope! We have clearly seen why this doesn't work in atomic description, tunneliing motion, etc. However, does that also means we can never, ever use classical description either? Nope! There are plenty of cases where the classical picture of electrons DO WORK, and in fact, is more convenient. Example? Particle accelerators, Drude model, and Boltzmann transport equation.

    This doesn't mean that the QM description fails in those cases. Just like in the Newtonian physics case for v << c, at some point, the classical description is more convenient when the right boundary conditions come up, when there's no significant overlap between the particles/electrons, etc.

    Zz.
     
  14. May 23, 2006 #13

    Mk

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    And hey, we're talking about in the ideal atom.
     
  15. May 23, 2006 #14

    LeonhardEuler

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    The wavefunction of the electron describes the probability that it is in a particular location. When the electron is bound to the nucleus of an atom and you measure position relative to the nucleus, the probability that an electron is at a given position does not vary with time. So, in a sense, it is not really moving at all. However, if you were to measure its position twice, you would likely find it in different positions each time. You might consider this "motion" in the sense that the particle has changed position. But this still does not show that the electron is moving around the nucleus when you are not measuring it because you must disrupt the wavefunction in order to carry out a measurement. It is best not to think of the electron as having a position before you measure it. It does have an "expected" position, or the average position in a probabalistic sense. This position does not vary with time in this case.
     
  16. May 24, 2006 #15

    rcgldr

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    Old man here, graduated high school back in 1970, only one semester of physics, and it was mostly mechanical. College degree in computer science, took calculus, differential equations, and linear algebra, but only one year of physics. So I rely on web sites and forums like this one to improve my education. In my work my main math speciaty is Reed-Solomon (Galios) type finite field math as used for error correction codes, I've done some curve fitting and data compression work as well.

    Ok, near specific path. The acclerators are programmed in advance to toggle polarity to accelerate electrons that have entered the proper timing window to be accelerated. The speeds are very close to c at SLAC.

    Has anyone come up with a good explantion while electrons can be fired one at a time at a double slit and end up with an interference pattern as if multiple electrons with wavelike properties were fired at the same time? This is going beyond the wavelike properties of light. Is there any reason to believe that the difference could be due to the fired electron interacting with the electrons that compose the double slit?
     
  17. May 24, 2006 #16
    Heh, for some unknown reason I was sure you were a high-school student :D Sorry.

    You mean you can choose how much you study a subject in High School? We don't get to do that till we are out of school and in college. Once we choose 4 subjects to study at 11th and 12th Grade, we have to study the whole syllabus of all the subjects. So everyone who takes Physics and Chemistry as subjects has to study the basics of Quantum mechanics (there's a little in both subjects, though more in Chemistry), but there's remarkably little wave mechanics.

    Just the theory of time-independent Schrodinger equation and other basic quantum phenomena such as quantization and HUP in 11th Grade, and linear superposition and radial probability densities in 12th Grade. But everyone has to study this minimum, we can't read more of Math by reading less of Physics. We probably won't be able to do that even in College here, our curriculum are not very flexible.

    As I said, the electrons are more classical when free. We would also have to examine Dirac's equation (which is the most elegant and beautiful theory in Physics I've seen, though I've not seen much yet) at velocities close to c, though I'm not sure whether that would make the electron more better or worse approximated by classical models.

    I believe the likes of Louis De Broglie and Erwin Schroedinger came up with very good explanations in the first quarter of the last century. As far as I know, firing one electron does not give rise to interference patterns, it gives rise to only one dot as would be expected of a particle. Only when many are fired do their combined impression form interference patterns, leading us to believe that the electron has the characteristics of a matter wave.

    I don't think the theory you advance is plausible. It's considered a much more fundamental phenomena rather than experimental errors as you suggest. No physicist worth his salt would claim the wave-particle duality to be false after a century of evidence to support it.
     
  18. May 24, 2006 #17

    rcgldr

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    It was common in private high schools, but I only had one semester in private junior high school, when my parents got divorced and ended back in public schools in west Los Angeles. Although it was rare in the late 1960's to have majors in public high shools in California, my high school was part of a group of high schools that did have majors, and longer breaks between pairs of classes so that kids could get bussed to a school that specialized in a particular subject. I just happened to be at the school that had physics, calculus (there were only 11 students out of 2500+ in that first calculus class), and a small computer (IBM 1130). I ended up more interested in programming than math, but kept my math major. As mentioned, I graduated during summer 1970. I consider myself lucky to have ended up at this high school.

    I meant firing one electron at a time, not just one. The issue is that you end up with interference (like) patterns, even though the electrons are fired one a time, so that they do not interfere with each other.

    So inspite of the fact that the fired electrons do not interact, you end up with an interference patttern similar (or the same?) as if they were interacting as wave interference. I've seen a few web sites that state that there isn't a good explanation for this, but these may be out dated.
     
  19. May 24, 2006 #18

    ZapperZ

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    This is now degenerating into a double-slit type of a discussion. Please read this thread, for example:

    https://www.physicsforums.com/showthread.php?t=108424

    There are many others. In all of these discussion, please note that the interference effects that we know and love is NOT due to one particle "interfering" with another. In the standard CI formulation, it is the SAME particle interfering with itself [I dislike that pedestrian description but that's what I'll go with].

    Zz.
     
  20. May 24, 2006 #19

    rcgldr

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    Sorry, didn't mean to stray off-topic, only mentioned the double slit issue to show that electrons behave "wierdly".
     
  21. May 24, 2006 #20

    ZapperZ

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    But why pick on electrons? Photons, neutrons, protons, buckyballs, etc... all show the identical behavior.

    Zz.
     
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