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Origin/structure of wave forms?

  1. Nov 13, 2003 #1
    We are about ¾ way through this “Hewitt” textbook in “Conceptual Physics”. Chapter 19 (Vibrations and Waves) explains that that there are two type waves shapes. These are transverse waves, and longitudinal waves. It seems to me that something either doesn’t add up, or is being left out. The book describes-

    The longitudinal wave (actual point-to-point locomotion) is produce by a compression/rarefaction of whatever particulate medium the wave is transmitted in. This is a one-dimensional linear plane (movement-to-180o reverse or opposite movement). An in/out or push/pull action creates this wave. Is this correct?

    The transverse wave (actual point-to-point locomotion) is produce by a Translational (transverse) movement from either high-to-low, side-to-side, right-to-left, or any other type movement-to-180o reverse or opposite movement on a transverse plane thru a medium that is transmitting the wave. Is this correct?

    Question #1-
    Is there a possibility that there is such a thing a Spherical Wave Motion?
    What causes me to ask this is, all matter that we are aware of here on earth, is made of some sort of orbital/spherical atomic particle. Is this correct? If so, the expanding then collapsing of the electronic orbits would produce a multi-dimensional (spherical) expanding/collapsing wave-form. If this is true, why is it not mention more commonly?



    Question #2-
    Is there a possibility that there is such a thing a Rotational Wave Motion?
    What causes me to ask this is, the example in the book show the shaking of one end of a rope while the other end is attached to a fixed point to demonstrate a transverse wave. This wave is produced only on a one-dimensional axis that appears to be able to be produced in any of the 2pie vectors. If the rope were to be spun in an orbital circle (like a jump rope normally is), wouldn’t this produce a wave form that at any visual point of view, look like both a frequency modulated and a amplitude modulated wave but is originated in a totally different manner than all others?

    I have posted question before on the class (internet forum) web page, just to either not get no response, or get one that reflects a “who cares attitude”.

    Anyone have a thought about this?

    Origin/structure of wave forms- barry.
     
  2. jcsd
  3. Nov 13, 2003 #2

    krab

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    You have reason to suspect your text of giving false info? Or are you asking whether your paraphrase of what you find in the text is correct? Anyway, looks right to me.
    I think you are confusing to different concepts. Wavefront shape, and wave type. If you stand in an empty field and shout, you are sending out spherical sound waves that are longitudinal. A radio transmitter also sends out spherical waves. These are electro-magnetic waves and so are transverse.
    What you describe is a 2D transverse wave. It can be thought of as composed of two motions x and y at 90 degrees to each other, but also 90 degrees out of phase. For electromagnetic (light) waves, this is called circularly polarized.
     
  4. Nov 13, 2003 #3

    turin

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    As far as I know.




    I'm having trouble understanding what you're saying. The wave has a direction of travel (through space). There are other directions (in space) perpendicular to the direction of travel. If the wave is the propogation of disturbances which are perpendicular to their direction of travel, then it is transverse.




    I don't know. It is an interesting thought. The wave would be a periodic variation in the volume of the spheres in space and time?




    In the context of the atomic scale, I don't think it would be feasible, since the atoms aren't really spheres (at least, not all of the states of the electron configuration are spherical).




    Yes. It is classified as spherical polarization. It is as you describe. There is also more generally elliptical polarization. For this, the amplitude would be different depending on the side you chose to view it from. Then, there is even a more general type than that, but I don't know what it's called. It is characterized by the variations in different directions occuring at different frequencies. All this can also be superposed with the longitudinal wave idea.
     
    Last edited: Nov 14, 2003
  5. Nov 13, 2003 #4
    Wouldn’t the waves forms (from both the above) radiate
    in all (untill reflected) directions so as to produce a
    3D outward transmission?
     
  6. Nov 13, 2003 #5

    selfAdjoint

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    There is a rotational propagation in certain materials. Take a long thin, fairly stiff band. Then you can put a twist in it and the twist will propagete down the band. Back in the 1960s, before microcircuits, one of the desk calculators had a memory based on this principle. rotational waves representing bits were sent down the bar and reflected back and forth until they eventually decayed, which hopefully wsn't until you finished your calculation.
     
  7. Nov 13, 2003 #6
    Is spherically symmetric radiation of [lamb]>>L (where L is the characteristic diameter of the radiator) practical? Brings to mind these little subwoofers made by Sunfire and other such companies.
     
  8. Nov 13, 2003 #7

    krab

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    Yes.

    Edit: OK. I guess that was a little terse. I'm guessing you have a problem with the fact that a 1D wave occupies 3D space? The 1D only refers to the dimension in which it varies. So consider spherical coordinates: radius, latitude and longitude. The longitudinal sound wave only varies with r; not with latitude or longitude. So even though it has spherical waves in 3D space, it is a 1D wave.
     
    Last edited: Nov 13, 2003
  9. Nov 13, 2003 #8

    krab

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    Easiest to think of this in a water wave analogy. Hold your finger in a large pool of water, and wiggle it up and down. You send out waves on the surface whose frequency depends upon how quickly you move your finger up and down. But now notice as you go slower and slower, the waves get lower in frequency, but also smaller. This is because as you note, if [lamb]>>L, the coupling is poor. (Going back to sound waves,) Speaker manufacturers compensate by boosting the low end (electronically, or by putting an acoustic resonance there). But you need speakers with really long cone travel.
     
  10. Nov 14, 2003 #9

    turin

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    It depends on the context. Since we're talking about the different modes of disturbance, then it is appropriate to include the source in our consideration. But, if you just want to take for granted the fact that you have a particular kind of wave, then you usually model it as a plane wave (these are a lot easier to deal with).
     
  11. Nov 14, 2003 #10
    I think I am beginning to understand. Can one say correctly that the topic here is one that will be investigated in a higher-level course? The course I’m in is “Conceptual Physics-101”. This seems to be addressing more of an elementary concept. I’ve answered questions before on exams that I’ve based my answer on (I think) a deeper train of thought, and got them wrong.
     
  12. Nov 14, 2003 #11

    turin

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    Yeah. From the sound of it, you are probably expected to make some bullsh!t assupmtions sometimes. The trick for a person like you taking a course like this, IMO, is to learn what these assumptions are.

    However, assuming that you are dealing with plane waves will probably follow you as far up as you go.
     
  13. Nov 14, 2003 #12
    Thanks everyone! I learned more here, than just the mechanics of wave-forms (which was my original question). I am beginning to understand that in the study of Physics, just like most other disciplines, there is an evolutionary process to deal with here. I’ll pull back the reins just a little and follow the directed path. Eventhough I want more now, I'll have to trust those who have been there before. Your conformations on both topics has helped!
     
  14. Nov 21, 2003 #13
    Light is an electromagneticall wave, it might be caused by charged particles spinning around eachother;

    nothing forbids that the charged particles inside the photone move faster than light. It's just so hard to imagine. And prove.
     
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