Radio waves question

Discussion in 'Physics & Math' started by Facial, Aug 28, 2006.

  1. Facial Valued Senior Member

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    Can they be created through voltage oscillations? ie, moving capacitor plates back and forth?

    And are they created in AC 60-Hz 120V electrical appliances (as in the US), in generating 60-Hz radio waves ubiquitous throughout the house?
     
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  3. perplexity Banned Banned

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    While all electrical activity radiates, "radio" usually infers high frequencies.
     
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  5. DaleSpam TANSTAAFL Registered Senior Member

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    Hi Facial,

    Ron is almost exactly correct. As he said, the electrical currents in your house do, in fact, radiate. That is why you can pick up a 60Hz hum in your speakers if you set your stereo equipment up wrong. But 60Hz is actually in the SLF (super low frequency) radio band which goes from 30Hz to 300Hz. Generally this band is only used for communication with submarines at depth. So 60Hz is kind of a radio wave "on a technicality", it is definitely not what most people think of wrt radio.

    -Dale
     
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  7. CANGAS Registered Senior Member

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    Like Maxwell said, any time varying electric field creates a magnetic field and vice versa. Electromagnetic radiation, alias radio waves is nothing else than a time varying electric field which creates a time varying magnetic field which creates a time varying electric field........

    When the charged plates move they create a time varying electric field; a stationary point in space observes the electric field drawing nearer and thereby getting stronger by the moment. At the stationary point in space a magnetic field is induced. This process repeats every Gillianth of a second.
     
  8. Mosheh Thezion Registered Senior Member

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    I would love to see the experiment that confirms that when you spread the distance between plates that electromagnetic radiation is produced.

    i have never seem it.

    spread the plates. and the voltage goes up... potential goes up.

    bringing them closer is when some energy should release, but its a question of how.

    if anyone can show reference to any experiment on the subject id love to see.

    i dont think anyone can.

    -MT
     
  9. Mosheh Thezion Registered Senior Member

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    2,650
    your assuming the plates are connected to some curcuit....

    if the plates are insulated and charged.. then their oscilation will result in no electron flow outside of the plates... no flow at all.

    just a varing tension.... voltage up.. voltage down.

    in which case.. moving the plates apart adds energy... bringing them close would allow potential to do work...

    but its a question of how it would work.

    in a perfect vacumm, it is likely that bringing the plates closer will increase the likelyhood of loss from the charged plates... i.e.. electrons bleeding off.. across the field.

    which then could manifest electromagnetic waves...

    but until electrons flow... i dont see any em waves being produced.

    again... show me a reference to an experiment showing as much, and i will consider it.

    -MT
     
  10. DaleSpam TANSTAAFL Registered Senior Member

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    Are you kidding MT? This is a common freshman level physics course lab experiment. Try Google, I am sure you will be able to find several examples with some variations of the course material posted on the web.

    If you move the plate smoothly (charges not accelerating) then the energy will all be released in the form of mechanical work done on the plates. You would get a small amount of radiation if the plates are made to accelerate.

    -Dale
     
  11. cato less hate, more science Registered Senior Member

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    I am not sure if everybody knows or not, but generally oscillation is not achieved by physically moving capacitor plates. it is normally a reaction of inductance and capacitance.
     
  12. perplexity Banned Banned

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    Deleted
     
    Last edited: Mar 13, 2007
  13. Mosheh Thezion Registered Senior Member

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    DALE..... the conversion of the tension across the dielectric between the plates could it is true... manifest as movement of the plates themselves.. towards each other...
    but that would not be electromagnetic radiation... and using caluculations too convert motion of a mass into a wave.. isnt really fact is it?
    energy equals energy... and we can say alot of things...

    but the fact of the matter is... moving the plates in itself does not change anything other than the potential energy of the field which is static.
    any field emitted... would be the result of loss...

    such as air particles between the plates... fliping and turning with the change in the field... this slight motion could manifest as wave energy...
    but such is not a direct conversion of the field energy.... its a side effect caused by the in-effiecncy of using a gas as a dielectric.

    the idea that an expanding or electric field automatically emits an electromagnetic field is not supported by anything.
    only a collapsing field or a moving electron can result in an output of electromagnetic radiation. and if no electron is allowed to flow, then i know of no evidense that says electromagnetics come into play... and no one has shown it to me.

    -MT
     
  14. DaleSpam TANSTAAFL Registered Senior Member

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    MT

    Perhaps you should have paid more attention in freshman physics. There is nothing mysterious or unknown here.

    If you move the plates apart at a constant velocity you are doing mechanical work on the plates. If you move the plates together at a constant velocity the plates are doing mechanical work. If you accelerate the plate in either direction you get some EM radiation. Any change in the electrical PE is exactly accounted for by the mechanical work plus any radiated energy.

    -Dale
     
    Last edited: Sep 4, 2006
  15. Mosheh Thezion Registered Senior Member

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    Dale... I DONT WANT TO INSULT YOU...

    so i will ask... when the plates are expanding... in what way, and by what mechanism does the energy of the electric field... somehow become radiated em radiation...??

    im sure that with your over abundance of confidence, you can find easy reference material to answer this.....


    hahaha.

    -MT
     
  16. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Correct, but reminds me of following exam question I often gave around mid year or later* to Freshman physics students:

    Assume two identical ideal capactors of capactance C, one charged to 2V and the other fully discharged prior to t = 0 when they are connected together by a perfect (non-disipating) switch.

    (1) In symbols, what is the energy stored in originally charged capacitor (prior to t = 0)?
    (2) Long after t = 0, what is the voltage on originally charged capacitor?
    (3) Long after t = 0, what is the energy now stored in originally charged capacitor?
    (4) Considering both capacitors as "the system," has the system energy been changed by closing the "perfect switch"?
    (5) For extra credit, in 100 words or less, discuss your answer to (4)'s relationship to the "conservation of energy" law and how an imperfect switch (with resistance) could change the results.
    ----------------------------------
    *Being some what sadistic, this question might be two months after that chapter was in book was done and we had been working images thru lenses / mirrors systems for the last week or two. I really wanted students to retain what they had learned.
     
    Last edited by a moderator: Sep 4, 2006
  17. Mosheh Thezion Registered Senior Member

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    2,650
    yo, billy.. maybe you can help dale and explain how seperating insulted plates will result in any em radiation.... and by what mechanism such em radiation is produced when a voltage rise occurs due to the increase in distance of two charged PLATES, unconnected plates in pure empty space... without any electron leakage.

    -MT
     
  18. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I think Dale has it correct in that acceleration of charge (assuming magnetic fields are always static or absent) is required for EM radiation like radio waves to be produced, however, if we extend from back side of each plate perpenducarly two long wires, it does get a little more complex in that it is beginning to resemble the charged/ discharged capacitor case of my exam question. I.e. there will be a current wave moving down the wire (which is sort of like another capacitor connected to the first main one) as the charge re distributes itself along the wire, charging up to the constantly changing voltage on the plates at plate connected end first, even with uniform plate speed. Perhaps you were thinking along these lines by including words "insulted plates"? These current redistribution waves will radiate, I think.
     
  19. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    23,198
    R.W. Wood, John Hopkin's great early physicists did a interesting and related experiment. I'm not sure of details anymore, but basically he put large electro static charge on the metal rim band of a plastic disk and spun the disk about its axis to show (by measuring it) that electrons did not need to flow thru metal to make a magnetic field and was also able to know exactly how fast they were moving (by storbe lights) and their quanity (by voltage and computed C). These "static" electrons obviously had a radial acceleration, but although I think this must ?/ may have produced radiation, I don't know much about the pattern, direction of propagation, rate etc.. Perhaps there is no radiation because of the perfect symmetry? - I.e. all the indivdual electron radiations add to zero, at least far from the disk? Interesting question I have thought of before but never answered. Perhaps he was first to make cyclotron radiation? but in much lower frequency band (ultra sonic)?
     
    Last edited by a moderator: Sep 4, 2006
  20. Mosheh Thezion Registered Senior Member

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    2,650
    1) IF YOU CONNECT ANY wires to the plates... then you will electron flow.. and so.. inductive fields.. and so em radiation...

    but what about without wire... when only the field is changing,, and electrons xcannot discharge at all... since they are trapped on the one plate, and are lacking on the other. thus..... no em waves.

    2) a spinning disk of plastic.. with charges on each side.. when spun...

    does not give off an em field... but as the disk spins.. the change in polarity relative to any instrument will show a wave pattern.. only because the elctrons in your instruments, are first being attracted to the positive charge, then repelled by the negative on.... back and forth... creating a em wave in your instrument, using charges and static field, which themselves dont change into em waves at all.

    the charge,, when in motion can do work... but when the disk stops... the charge value on the disks may not have changed... showing that it was not a conversion of a static electric field iinto em waves....
    but instead was the conversion of the work applied as spin, to create em waves, using the effects of moving and oscilating charges on the disk.


    so basically.... im still waiting.

    -MT
     
  21. DaleSpam TANSTAAFL Registered Senior Member

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    I will help you out since you apparently forgot the address of Google (for future reference it is: www.google.com). Here is a link describing the Radiation From Accelerated Charged Particles. This radiation is also known as bremsstrahlung radiation and should have been covered in any basic physics course. As you can see, the radiated power is zero for a non-accelerating charge and increases as the square of the acceleration. So to obtain the total radiated energy simply integrate the square of the acceleration of the moving plate through time and over all of the charges on the plate. Note that this is non-conservative so it needs to be calculated for each different motion pattern. If the movement is smooth (negligible acceleration) then there will be negligible energy radiated.

    In addition to the radiated energy there is also an Electrical Potential Energy associated with the distance from the stationary plate, this is also covered in any first-year physics course. The total electrical potential energy can be obtained by integrating the given formula over the two plates. Note that this is conservative so it only needs to be evaluated at the beginning and end points of the motion. This energy change will occur regardless of wether or not the movement is smooth.

    The work done on the moving plate is equal to the sum of these two terms. So, in the case of a smoothly departing plate there will be no radiation and all work done on the plate will increase its electrical potential energy. However, in the case of a suddenly departing plate there will be radiation during the acceleration and the work done on the moving plate will be greater than the increase in electrical potential energy.

    All of the concepts required to figure this out are freshman-level physics course material. You apparently needed to take better notes in class. I hope whoever paid for your education realized that they got ripped off.

    -Dale
     
    Last edited: Sep 4, 2006
  22. Mosheh Thezion Registered Senior Member

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    2,650
    http://www.astro.utu.fi/~cflynn/astroII/l3.html

    no where in that link... is there anything that says....

    when two charged bodies are moved apart... that em radiation is produced.

    its funny how you keep mentioning first year physics....

    is that as far as you got??

    thats to bad... because i am self taught... i didnt wait for people to teach me what we real.... i taught myself, and motivated myself to learn the subject at hand.

    you... while being able to mis quote alot of science.

    are un able to answer the question.

    by what mechanism.... specifically.. does the seperation of charged, insulated plates, result in em radiation, when... no electron have dscharged. or moved.

    show me the radiation dale.....??

    show me anything, that will show that an expanding field, automatically produces em waves.... which in this case... would'nt be a wave would it...??

    it would be 1/4 of the wave.... since all we have is two plates seperating...

    thus all we are doing is the first 1/4 of a wave....

    show me how, this growing potential field, has yet to produce anything????

    when it collapses... maybe... due to electron motion and radiation.

    but not when the plates are moving apart.


    its easy to act smart dale... and its easy to think you are highly educated...

    but you act as if i am stupid.

    yet you cannot show me that a changing electric field automatically results in a em radiation emission... when dealing with electrostatic fields of potential.

    remember... we dont have moving electrons... and we dont have photons.

    all we have is two masses.. charges on them.. and seperation.

    please... ok... im stupid.

    explain it to me, if you can.... hahahaha

    -MT
     
  23. DaleSpam TANSTAAFL Registered Senior Member

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    Bremsstrahlung radiation. Did you miss it last time I said it?


    And how exactly do you propose to move the masses without moving the charges on them?

    When you move the masses you move the attached charges. When the attached charges accelerate they emit bremsstrahlung radiation.

    If you haven't even taken a basic physics course then it is ridiculous for you to assume that you have some great knowledge that everyone else has missed. I have no problem with self-taught people, I have a problem with arrogant ignorance like yours. Maybe next time you should continue your education instead of assuming that you already finished.

    -Dale
     

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