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Electromagnetic waves

  1. May 6, 2012 #1
    So what i understand from my professor about the electromagnetic waves is this :

    an electric charge has an electric field E
    a moving electric charge induces a magnetic field B
    The electromagnetic wave produced is due to the moving charge which has both components E and B...

    But my question is, why does this ONLY work in an accelerating charge? doesn't a regular charge not accelerating (such as a current) produce a magnetic field as well as an electric field ?


    Thanks
     
  2. jcsd
  3. May 8, 2012 #2
    Hi inmyblood,

    A non-accelerating charge produces merely a displacement current or perturbations in the E and B fields that are not traveling waves. You can think of them as evanescent waves, disturbances that dissipate very quickly if the charge stops moving.

    A nice equation showing how both charge velocity and acceleration affect the magnetic field can be found in "Classical Electomagnetism via Relativity" by W. G. V. Rosser p. 38-41 (referenced from Jefimenko's book "Causality, Electromagnetic Induction and Gravitation"):

    [tex]H = \frac{q}{4 \pi s^3}([v][1 - \frac{v^2}{c^2}] - \frac{1}{c[r]}[r]\ \mathsf x \ (R\ \mathsf x \ [a]))\ \mathsf x \ [r][/tex]

    (H is related to B through the ratio given by the vacuum permittivity constant [itex]\mu_0[/itex])
     
    Last edited: May 8, 2012
  4. May 8, 2012 #3
    Steady current produces constant magnetic field, which doesn't induce Electric field. Accelerating charge however produces variable magnetic field, which then in turn produces variable Electric field.
     
  5. May 8, 2012 #4
    Please be careful there. Strictly speaking the moving charge produces both the electric and magnetic fields or rather, their fluctuation. (Sorry, I omitted the related equation for the electric field)

    [tex]E = \frac{q}{4 \pi \epsilon_0 s^3}(R[1 - \frac{v^2}{c^2}] + \frac{1}{c^2}[r]\ \mathsf x \ (R\ \mathsf x \ [a]))[/tex]


    Though as a side effect, some other charge (or a photon) could be producing magnetic field fluctuations that move the charge in the equation resulting in added E and B field components. But we're probably most interested in the one charge to not complicate and confuse things, aren't we?
     
    Last edited: May 8, 2012
  6. May 8, 2012 #5
    Thanks PhilDSP!

    So If i understand correctly an EM wave has to have both the magnitude of the Electric Field as well as its Magnetic field varying in space to be considered an EM wave ?

    Also regarding the Magnetic waves may you check my question about mutual inductance ?

    showthread.php?t=603580

    Thanks :)
     
  7. May 8, 2012 #6
    Is that what PhilDSP sad? I didn't get it. I thought you asked why steady current doesn't produce EM wave.
     
  8. May 9, 2012 #7
    That's probably not an especially good way to classify an EM wave. I think generally a single non-accelerated moving charge will also have E and B fields varying in space or across space. We should notice the curl in the equation for the H field. That means the field values will curve across space.

    But a conglomerate of moving charges, a current, might be steady over a certain region of space and then produce fields that don't varying in time at one position.

    A different classification is that the EM wave will be self-propelled while displacement current will not be.
     
    Last edited: May 9, 2012
  9. May 17, 2012 #8
    The antenna TV amplifier feds up electricity to the antenna.Does eletricity pull EM waves?
     
  10. May 17, 2012 #9
    Provided that photons have not mass,why only SW reflected by ionospere?
    Why radars works only with UHF waves? Why submarines communicate with LW?
    And many more questions arise.
     
  11. May 18, 2012 #10
    Why when you put an AM radio close to an elektricity conductor the signal becomes stronger?
     
  12. May 18, 2012 #11

    sophiecentaur

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    You can only answer these questions by getting to know the basics of EM theory. There is no quick 'arm waving' way of explaining something so complicated. You will be disappointed if you hope for a simple explanation.
     
  13. May 18, 2012 #12

    davenn

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    no it doesnt.
    A power supply feeds electricity up to an amplifier at the antenna
    that DC voltage is used to make the amplifier work.

    there is no electrical pulling of EM waves

    Dave
     
  14. May 18, 2012 #13

    davenn

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    the conductor acts as a long antenna and helps to pick up the radio signal better,
    it is then induced into the radio's internal antenna when the radio is brough close to the conductor.
    The conductor ( a piece of wire) doesnt need electricity in it for it to act as an antenna
    any long piece of wire will have the same effect

    Dave
     
  15. May 21, 2012 #14
    why different colours have different refractive index?
     
  16. May 21, 2012 #15

    sophiecentaur

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    The speed of the light through a medium will depend on how it interacts with the structure it passes through. Different colours (at least, the spectral colours) just correspond to different wavelengths. When EM waves interact with matter it is always wavelength dependent so it is not surprising that different wavelengths of light should be slowed down by different amounts as they pass through a transparent medium - as far as the WHOLE of the EM spectrum ("DC to Daylight") is concerned, there is a vast range of effects that a given medium can have on the different wavelengths involved.
     
  17. May 22, 2012 #16
    If you touch antenna you will feel the sting.Amplifier is just a frequency filter.
     
  18. May 22, 2012 #17

    davenn

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    no thats not quite correct. The amplifier AMPLIFIES the signal, else it wouldnt be called an amplifier. Generally mast head amplifiers are used in weak signal areas or where a single antenna feeds a block of accomodation units.

    That tingle you feel is coming from the TV. Many of the TV's that have a voltage feeding out of them will be enough for you to feel a slight tingle.

    Some of the older TV's that had live chassis, had quite a substantial voltage on the antenna socket. And altho the sockets had bypass and DC blocking capacitors, you could still get a strong tingle. This had nothing to do with feeding power to amplifiers.

    cheers
    Dave
     
    Last edited: May 22, 2012
  19. May 22, 2012 #18

    davenn

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    I will also take the opportunity to answer a couople of your earlier questions
    well give you a little insight anyway....

    actually RADAR will work over virtually any radio frequency
    but the main reason why they use microwave frequencies, usually greater than 1000MHz, is because as the frequency is increased the resolution of the RADAR gets better. That is they are able to discriminate between smaller and smaller objects.

    I remember many years ago, the local airport near where I used to live had RADAR on 600MHz.
    up till 1999, FAA in the USA used 1330 and 1350 MHz freq's for airport radars. dont know how much that's changed in the last 10 odd years

    Weather radar commonly uses 3000MHz (3GHz)

    from what I understand, VLF ( Very Low Frequency) radio signals penetrated the sea much easier than higher freq's. This allowed submarines to remain submerged and still receive communications.

    cheers
    Dave
     
  20. May 22, 2012 #19

    sophiecentaur

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    If you touch an antenna and you "feel the sting" then your head amplifier power supply is faulty - that's the mains you can feel. You might think about changing it!
    Some basics: An amplifier is an amplifier and a frequency filter is a frequency filter: two different beasts. The head amp is a wide band device (no filtering) which handles all the possible channels you might want to receiv with your selective receiver. The reason that you put a head amp up at the aerial is that it has a better noise figure than the receiver and is providing amplification at the far end of the feeder. Both of those functions improve the carrier to noise ratio at the input to the set when your received signal is below the desirable level.
    The voltages on a T V receiving antenna are in the region of a few tens of milivolts. No "sting" possible.
     
  21. May 22, 2012 #20

    davenn

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    not entirely correct. having serviced TV's for more than 30 years. I can definitively tell you that you will receive a tingling from the antenna socket or any antenna connected to many many makes and models of TV's. This is normal ... it is NOT a fault condition
    I will qualify that by saying that generally it would be usually felt by the more sensitive skin areas say, under the arms, rather than on the thicker / drier skin of the hands or fingers

    Dave
     
  22. May 23, 2012 #21

    sophiecentaur

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    The volts come from the TV and not from the aether. They aren't sucked in from elsewhere.

    I was discussing the situation with a naked antenn and head amp.

    That tingle can also be felt on microphones and electric kettles when there's poor earthing. Did you ever measure those volts, btw?
     
  23. May 23, 2012 #22
    I have known all that you mentioned,my question was exactly about EM waves properties:
    Why UHF and microwaves are more reflected by metal surfaces and other objects?
    Why MW and LW are more penetrating?
    Why only SW reflected from the ionosphere?
    If we consider that the photons have size or mass,then the above and many more can be explained.
     
  24. May 23, 2012 #23

    sophiecentaur

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    For starters, photons cannot have mass or they couldn't travel at c, could they?

    The other questions cover virtually the whole of em theory. There isn't a simple answer to include them all at once.
     
  25. May 24, 2012 #24
    For starters, photons cannot have mass or they couldn't travel at c, could they?
    Why not? In a transparent material velocity is lower.
     
  26. May 24, 2012 #25
    If photons have rest mass it must be exceedingly small. Mass in an electromagnetic particle causes frequency dispersion of radiation when the particle interacts with another particle. Experimentally, in a vacuum no dispersion has been found to very, very small limits.
     
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