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Electromagnetic emission...

Discussion in 'General Electronics Discussion' started by Externet, Jan 13, 2017.

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  1. Externet

    Externet

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    Aug 24, 2009
    Hi all.

    - A magnet produces a magnetic field that can be detected by a proper instrument, perhaps 1 metre away if powerful enough; Yes or no ?
    - If that magnet has a coil wound on it, and that coil is fed with AC, its field becomes magnetic + electromagnetic, and can be detected by the proper instruments, perhaps 1 metre away; Yes or no ?
    - If the magnet is removed from the coil and an iron core is placed instead, the coil fed with AC will produce an electromagnetic field only, and can be detected by the proper EM instrument; Yes or no ?
     
  2. duke37

    duke37

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    Jan 9, 2011
    Adding an AC coil to a magnet will vary the strength of the field
    Remove the magnet and there will be AC field only. This can be detected 1m away or more. The detection device is called a radio receiver. Medium wave radios often use what is called a loopstick antenna (US) which is a multiturn loop with a ferrite core to enhance performanence.

    Transmitter antennas and receiver antenna differ in that power levels differ but the overall attenuation will be the same if they are interchanged.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    yes

    yes, but it's not the magnet that produces the EM field ... the coil of wire on its own when fed with AC will radiate an EM field .... that can be picked up with a radio receiver

    It is possible/probable that the magnetic core will limit the range of the EM radiation rather than enhancing it

    same as my previous response, but this time the iron core will be magnetised by the coil of wire


    Dave
     
  4. Externet

    Externet

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    Aug 24, 2009
    Thanks.
    Then, driving a plain speaker creates an acoustic + magnetic + electromagnetic fields.

    If the speaker cone and the magnet are removed, leaving only the iron core; gluing the coil to the iron core so nothing moves, when fed by audio it will create only an electromagnetic field that can be picked/detected/sensed/received by another coil nearby; yes or no ?

    Is there any experiments/tests/documents that you know done on that ?

    Like placing a dynamic microphone or pickup coil nearby of that now silent crippled speaker and use the perfectly good signal towards an amplifier ?
     
  5. davenn

    davenn Moderator

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    well the speaker magnet is already producing a magnetic field ... I have never tried to detect an EM field from a speaker, but yes, it will exist



    I would expect it to ... probably not overly strong tho
     
    Externet likes this.
  6. duke37

    duke37

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    Jan 9, 2011
    What you are making is a transformer with a shorted turn and a poor core (no laminations) with very little coupling to the secondary.

    The speaker tries to keep all the magnetic field between the magnet pole pieces but some escapes. Do not put a speaker near to a CRT TV or you will see the stray field affecting the color (US).
     
  7. Alec_t

    Alec_t

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    A good quality dynamic mic is unlikely to be sensitive to external magnetic fields, since its moving coil will be buried between concentric magnet poles and thus screened.
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    In this case the magnet will not interact with the EM wave produced by the coil. The magnetic field produced by a magnet is completely different than that produced by an EM wave. The EM wave contains photons, the magnet does not. If you were to vibrate the magnet fast enough then it would interact with the EM wave.
    Thanks
    Adam
     
  9. Externet

    Externet

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    Aug 24, 2009
    Thanks, gentlemen. Good responses as expected.

    duke37 : Yes, that is a well known behavior for a CRT next to a speaker. The magnetic field distorts the raster and when active, the EM makes it flicker at the cadence of audio.

    Adam : can you explain why electromagnetic waves "contain" photons and "the magnet does not", even when put to spin ?
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    The EM wave carries energy in the form of photons, this happens when an electron is accelerated, hits another electron or pops back into the valence shell of an atom. The energy is radiated as a photon which reacts with other materials disturbing that materials electrons which then release their own photons, this is how the energy passes from one point to another. You have no accelerating electrons in a magnet unless you give it energy. I suppose you could say anything above zero degrees Kelvin emits photons, but just for now and in the context of your post the magnet doesn't release any photons.
    Thanks
    Adam
     
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