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Question about a magnetic field

  1. Apr 27, 2015 #1
    The scenario is as follows,

    We have a rare Earth permanent magnet with some arbitrary strength magnetic field.
    It is mounted horizontally at the end of a vertical spindle so the magnet forms a 'T'.
    The spindle is then attached to a motor which spins the permanent magnet at 50,000 revolutions per Second. (That's 3,000,000 RPM. Let's just pretend we have motors that can go that fast and things won't self destruct in the process.)

    The question is,

    If I have a radio receiver tuned to 50 KHz some distance away, am I going to receive a 50 KHz RF signal?
  2. jcsd
  3. Apr 28, 2015 #2
    Sure. Here's a closely related example. http://helios.gsfc.nasa.gov/solarmag.html

    In the absence of charge, an alternating magnetic field is associated with an alternating electric field; an electromagnetic wave.
  4. Apr 28, 2015 #3
    I have some follow on thoughts & questions..

    Venturing to assume that a rapidly rotating permanent magnet will radiate an RF signal, perhaps indistinguishable to a signal transmitted from a radio transmitter, then given..

    1. Higher frequencies have more energy than lower frequencies.
    2. The strength of the permanent magnets field is fixed.

    If the magnet is then made to rotate at twice its original speed, now 100,000 revolutions per Second, (100 KHz) it should now be transmitting a signal with twice the energy? From the same fixed strength magnet? I am to believe the increased energy of the radiated signal was derived from the energy applied to achieve the higher rotation speed?

    Now, let's make up a standard rotation value of 100,000 revolutions per Second and set up an experiment. On one end of a very large room we have the permanent magnet that we can rotate at 100 K/Sec, and we have a battery powered transmitter that transmits an RF signal of 100 KHz of which we can vary its output power. At the far end of the room we have the receiver antenna tuned to 100 KHz connected to a power meter. Now we turn on the motor spinning the magnet and measure the signal strength at the receiver at the other side of the room. For this scenario let's say it is -90dBm. Now we turn off the spinning magnet and turn on the battery powered transmitter. We should be able to adjust the output power of the battery powered transmitter to where the signal level at the receiver is the same as it was for the rotating magnet. Now we measure the output power of the battery powered transmitter. Again for this scenario let's say it is 25 mW. Using a standardized rotation speed or frequency, we should be able to make a direct relationship between a magnets field strength and its equivalent power in Watts?

    Also, going on the premise that the rotating magnet can radiate an electromagnetic field, let's now slow that magnet down to a stop. The same magnetic field that radiated the 100 KHz signal is now stationary. Is it not correct to think that this stationary magnetic field is actually a standing 0 Hz electromagnetic potential?

    If this does not make sense, tell me where I'm getting things mixed up.
  5. Apr 28, 2015 #4


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    EM radiation usually occurs when a charge is accelerated, but it also seems to occur if a magnetic pole is accelerated, so I think you are correct. Notice, however, that 50kHz corresponds to a large wavelength, 6 km, so the magnet would need to be long in order to obtain much radiation. When the receiver is near the magnet, you are mainly detecting the ordinary induction fields rather than radiated fields. The radiated energy will be supplied by the motor.
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