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The most common method of generating electromagnetic radiation is by using oscillating charges, for instance in antennas. However, I couldn't think of an example where varying magnetic fields are deliberately used for creating electromagnetic radiation. Things like DC electric motors and generators produce EM radiation as an accidental by-product but it's not really their primary objective. So is there any better example in which a varying magnetic field is primarily used for generating EM waves?

To be more specific I'm looking for examples of sources of EM radiation which are otherwise only sources of magnetic fields (and not electric fields) when stationary.

Disclosure: This question was a puzzle posed by our antenna theory professor last week and I'm not sure whether DC motors/generators is the example he is looking for.

  • Isn't an oscillating charge also a varying magnetic field? – TimRias Sep 12 '19 at 16:07
  • @mmeent An oscillating charge produces a varying magnetic field. It isn't a varying magnetic field by itself. –  Sep 12 '19 at 16:08
  • In that sense an oscillating charge also is not a varying electric field, and only produces one. – TimRias Sep 12 '19 at 16:10
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    search "loop antenna". The same antenna can be used to receive and to transmit. – verdelite Sep 12 '19 at 16:11
  • @mmeent True indeed. I somehow considered electric fields to be more intrinsic to oscillating charges. I need to rethink this. –  Sep 12 '19 at 16:13
  • I guess what you were looking for are examples of EM radiation generated by varying currents, and not varying charge configurations. If so, you might wan to edit the function to reflect this. – TimRias Sep 12 '19 at 16:16
  • @mmeent Umm, I think I'm looking for examples of EM radiation generated by varying magnetic fields rather than currents as mentioned in the question... –  Sep 12 '19 at 16:18
  • EM radiation is a varying magnetic field, it does not generate one. – TimRias Sep 12 '19 at 16:22
  • @mmeent Say for example a spinning bar magnet can give rise to EM radiation (cf. here and here). It is true that EM radiation is a sequence of varying electric and magnetic fields (orthogonal to each other)...but it doesn't mean we can't start with one varying field and generate the other. –  Sep 12 '19 at 16:29
  • In some sense, a bar magnet is a stationary current. More importantly, a bar magnet is a source for a magnetic field, not itself a magnetic field. – TimRias Sep 12 '19 at 16:36
  • @mmeent So perhaps I'm looking for examples of sources of EM radiation which are otherwise only sources of magnetic fields (and not electric fields) when stationary. –  Sep 12 '19 at 16:41
  • Use DeepL to read the German Wikipedia about Magnetantenne. In short: A magnetic antenna in the narrower sense - a frame antenna - consists of one, rarely a few windings with as large a coil area as possible, and possibly a capacitor. Without a capacitor such an antenna is very broadband; with a capacitor it becomes a resonant circuit and very narrowband. – HolgerFiedler Sep 13 '19 at 05:23

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In MRI (magnetic resonance imaging) the coils that are used both to transmit energy into the patient and to receive information and energy from the patient are antenna structures whose primary purpose is to transmit and receive signals in the magnetic field. They are primarily built using loops, magnetic dipoles, rather than lines, electric dipoles. These operate in the RF (radio frequency) regime, however whether you want to call what they produce "radiation" or not is a matter of taste. They operate primarily in the near field, with as little energy going to the far field as possible.

Dale
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When you wiggle a magnet in your hand, the magnet produces EM waves.

Ján Lalinský
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  • Indeed, we figured that out in the comments. However, a bit more detailed answer might be helpful for future viewers. Oh look, here's a bounty! ;) –  Sep 12 '19 at 17:09