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I Radiation from a Square Loop of Current

  1. Apr 24, 2016 #1
    It is well known that a circular loop of steady current does not radiate, even though each individual charge is undergoing centripetal acceleration.

    How about a loop of steady current that is shaped in a square? Does this structure radiate?
  2. jcsd
  3. Apr 24, 2016 #2
    Imagine the static field of the charges in the wire. The lines of force point radially outwards everywhere, and are always normal to the wire. Even when it goes round a corner they are normal to the wire. To have radiation, they need to be not normal but pushed to one side a bit. In this way a transverse component of field is created, the radiated E-field. This only occurs if the current is varied, as with AC or at switch-on.
    Last edited by a moderator: Apr 26, 2016
  4. Apr 24, 2016 #3

    Vanadium 50

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    You get radiation when you have a changing dipole (or higher) moment. So nothing with a steady current will radiate.
  5. Apr 25, 2016 #4
    Further to my reply, if the electrons can move fast enough with a steady current it is possible to obtain cyclotron radiation. But electrons in a wire are only travelling at, say, 1 mm an hour. In a vacuum tube it is possible for them to go round millions of times a second, and then cyclotron radiation can be seen at the rotation frequency.
  6. Apr 26, 2016 #5
    and i guess radiated power will be given by Larmor's formula ?
  7. Apr 27, 2016 #6
    Yes, taking the radial acceleration.
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