# Radiation from a Square Loop of Current

• I
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?

tech99
Gold Member
How about a loop of steady current that is shaped in a square? Does this structure radiate?
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.

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Staff Emeritus
You get radiation when you have a changing dipole (or higher) moment. So nothing with a steady current will radiate.

tech99
Gold Member
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.

debajyoti datta
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.
and i guess radiated power will be given by Larmor's formula ?

tech99
Gold Member
and i guess radiated power will be given by Larmor's formula ?