# I Acceleration of a metal piece due to dipole radiation magnetic field

#### Wayne Lai

Summary
A question about whether the eddy current created by the changing magnetic field will accelerate the metal that it take place (take dipole radiation and thin round metal disk for example)
Recently I am learning about electrodynamic radiation and its various types, and it occur to me that since the form of the magnetic field created by the dipole radiation is some combination of cos(wt), 1/r, and cos(kr) (take the approximation of r >> c/w)

Therefore, if there is a metal placed in the field, will it be accelerate by the field?

I've done some simple calculations on the case of a thin round metal disk, which the normal vector of its surface is parallel to the tangent line of magnetic field.

By calculate the emf that create by the changing magnetic field value (regarding the time changing and the displacement), I obtain the eddy current for each radius.

By integrating these factors, I get the the total magnetic moment M, thus by using the rule F=-dU/dr and U=M*B, I find the force will be proportional to the sum of v/r^3 and (v/r)*k^2 (which I take the average of time and position on the (cos^2)s).

Could anyone tell me whether this result is correct or not, please?

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#### Henryk

Gold Member
Hi
I didn't follow your calculations, but the answer to your question is YES. If you place a metal object in a alternating magnetic field, the eddy current induced will produce a force on that metal object and the force will be directed away from the alternating magnetic field source. This is a consequence of the Lenz principle: eddy currents induced oppose the change of the magnetic field.
With a strong enough magnetic field, the effects can be quite dramatic. Have a look at this video

"Acceleration of a metal piece due to dipole radiation magnetic field"

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