Hi all, I am working a problem including calculation of magnetic
forces on a conductor with induced current in it. I used some FEM
softwares and find the B (magnetic flux density) field and J (current
density) field. Can I simply use the Lorentz force equation F = JxB
to find the force on the conductor?

I was using it initially, but now I doubt about it. Because in a
simplified problem of finding the force on a conductor in a magnetic
field, B is the external field which is not affect by the current in
the conductor. However in my case, the B filed (a combined results of
the source current and induced current) is different when the
conductor is taken away.

So where I was wrong in this?

Your are right, normally you can use the unpertubated (undistorted) B-
wriht;

field for calculating the force, it does not matter that that
One can't calculating anything.. and that that?
structure L changes the B-field (because of superposition of two B-
fields).

When structure L does not change the current density in structure K,
you can use the JcrossB formula. However when it does (significantly,
now the "driving" B-field does change), you cannot use it and you need
to setup a model with mutual coupling.