# EM field and its sources

1. Oct 29, 2008

### fisico30

EM field and its sources....

hello forum,

a simple question about EM field and its sources.
In a time-varying situation, maxwell equations tell that the E field has the time-changing charge density rho as a source, while the B field has the time-changing current density J.

But both fields are actually and fundamentally generated by charges, static, moving, or accelerating (i.e. rho).
Why is the source distinction useful? After all, the continuity eqn link rho and J.

On the other hand, in antenna theory, the E field (and the B) is generated by the time-varying current source in the antenna. There is no need to talk about the charge density rho..... J is fine and enough.

Why in electrodynamics we just don't use J?
thanks!