As a parallel$-$plate capacitor with circular plates $21$ cm in diameter is being charged, the current density of the displacement current in the region between the plates is uniform and has a magnitude of $20$ A$/$m $.($a$)$ Calculate the magnitude B of the magnetic eld at a distance r $=43$ mm from the axis of symmetry of this region. $($b$)$ Calculate dE$/$dt in this region.