Uniform displacement$-$current density. The figure shows a circular region of radius $R=3\mathrm{.}20cm$ in which a displacement current is directed out of the page.The displacement current has a uniform density of magnitude $J_d=4\mathrm{.}50\frac{A}{m^2}.$ What is the magnitude of the magnetic field due to the displacement current at radial distances $($a$)1\mathrm{.}80$ cm and $($b$)4\mathrm{.}70$ cm $?$