A wire $4\mathrm{.}00$ m long and $6\mathrm{.}00$ mm in diameter has a resistance of $30$ m$.$ A potential difference of $23\mathrm{.}0$ V is applied between the ends. $($a$)$ What is the current in the wire? $($b$)$ What is the magnitude of the current density? $($c$)$ Calculate the resistivity of the wire material.