Circular Current Loop

$($a$)$ Explain the concept of current density in a circular wire carrying a current $\backslash ($ I $\backslash ).$ Define surface current density $\backslash ($ K $\backslash )$ and its relationship to total current $\backslash ($ I $\backslash ).$

$($b$)$ Given a circular wire of radius $\backslash ($ r$\_\{0\}\backslash )$ carrying a current, derive the expression for the magnetic field at a distance $\backslash ($ R $\backslash )$ from the axis of the wire.

Please include all steps and sketch.