A Long Straight Wire of Radius R Carries a Current $\mu$

Multiple Choice

A long straight wire of radius R carries a current density J = kr A/m² where k is a constant. The magnetic field for r > R is (Hint: Current enclosed .)

A) $\mu$ ₀kR³/3r
B) 2 $\pi$$\mu$ ₀kR³/3r
C) 2 $\pi$$\mu$ ₀kR²/r
D) 2 $\pi$$\mu$ ₀kR/2
E) none of the above