An infinitely long uniform solid wire of radius $a$ carries a uniform dc current of density $\overrightarrow{j}$.
The magnetic field at a distance $r$ from the center of the wire is proportional to
- $r$ for $r\lt a$ and $\frac{1}{r^2}$ for $r\gt a$
- $0$ for $r\lt a$ and $\frac{1}{r}$ for $r\gt a$
- $r$ for $r\lt a$ and $\frac{1}{r}$ for $r\gt a$
- $0$ for $r\lt a$ and $\frac{1}{r^2}$ for $r\gt a$