An electric current $I=0\mathrm{.}65A$ is flowing in a long wire. Consider a rectangular area with one side parallel to the wire and at a distance $c=0\mathrm{.}032m$ away from the wire. Let the dimensions of the rectangle be $a=\frac{0\mathrm{.}03}{b}ar(I)m$ and $b=0\mathrm{.}081m.$

Part $($a$)$

Express the magnitude of the magnetic field generated by the wire $B$ at a distance $r$ in terms of I and $r.$