Using Ampère's law, the magnetic field at a perpendicular distance r from a long straight wire carrying current I is:

1. Choose a circular amperian loop of radius $r$ around the wire; B is tangential and constant on it. 2. Then $\oint \mathbf{B}\cdot d\mathbf{l} = B(2\pi r)$. 3. Ampère's law: $B(2\pi r) = \mu_0 I$. 4. Solve: $B = \dfrac{\mu_0 I}{2\pi r}$ (the $1/2R$ form belongs to a loop centre, not a straight wire). _Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.150_