The magnitude of the magnetic field at perpendicular distance r from an infinite straight wire carrying current I is

1. Use Ampere's law on a circle of radius r centred on the wire. 2. By symmetry, B is tangential and constant on the circle: ∮B·dl = B·(2πr). 3. Enclosed current is I, so B·(2πr) = μ₀I. 4. Therefore B = μ₀I/(2πr). _Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", §4.7_