Ampere's Circuital Law
Maxwell's fourth equation (in magnetostatics) relating the integrated magnetic field around a closed Amperian loop to net electric current passing through the loop.
Governing formula
∮ B · dl = μ₀ I_enclosed
SI unit
T·m (Tesla-meters)
In depth
Ampere's law allows straightforward calculation of magnetic fields surrounding highly symmetric current configurations, such as long straight wires, solenoids, and toroids.
Examples in the real world
Deriving the circumferential magnetic field surrounding a long straight current-carrying wire at distance r: B = μ₀ I / (2π r).