The line integral of the magnetic field around any closed loop is proportional to the total current passing through that loop.
Right-hand grip rule: thumb along I, fingers curl the way B circles.
∮ B · dl = μ₀ × I(enc)
B
magnetic field (teslas)
I(enc)
current enclosed by the loop (amperes)
μ₀
permeability of free space = 4π × 10⁻⁷ H/m
In plain English: Current creates a magnetic field circling around it — wrap your right hand around the wire with the thumb along the current and your fingers show the field direction.
Where you'll meet it: Gives the field of straight wires, solenoids and toroids — the design math behind inductors, electromagnets and clamp meters (which measure current by its magnetic field alone).