Magnetic Field of a Straight Wire

Formula

Description

A long straight wire carrying current produces a magnetic field that circles the wire and falls off inversely with distance. The field lines form concentric circles whose direction follows the right-hand rule. This is the starting point for analysing the fields of coils, the coupling between adjacent traces, and magnetic interference from current-carrying conductors.

Variables

• B — Magnetic flux density (T)
• I — Current in the wire (A)
• r — Radial distance from the wire (m)
• µ₀ = 4π×10⁻⁷ H/m —

Practical Notes

At 1 cm from a 1 A wire, B ≈ 20 µT—comparable to Earth’s field. This circulating field is why current loops should enclose minimal area to limit radiated emissions.