Frequently Asked Questions
What is the formula for B around a wire?
B = μ₀·I / (2π·r), where μ₀ is the permeability of free space, I is current, and r is the perpendicular distance from the wire.
What is μ₀?
The permeability of free space: 4π × 10⁻⁷ T·m/A. Exact by definition in the pre-2019 SI; now derived from fixed e and h.
Does this work for a coil?
No - this formula is for a single straight wire. Coils, solenoids, and toroids have their own formulas (B = μ₀nI for a long solenoid, for example).
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