Magnetic Effects of Current

📋 Teacher notes

• •A current-carrying wire is always surrounded by a magnetic field — the field lines form CONCENTRIC CIRCLES centred on the wire, in a plane perpendicular to it.
• •Right-hand GRIP rule: point your right thumb in the direction of conventional current flow; your curled fingers show the direction of the field.
• •Field strength B = μ₀I/2πr decreases with distance from the wire — double the distance, half the field.
• •μ₀ = 4π×10⁻⁷ T·m/A is the permeability of free space — it sets how strongly current produces a magnetic field in a vacuum (or air, to a very good approximation).
• •Undergraduate note: this is the direct result of Ampère’s circuital law, ∮B·dl = μ₀I_enc, applied to a circular path around the wire — the field magnitude is constant on that path by symmetry, which is exactly why it simplifies to B(2πr) = μ₀I.