🧲 Solenoid — current in the windings, a uniform field threading the core

A long solenoid carries current I through n turns per metre. Inside, every turn's field adds to a uniform axial field B = µ₀·n·I; outside, the return field spreads thin and, for a long coil, collapses toward zero.

Walkthrough 1 / 4

Current I flows through the copper winding — follow the blue bead around one turn.

Turns per metre n 500 /m

Current I 3.0 A

B_inside = µ₀·n·I = 1.88 mT · B_outside → 0.11 mT
µ₀ = 4·π×10⁻⁷ T·m/A · the interior field is uniform & along the axis

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