🌀 Eddy Currents — Magnetic Braking
Drop a magnet through a conducting tube. The changing magnetic flux induces circulating eddy currents (Faraday's law: ε = −dΦ/dt). By Lenz's law these currents create a braking force F ∝ σv opposing motion — the magnet falls far slower than free-fall.
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The braking force on a magnetic dipole moving with velocity v through a conducting tube of conductivity σ and wall thickness d is approximately Fbrake = k·σ·d·B₀²·v, where k is a geometry factor. At terminal velocity the braking force equals gravity: mg = k·σ·d·B₀²·vt, giving vt = mg / (k·σ·d·B₀²).
The swirling eddy current loops visualised around the tube walls carry induced currents that dissipate energy as Joule heat I²R — all kinetic energy is converted to heat by the time the magnet exits. Plastic has σ ≈ 0 so no braking occurs at all.
Did you know?
Maglev brake systems on roller-coasters and trains use this principle — no contact, no wear, and braking force increases automatically with speed. The classic "slow magnet in copper tube" experiment is a staple of physics demonstrations worldwide.