Physics
Electromagnet and filings
Real iron filings around a coil: the pattern grows itself.
What happens
A coil with an iron core and a tray of real iron filings, every one a rigid needle that magnetizes, feels its neighbours and tips over. Raise the current and the tray erupts into bristles arching from the pole; switch the current off and friction holds the pattern; shake the tray and it starts over, never the same twice. Drag the coil to move it across the sheet; pull a core end (or hold Shift) to rotate it. The right mouse button orbits the scene.
Equations used in the module
Equations B = μ₀ μ_r n I (inside a long coil) Φ = B A, F ≈ B² A / (2μ₀) (pull at the pole) saturation: μ_r falls as B grows
