Why does a magnet fall slowly through a copper tube if copper is not magnetic?

The moving magnet changes magnetic flux through the conducting tube and drives circulating eddy currents. By Lenz's law, their magnetic field opposes the change that produced them, creating drag. In the low-speed linear model used here, drag grows with speed until it balances the magnet's weight at a terminal velocity.

Does an eddy current brake work at low speed?

Its braking force falls toward zero as relative speed approaches zero, so an eddy current brake cannot by itself provide a static holding force. Many practical systems therefore combine eddy-current braking with friction, regenerative braking or a mechanical holding brake, depending on the application.

Where does the mechanical energy go?

In this model, the mechanical power removed by magnetic drag is converted to Joule heating in the conductor. Real systems can also have smaller additional losses, but resistive heating from the induced currents is the principal energy sink represented by the simulator.

Does braking force always keep increasing with speed?

No. This simulator intentionally uses a low-speed linear-drag approximation. In real eddy-current brakes, field distortion, finite geometry and skin effect can make the force depart from linear behavior; many high-speed designs show a peak braking force at a characteristic speed. The location and shape of that peak are geometry dependent and are not predicted by this model.

Drawing No. EH–EE–006 // Electrical Engineering

Eddy Current & Magnetic Braking Simulator

Reviewed August 2026

A magnet dropped down a copper pipe takes seconds to fall a metre. Nothing touches, nothing is magnetic except the magnet, and yet the drag is real and grows with speed. Change the field, the metal and the geometry and watch the induced current loops, the braking force and the heat appear.

Educational scaling model. The tube, disc and decay modes use a low-speed linear-drag approximation in which induced currents do not significantly reshape the applied field. The dimensionless coupling factor k collects unresolved field shape, current-return path, air-gap and finite-geometry effects and must be calibrated separately for a real geometry. The model does not predict the high-speed force peak of a real brake. The skin-depth tab is a separate sinusoidal-field illustration for non-magnetic conductors.

How eddy current braking works

A changing magnetic flux induces closed currents inside a conductor. Those eddy currents create their own magnetic field, and Lenz's law makes the resulting force oppose the relative motion that created them. The mechanical energy removed by that drag is dissipated mainly as resistive heat in the conductor.

1. Change the fluxMove a magnet past a conductor or move the conductor through a magnetic field.

2. Induce eddy currentsFaraday's law drives circulating currents through the conducting material.

3. Oppose the motionLenz's law sets the current direction so the magnetic reaction resists the change.

4. Dissipate energyMechanical power removed by the brake becomes predominantly Joule heating in the conductor.

Magnet falling through a conducting tube

induced current rings above and below the magnet

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Parameters

Live readout

What to watch for

B squared in the low-speed model

Within this simplified regime, drag scales with B² and linearly with conductivity and effective thickness. Halving B therefore reduces the modelled force to one quarter. Real magnetic circuits also have gap, saturation, thermal and geometry limits, so this B² law should not be extrapolated without checking the actual brake.

Terminal velocity is a force balance

In the calibrated linear model, the falling magnet approaches the speed at which magnetic drag equals weight. More mass raises that terminal speed; stronger coupling lowers it. The absolute value depends strongly on the real tube/magnet geometry represented here only by k.

The brake fades near zero speed

In the linear regime, braking force tends toward zero with relative speed, so the speed-decay model is exponential and has no static holding force. Applications that must stop precisely or hold position need another mechanism for the final stop/hold.

Copper is not automatically better

Copper conducts about 1.6 times better than aluminium but is three times denser. For a rotating disc, where the metal has to be accelerated too, aluminium often wins on braking per kilogram even though it loses on braking per millimetre.

Mechanical energy becomes mainly Joule heat

Watch the power readout. The model sends the mechanical power removed by magnetic drag into resistive heating of the conductor. Real assemblies can have additional smaller loss paths. Conductivity also changes with temperature, so thermal design matters in continuous or repeated braking.

Skin depth is an exponential scale

The last tab shows the standard sinusoidal-field skin-depth concept: current density falls to about 37% of its surface value after one δ. Material deeper than several skin depths contributes progressively less, not literally zero. In a moving eddy brake the effective excitation frequency depends on speed and magnetic-pole geometry, so this tab is illustrative rather than directly coupled to the other modes.

Where this is used

Application	Why eddy braking	What it is paired with
Roller-coaster brake runs	Contactless and wear-free at the braking interface; permanent-magnet systems can be designed normally-on without external braking power	Independent safety architecture and friction/mechanical brakes where final stopping or holding is required
Rail retarders and some high-speed rail braking systems	Can shed substantial energy at speed without frictional contact at the braking interface	Friction and/or regenerative braking, especially at low speed and for holding
Gym and rehabilitation equipment	Smooth, silent, infinitely adjustable by moving the magnet closer	Nothing — resistance falling to zero at rest is desirable here
Analogue energy meters and speedometers	The disc's drag is proportional to speed, so the deflection reads velocity directly	A hairspring providing the restoring torque
Dampers on instruments and balances	Kills oscillation without adding stiction	Nothing; damping that vanishes at rest is exactly what is wanted
Metal separation and sorting	Non-ferrous metals are repelled by a rapidly changing field and thrown clear of the stream	Conveyor and magnetic drum

The pattern. Eddy braking is chosen wherever wear, dust, contamination or fade matters more than the ability to hold a stationary load.

Frequently asked questions

Practical interpretation of the model and its limits.

Sources and technical basis

Faraday's law and Lenz's law set the direction and energy flow. The force/torque equations used by the interactive model are deliberately reduced low-speed scaling relations with a calibration factor; high-speed behavior requires a geometry-specific model.

OpenStax — University Physics Volume 2Electromagnetic induction, Lenz's law, eddy currents and magnetic damping.NIST — 2022 CODATA fundamental constantsCurrent CODATA reference for the vacuum magnetic permeability used in the skin-depth equation.NIST — Alloy DataTransport-property data and source literature for metallic materials. Conductivities in this simulator are representative room-temperature presets; actual alloy, purity and temperature matter.Chen et al. (2019) — peak braking force and critical speedAnalytical high-speed eddy-current braking study including skin effect, illustrating why real force can depart from the simulator's linear-speed approximation.