How Does a Brake Work Without Touching Anything? Magnets and a Moving Sheet
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A changing magnetic field drives circulating currents inside a conductor, and those currents produce a field of their own that opposes the change. The effect brakes, heats and detects.
Why the currents appear
A conductor experiencing a changing magnetic field has a voltage induced around any closed path within it, which drives current around that path, and in a solid block those paths are loops of circulating current within the metal rather than a defined circuit. The direction of each loop is such that the magnetic field it produces opposes whatever change caused it, which is a general rule and is why the effect always resists. A magnet moving past a conductor therefore meets a force opposing its motion, and a conductor moving past a magnet meets the same.
The simple demonstrations
Two classroom experiments show it unambiguously:
- •A magnet dropped down a copper tube falls slowly, taking seconds
- •A non-magnetic slug of the same size drops straight through
- •The tube is not magnetic, which surprises people
- •A swinging metal plate between magnet poles stops almost at once
- •Cutting slots in that plate to break the loops restores the swing
- •Both effects vanish if the motion stops
Where it brakes and where it heats
The same effect is exploited for two opposite purposes. Braking uses the opposing force directly, with no contact and therefore no wear, which is why it is used on roller coasters, on heavy vehicles as a supplementary retarder, on trains, and on exercise machines where resistance is set by moving a magnet closer. Its limitation is that the force falls to nothing as speed falls, so it cannot hold anything stationary and a friction brake is still needed at the end. Heating uses the energy those currents dissipate, which is the basis of induction hobs, of industrial induction furnaces melting metal without a flame, and of induction heating used to shrink-fit bearings.
Why coins and metals are sorted this way
A useful application separates materials that look identical and is worth explaining because it is encountered daily. A conductor moving past a magnet experiences a force, and how strong that force is depends on how well the material conducts and on its density, so different metals moving at the same speed are deflected by different amounts. Vending machines use that to reject slugs that match a coin in size and weight but not in composition. Recycling plants use a rapidly rotating magnetic drum to fling aluminium off a conveyor while plastics and glass continue straight, which separates non-magnetic metal that a simple magnet cannot pick up at all.
Where it is a nuisance
The same currents waste energy wherever a changing field meets a conductor that is not meant to carry current, and dealing with that shapes how electrical machines are built. A transformer core sitting in a rapidly changing field would carry large circulating currents and heat up, so cores are built from thin sheets insulated from each other and stacked, which breaks the loops while leaving the magnetic path intact. Motors and generators use the same construction for the same reason. Metal detectors work by deliberately inducing such currents in buried objects and detecting the field they produce, which is the effect turned into an instrument.
The takeaway
A changing magnetic field drives circulating currents inside a conductor, and those currents produce a field opposing the change, so a magnet and a conductor moving relative to each other always resist. That gives contactless braking with no wear, which cannot hold anything still since the force vanishes at rest, and it gives induction heating. Transformer cores are built from insulated sheets to break the same loops.