Eddy Currents (Magnetic Braking)
Solid plate damps quickly in B; slotted plate barely damps. Lenz dissipation.
Key Notes
Eddy currents are circulating currents induced in a BULK conductor whenever flux through it changes — not in a wire loop, but in the material itself.
By Lenz's law, they always flow so as to oppose the change in flux — producing a force or torque that resists the motion or change creating them.
They dissipate energy as heat (I²R), which can be a loss or a feature depending on context.
Bad eddy currents: in transformer cores, motor armatures, induction-coil bodies — energy is wasted as heat. Mitigation: LAMINATE the core into thin insulated sheets so eddy paths are short.
Good eddy currents: induction cooktops (heat pots directly), electromagnetic brakes (no contact, no wear, smooth braking), metal detectors (sensing induced eddies), induction furnaces.
A swinging metallic plate between magnet poles is heavily damped by eddy currents — its kinetic energy is converted into heat. Slot the plate and the damping disappears.
Power dissipated by eddy currents in laminated cores scales as (B·f·t)² where t = lamination thickness, so thinner laminations dramatically reduce losses.
Formulas
Eddy-current power (qualitative)
B = flux density amplitude, f = frequency, t = lamination thickness, ρ = resistivity.
Force on a moving conductor
Eddy-brake retarding force — grows linearly with velocity (small v) until field saturation.
Faraday's law (in conductor)
Same Maxwell equation — applied to closed paths within the bulk material.
Important Points
Laminated cores in transformers: thin insulated steel sheets stacked together — interrupts large eddy loops, drastically cuts losses.
Eddy-current brakes are used in trains, roller coasters, free-fall amusement rides — no mechanical contact means no wear and consistent performance.
Induction cooktops only work with FERROMAGNETIC cookware (iron, steel) because the high resistivity and ferromagnetism amplify hysteresis and eddy heating. Aluminium and copper pots do NOT heat well.
A solid copper plate falling between magnet poles is strongly braked; a slotted plate of the same mass falls almost freely — slots break the eddy paths.
Eddy heating is the basis of induction furnaces used to melt metals (no flame, very fast).
Eddy currents have NO single 'circuit' — they form 3-D current distributions within the bulk.
Eddy Currents (Magnetic Braking) notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.