Question
What are eddy currents? Explain how they are produced. Give three applications and two methods of minimising unwanted eddy currents.
(NCERT Class 12, Electromagnetic Induction)
Solution — Step by Step
Eddy currents (also called Foucault currents) are loops of electric current induced within a bulk conductor when it is exposed to a changing magnetic field. They flow in closed loops within the conductor, in planes perpendicular to the magnetic field.
They are called “eddy” currents because they resemble eddies (swirling patterns) in water.
By Faraday’s law, whenever the magnetic flux through a conductor changes, an EMF is induced. In a bulk piece of metal (not a wire), this EMF drives currents that flow in circular paths within the metal body itself.
The direction of eddy currents follows Lenz’s law — they always flow in a direction that opposes the change in flux that caused them. If a metal plate is moved into a magnetic field, eddy currents create their own field to oppose the entry, producing a retarding force.
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Electromagnetic braking: Used in trains and roller coasters. When a conductor moves through a magnetic field, eddy currents create an opposing force that slows it down — no physical contact needed, so no wear and tear.
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Induction furnace: A metal piece placed in a rapidly changing magnetic field develops large eddy currents. The resistance of the metal converts this current into heat ( heating), melting the metal. Used in steel and glass industries.
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Electromagnetic damping: In galvanometers, the coil is wound on a metallic frame. When the coil swings, eddy currents in the frame oppose the motion, causing the needle to settle quickly (dead-beat galvanometer).
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Speedometers: A rotating magnet induces eddy currents in an aluminium drum. The torque on the drum is proportional to speed.
In transformers and electric motors, eddy currents waste energy as heat. Two methods to reduce them:
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Lamination: The iron core is made of thin, insulated sheets (laminations) stacked together instead of a solid block. Eddy currents are confined to each thin sheet, drastically reducing their magnitude since resistance increases.
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Using high-resistivity materials: Cores made of ferrites or silicon steel have higher electrical resistance, which reduces the magnitude of eddy currents (; higher → lower ).
Why This Works
Eddy currents are a direct consequence of Faraday’s law applied to bulk conductors. A changing magnetic field doesn’t care whether the conductor is a wire loop or a solid metal block — it induces EMF either way. In a solid conductor, the “loops” are just paths through the metal where current can flow.
The energy dissipated by eddy currents comes from the kinetic energy of the moving conductor (in braking) or from the AC source (in transformers). This is why lamination is so important — without it, a significant fraction of the energy in a transformer would be wasted as heat in the core.
Alternative Method
To remember whether eddy currents are useful or harmful, think of context:
- Moving parts (brakes, dampers, speedometers): eddy currents are useful — they provide controlled resistance to motion
- Static cores (transformers, motors): eddy currents are harmful — they waste energy as heat and must be minimised
CBSE board exams consistently ask: “What are eddy currents? Give two applications” (3 marks) or “How are eddy currents minimised in a transformer?” (2 marks). NEET asks application-based MCQs — “Which of the following uses eddy currents?” with options like induction furnace, dead-beat galvanometer, etc. Both applications and prevention methods are scoring topics.
Common Mistake
Students often write that lamination “prevents” eddy currents. Lamination does NOT prevent eddy currents — it reduces them. Small eddy currents still flow within each lamination. The key is that these currents are much smaller (each sheet has higher resistance due to its thinness and the insulating layers between sheets), so the power loss () is dramatically reduced. Use “minimise” or “reduce,” not “prevent” or “eliminate.”