Q.A thick sheet of copper is allowed to swing like a pendulum between the pole pieces of a strong magnet. Explain why it comes to rest after only a few oscillations, and explain why cutting narrow slots in the sheet (so that it looks like a comb) lets it swing for a much longer time.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Eddy Currents
Eddy Currents: The Intuition
Imagine you have a solid metal plate — say, copper or aluminium — and you bring a strong magnet near it. If you move the magnet quickly, something strange happens: the metal plate seems to resist the motion. It feels like the plate is trying to push back against the magnet. And if you keep moving the magnet, the plate gets warm.
Why? The metal is a conductor. When the magnetic field through any part of it changes, that changing field induces an electric field inside the metal itself. That induced electric field pushes the free electrons in the metal, making them flow in closed loops — like tiny whirlpools of current. These are eddy currents.
The name comes from the way they swirl, like eddies in a river. They are not confined to a wire; they circulate freely within the bulk of the conductor.
The Precise Statement
Eddy currents are loops of electrical current induced within a conductor by a changing magnetic field, due to Faraday's law of electromagnetic induction. They flow in closed paths perpendicular to the direction of the magnetic flux.
The key points:
- They arise only when the magnetic flux through the conductor changes — either because the magnet moves, the conductor moves, or the field itself changes with time.
- They are not limited to a specific path; they spread throughout the conductor, following the path of least resistance for the induced electric field.
- Their magnitude depends on three things: the rate of change of flux, the conductivity of the material, and the thickness of the conductor.
Why They Matter: Two Major Effects
1. Heating (Joule Heating)
Eddy currents are real currents, so they encounter electrical resistance. As they flow, they dissipate energy as heat. This is why the metal plate gets warm when you move a magnet near it.
In many devices — like transformers and electric motors — eddy currents are wasteful. They convert useful electrical energy into unwanted heat, reducing efficiency. Engineers fight this by laminating the core: slicing it into thin sheets insulated from each other, which breaks the path of the eddy currents and shrinks them.
2. Electromagnetic Damping
Remember the feeling of resistance when you moved the magnet? That is Lenz's law in action. The eddy currents create their own magnetic field that opposes the change that caused them. So if you try to move a magnet toward a metal plate, the induced eddy currents produce a field that pushes the magnet back. If you try to pull it away, they pull it back.
This is electromagnetic damping — a non-contact braking force. It is used in:
- Magnetic brakes in trains and roller coasters (no friction, no wear)
- Galvanometers (a metal former around the coil damps the pointer's oscillations)
- Energy meters (a rotating aluminium disc is slowed by a permanent magnet)
A Simple Experiment to See It …
The solid sheet develops large eddy currents that damp its swing quickly; slotting breaks up their loops, so it swings longer. …
Why the solid sheet stops quickly. As the copper sheet swings through the region between the magnet's pole pieces, the flux linked with different parts of the sheet keeps changing. Being a solid conductor with plenty of cross-sectional area available, large eddy currents are induced within its bulk. By Lenz's law, these currents create a magnetic force that opposes the sheet's own motion through the field -- a retarding (damping) force -- which does negative work on the sheet at every swing, rapidly draining its mechanical (kinetic + potential) energy and converting it into I2R heat within the sheet. The oscillation dies out after only a few swings. …
Apply Lenz's law to identify the damping force from eddy currents, then explain slotting as a resis …
- Attributing the damping to ordinary air resistance/friction rather than eddy-current braking. …
- CBSE 2024Set A1 markMCQQ.On oscillating any metallic sphere in the magnetic field, its oscillatory motion is (A) Accelerated (B) Damping (C) Uniform (D) None of these
›Reveal solutionSolution
Eddy currents induced in the moving metal oppose the motion, so the oscillation is damped (electromagnetic damping).
When a metallic sphere oscillates in a magnetic field, the flux through the metal changes continuously. By Faraday's law this induces circulating eddy currents in the body of the metal. By Lenz's law these currents always flow in a direction that opposes the change producing them — i.e. they oppose the motion of the sphere.
…
- CBSE 2024Set ANNUAL1 markMCQQ.Eddy currents are produced in(a) induction furnace(b) electromagnetic brakes(c) speedometer(d) all of these.
›Reveal solutionSolution
Eddy currents are induced circulating currents in any bulk conductor experiencing a changing flux, and they are deliberately exploited in all three listed devices.
Eddy currents arise whenever a conductor experiences a time-varying magnetic flux (Faraday's law), inducing loops of current within the body of the conductor itself (not confined to a wire), which by Lenz's law oppose the change and dissipate energy as heat (I2R loss) or produce a retarding force.
- Induction furnace: a high-frequency alternating magnetic field induces strong eddy currents in a metal sample placed inside a coil; the resistive (I2R) heating from these currents melts the metal — used industrially to melt metals without any external heat source touching them. …
- CBSE 2022Set ANNUAL1 markMCQQ.Eddy currents are used in :(a) Magnetic braking in trains(b) Induction furnace(c) Electromagnetic damping(d) All of the above
›Reveal solutionSolution
Eddy currents are induced circulating currents in a bulk conductor moving through (or exposed to a changing) magnetic field; their heating and opposing-force effects are put to practical use in several devices.
Eddy currents are induced whenever a conductor experiences a changing magnetic flux, and by Lenz's law they oppose the change that produces them. This gives useful applications:
- Magnetic braking in trains: electromagnets are switched on near the rails/wheels; the induced eddy currents oppose the motion, bringing the train to a smooth stop without mechanical contact wear. …
- CBSE 2022Set ANNUAL1 markQ.Changing magnetic fields can set up current loops in nearby metal bodies. They dissipate electrical energy as heat. Such currents are ____.
›Reveal solutionSolution
Currents induced in the bulk of a conductor by a changing magnetic flux, which dissipate energy as heat, are called eddy currents.
Whenever the magnetic flux linked with a piece of metal (not just a wire loop) changes with time, Faraday's law of electromagnetic induction still applies locally within the bulk of the conductor. This sets up circulating (swirling, or 'eddy'-like) induced currents inside the metal body itself. Because the metal has finite resistance, these currents dissipate electrical energy as heat (I²R loss) — this is used usefully in ind …
- CBSE 2020Set 55/1/11 markQ.Laminated iron sheets are used to minimize ___________ currents in the core of a transformer.
›Reveal solutionSolution
The core of a transformer is laminated with insulated iron sheets to reduce eddy currents, which otherwise cause wasteful heating and energy loss.
The key idea is that a changing magnetic field in the core induces circulating currents within the iron itself — these are called eddy currents. If the core were a solid block of iron, these currents would be large because the iron offers a low-resistance path. The currents would flow in big loops, generating heat (I²R loss) and reducing the transformer’s efficiency.
Laminating the core — stacking thin sheets of iron, each coated with a thin insulating layer — breaks the core into many small, isolated pieces. This forces the eddy currents to stay within each thin sheet instead of flowing across the entire core. Because the cross-sectional area available for current flow is drastically reduced, the resistance to eddy currents increases, and the magnitude of the induced current drops sharply. The result is much lower energy loss as heat.
Let’s walk through the reasoning step by step.
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What causes eddy currents?
A transformer works by passing an alternating current through the primary coil, which creates a changing magnetic flux in the iron core. According to Faraday’s law of electromagnetic induction, this changing flux induces an emf in any conductor it links — including the core material itself. Since iron is a conductor, this induced emf drives circulating currents within the core.
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Why are eddy currents a problem?
These currents flow in closed loops inside the core, dissipating electrical energy as heat (Joule heating). This heat is wasted energy — it doesn’t contribute to the transformer’s output — and can also raise the temperature dangerously, damaging insulation or reducing the transformer’s lifespan.
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How does laminating help?
Instead of a single solid block, the core is built from many thin sheets of iron, each separated by a thin insulating layer (often varnish or an oxide coating). The insulation prevents current from flowing from one sheet to the next.
- In a solid core, eddy currents can flow in large loops spanning the entire cross-section.
- In a laminated core, each sheet is electrically isolated, so eddy currents are confined to the tiny cross-section of that single sheet. …
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