Eddy Currents — A Complete Guide
Eddy currents are one of those topics where physics directly meets the gadgets in our daily lives. Induction stoves, electromagnetic brakes on metro trains, magnetic levitation, even the airport metal detector — each runs on the same principle. JEE Main and NEET regularly slip in conceptual questions on eddy currents because they test whether a student really understands Faraday’s law in three dimensions.
We’ll work through what eddy currents are, the physics behind them, why we sometimes want them and sometimes don’t, and where they show up in real applications. The chapter is more conceptual than numerical — most exam questions are MCQs that test pattern recognition rather than calculation.
Key Terms & Definitions
Eddy Current. A circulating current induced in a bulk conductor (a sheet of metal, a rotating disc, an aluminium block) when the magnetic flux through it changes. Unlike currents in wires, eddy currents flow in closed loops within the body of the material — like miniature whirlpools.
Lenz’s Law. The induced current always flows in a direction that opposes the change in flux that produced it. For eddy currents, this means the induced loops oppose the relative motion between the conductor and the field.
Skin Depth. At AC frequencies, eddy currents concentrate near the surface of the conductor. The depth at which current density falls to of the surface value is the skin depth.
Laminations. Thin insulated sheets stacked together (used in transformer cores) that break up eddy current loops, reducing energy losses.
Methods & Concepts
Why Eddy Currents Form
Imagine a copper plate sliding into a region with a magnetic field perpendicular to the plate. As the plate moves in, the flux through any imagined loop drawn on the plate increases. By Faraday’s law, an EMF appears around that loop, driving a current. Since the plate is a continuous sheet, infinitely many such loops exist — together they form swirling currents called eddy currents.
The induced current creates its own magnetic field that opposes the change — by Lenz’s law, this opposes the relative motion between the plate and the magnet. The plate experiences a drag force.
The Drag Force Insight
When you drop a magnet through a copper tube, it falls slowly because eddy currents in the tube create a retarding force. The magnetic field of the eddy currents pushes back on the magnet, and the magnet pushes forward on the eddy currents (Newton’s third law). The net effect: heat is dissipated in the tube and the magnet’s KE drops.
This is the same physics as the magnetic brakes used on roller coasters and freight trains — no friction pads, no wear.
Reducing Eddy Currents
In transformer cores, eddy currents are unwanted — they waste energy as heat. Two strategies reduce them:
- Laminations. Stack thin insulated sheets so loops can’t form across the bulk. Each lamination has small loops, but resistance dominates.
- High-resistivity materials. Silicon steel, ferrites. Higher means smaller , less heating.
Solved Examples
Example 1 — Conceptual (NEET pattern)
A copper plate is held in a magnetic field. It is slowly heated. What happens to the eddy currents induced when the plate is moved?
The plate’s resistivity increases with temperature, so falls. For the same EMF, induced current decreases. Eddy currents weaken with rising temperature.
Example 2 — Energy Dissipation (JEE Main pattern)
A solid metal disc rotates in a magnetic field perpendicular to its plane. Why does it slow down?
Eddy currents form in the disc due to changing flux at each point. By Lenz’s law, the magnetic force on these currents opposes the rotation, producing a torque that brakes the disc. The lost rotational KE shows up as heat from dissipation in the disc.
Example 3 — Pendulum (JEE Advanced pattern)
A copper plate swings as a pendulum between the poles of a strong magnet. Why does it stop quickly, while a slotted copper plate (with vertical cuts) keeps swinging?
The solid plate develops large eddy current loops that dissipate energy fast. The slots in the second plate break these loops into thin strips with much smaller cross-sectional area, dramatically cutting the induced current and reducing damping.
Exam-Specific Tips
JEE Main weightage: 1 conceptual MCQ in roughly half of all shifts. NEET: Almost every year, asked as a “match the column” or “applications” question. CBSE Class 12: A 2-mark or 3-mark question on uses or methods of reducing eddy currents.
When asked “why” a particular gadget uses eddy currents, link the answer to either heating (induction cooktop, dental drills) or braking (electromagnetic brakes, galvanometer damping).
Applications Reference Table
| Application | Eddy current is | Mechanism |
|---|---|---|
| Induction cooktop | Wanted | heating in iron pot |
| Magnetic brake (trains) | Wanted | Lenz’s drag force |
| Galvanometer damping | Wanted | Slows pointer to rest |
| Metal detector | Wanted | Senses induced field |
| Transformer core | Unwanted | Waste heat → use laminations |
| Motor armature | Unwanted | Reduces efficiency → use laminations |
| Inductor core | Unwanted | Increases loss → use ferrite |
Common Mistakes to Avoid
Confusing eddy currents with regular induced currents. Both come from Faraday’s law, but eddy currents flow in closed loops within bulk conductors, not along wires.
Forgetting Lenz’s law direction. The induced field always opposes the change in flux — this is the source of the drag force.
Saying laminations “cut the field”. They don’t — they cut the current loops. The magnetic field passes through laminations almost undisturbed.
Assuming eddy currents always cause loss. In an induction stove or brake, that “loss” is exactly the useful effect.
Ignoring the role of resistivity. A perfect conductor would have infinite eddy currents and no power dissipation. Real materials have finite , which determines the loss.
Practice Questions
Q1. Why are transformer cores laminated rather than solid?
To break large eddy current loops into many small ones, increasing effective resistance and reducing heat loss.
Q2. Why does a magnet fall slower through a copper pipe than through a plastic pipe?
The copper pipe develops eddy currents that exert a Lenz-law drag force on the falling magnet. The plastic pipe is non-conducting, so no eddy currents form.
Q3. In an induction cooktop, why must the cookware be ferromagnetic?
Ferromagnetic materials have high magnetic permeability, which concentrates the AC flux and produces strong eddy currents that heat the pot. Aluminium pots barely work.
Q4. Why does a galvanometer pointer come to rest quickly without oscillating?
The aluminium frame holding the coil develops eddy currents as it rotates, providing critical damping.
Q5. What happens to eddy currents at higher frequencies?
They concentrate near the surface (skin effect) and can be more intense due to faster flux changes.
Q6. Why is silicon steel preferred over plain iron in transformer cores?
Higher resistivity reduces eddy current loss while preserving high permeability for the main flux.
Q7. How does an electromagnetic brake on a moving train work?
A magnet on the train induces eddy currents in the rail (or vice versa). The Lenz drag force decelerates the train without mechanical contact.
Q8. Can eddy currents flow in a vacuum?
No — they require a conductor. No charges, no current.
FAQs
What’s the difference between eddy currents and self-induction currents? Self-induction is a single-loop concept (current in a coil opposes its own change). Eddy currents are bulk-conductor loops that swirl in three dimensions.
Why don’t eddy currents heat all metals equally? Heating . High- metals have weak heating; high-resistivity metals heat more. Iron heats well; copper barely does.
Are eddy currents dangerous? Generally no, but in MRI machines they create patient discomfort and image artefacts. Specific gradient designs minimise them.
Do eddy currents violate energy conservation? Never — the energy dissipated as heat comes from whatever is moving the magnet or carrying AC current. Lenz’s law is energy conservation in action.
How do metal detectors at airports use eddy currents? A coil emits a pulsed magnetic field. Any nearby metal develops eddy currents whose decaying field is detected by the same or another coil. Different metals decay at different rates, helping identify them.
Can eddy currents be used for non-destructive testing? Yes — eddy current testing finds cracks in metal parts. A crack distorts the eddy current pattern, which a probe coil senses. Widely used in aviation and pipelines.