Eddy Currents and Induction Applications
In 1971, Westinghouse Electric Corporation demonstrated the world's first commercial induction range at the National Association of Home Builders show. The cooktop heats ferromagnetic cookware via eddy currents induced by an alternating magnetic field at 20–40 kHz. Efficiency is 90%, compared with 74% for gas. In Australia, induction cooktop sales reached 35% of new cooktop sales by 2023 — up from just 5% in 2015 — all powered by the same eddy current physics that Léon Foucault first observed in 1855.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A strong magnet is dropped down a vertical copper pipe and a vertical plastic pipe of the same dimensions.
- In which pipe does the magnet fall faster?
- What force acts on the magnet in the copper pipe that does not act in the plastic pipe?
- If the copper pipe were cut lengthwise (no longer a closed conducting loop), how would the magnet's fall change?
Warm-up — eddy currents are produced when a conductor experiences…
Know — Eddy Currents
- Eddy currents are induced currents flowing in loops within conductors
- They are produced by changing magnetic flux
- They create magnetic fields that oppose the change (Lenz's Law)
Understand — Applications and Losses
- Undesirable: heat loss in transformers, cores, and motors
- Desirable: magnetic braking, induction cooktops, metal detectors
- Laminations reduce losses by breaking conduction paths
Can Do — Analyse and Explain
- Explain magnetic braking using Lenz's Law
- Explain how induction cooktops heat metal pans
- Describe how lamination reduces transformer losses
Copper is a magnetic material, which is why a magnet slows inside a copper pipe.
Laminating a transformer core reduces its ability to carry magnetic flux.
Core Content
Swirling currents that oppose change
Drop a strong neodymium magnet into a thick copper tube and watch: instead of accelerating under gravity, it drifts down in slow motion, taking 3–4 seconds to traverse a 30 cm tube. Copper is not magnetic — no permanent attraction explains this. What slows the magnet is that its changing magnetic field induces swirling loops of current (eddy currents) throughout the copper. By Lenz's Law, these currents create an opposing magnetic field that pushes up against the falling magnet, reaching a terminal velocity far below free-fall speed.
Example — magnet falling through a copper tube:
- The magnet's moving field induces eddy currents in the tube walls.
- These currents create an opposing magnetic field (Lenz's Law).
- The opposing field repels the falling magnet, slowing it to terminal velocity.
- If the tube is cut lengthwise, eddy currents cannot flow in complete circuits — the magnet falls nearly as fast as in free air.
Eddy currents require a closed conducting loop. Anything that breaks or restricts the loop — a cut, a slot, an insulating layer — reduces eddy currents proportionally.
Eddy currents: induced loops of current in bulk conductors when flux changes. Lenz's Law: they oppose the change → drag force on the moving source. Require a closed conducting loop — any cut or slot eliminates them. Copper tube example: magnet drifts slowly to terminal velocity.
Pause — copy the highlighted eddy current definition and Lenz's Law link into your book before moving on.
A magnet falls through a copper tube. If the tube is cut lengthwise so it is no longer a closed conducting loop, the magnet will…
Harnessing induction for friction-free deceleration
We just saw that eddy currents oppose the change in flux that creates them. That raises a question: where does the kinetic energy of the slowing object go? This card answers it → KE converts to electrical energy in the eddy currents, then to heat — fully conserved, no friction required.
Magnetic braking converts the kinetic energy of a moving conductor into electrical energy (via eddy currents) and then into heat. It is smooth and wear-free because there is no physical contact between the braking components.
Applications: trains, roller coasters, gym equipment, sensitive laboratory balances.
As a conductor moves through a magnetic field, eddy currents are induced. By Lenz's Law, these create an opposing force (drag). The conductor slows — losing kinetic energy. That energy is converted to electrical energy in the eddy currents, which is then dissipated as heat ($I^2R$ heating). Energy is fully conserved: no mechanical energy disappears, it is transformed.
The braking force is proportional to speed: faster motion → greater rate of flux change → larger eddy currents → stronger opposing force. This is why magnetic braking is strongest when the object moves fastest — it is self-regulating.
Magnetic braking: KE → electrical (eddy currents) → heat ($I^2R$). Braking force $\propto$ speed (self-regulating via Lenz's Law). Advantages: no contact, no friction, no wear. Applications: trains, roller coasters, laboratory balances.
Pause — copy the energy chain and the self-regulating property into your book before moving on.
In magnetic braking, the kinetic energy of the moving object is ultimately converted into…
When eddy currents waste energy
We just saw that eddy currents are useful in magnetic braking. That raises a question: when are they harmful and how do we reduce them? This card answers it → in iron cores (transformers, motors) they waste energy as heat; lamination and silicon steel cut the losses.
In transformers and motors, eddy currents in the iron core waste energy as heat. Two main strategies reduce these losses.
- Lamination: The core is built from thin sheets of iron, each insulated from the next. This confines eddy currents to each thin lamination instead of flowing in large loops through the whole core. Smaller loops mean much higher effective resistance, so far less current and far less heating ($P = I^2R$, but $I$ is drastically reduced).
- High-resistance iron alloys: Some cores use silicon steel, which has higher resistivity than pure iron, further reducing eddy current magnitude.
Without lamination, transformer cores would overheat and waste significant energy. This is why all practical transformer cores are laminated — never solid blocks of metal.
Induction cooktops (not syllabus-required but common in exams): A coil beneath the ceramic surface carries high-frequency AC (20–50 kHz). The rapidly changing field penetrates the ferromagnetic base of the pot, inducing large eddy currents that produce heat directly in the pan via $I^2R$. The cooktop surface itself stays cool.
Metal detectors: A transmitter coil creates a changing magnetic field. Eddy currents induced in nearby metal create their own field, detected by a receiver coil. Non-metals produce no eddy currents and go undetected.
Lamination: thin insulated iron sheets → small eddy-current loops → high resistance → small $I$ → less $I^2R$ heating. Silicon steel: higher resistivity than pure iron → further reduces eddy current magnitude. Both strategies reduce core losses without affecting magnetic flux.
Pause — write the lamination mechanism (small loops → less heating) and silicon steel benefit into your book before moving on.
Three of these statements about transformer core lamination are correct. Pick the odd one out.
Using the induction interactive: which scenario produces the largest induced eddy current in a conductor?
Induced EMF (Faraday): $\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}$
Power lost to eddy currents: $P = I^2 R$ — reducing $I$ by lamination dramatically cuts $P$
Lenz's Law: The induced current always opposes the change in flux that produced it
Apply Faraday's and Lenz's Laws to real-world scenarios
- A roller coaster uses magnetic brakes at the end of the ride. Explain why the braking force is strongest when the coaster is moving fastest.
- Transformer cores are laminated. Explain what would happen if a solid iron core were used instead, and why this would reduce efficiency.
- (Enrichment) An induction cooktop does not heat a glass or ceramic pot. Explain why only metal pans work.
Complete the sentence: In magnetic braking, the braking force is proportional to the _____ of the moving object.
Classic demonstration — explain the difference
A solid copper pendulum swings through a magnetic field and quickly comes to rest. The same pendulum with slots cut through it (like a comb) swings for much longer. Explain why, using your knowledge of eddy currents.
A solid copper plate and a slotted copper plate of the same size swing through the same magnetic field. Which set of descriptions is correct?
Key connections
- Eddy currents are induced by changing magnetic flux in conductors (Faraday's Law).
- By Lenz's Law, they create opposing magnetic fields — basis of all magnetic braking.
- Useful: magnetic braking (trains, rollercoasters), induction cooktops, metal detectors.
- Unwanted: heat losses in transformer and motor cores — reduced by lamination.
- Lamination reduces losses by breaking up large eddy-current loops, drastically increasing effective resistance.
Key Definitions
- Eddy current: induced loop of current in bulk conductor
- Magnetic braking: drag force from eddy currents opposing motion
- Lamination: insulated thin layers to limit eddy current loops
Key Relationships
- $\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}$ (Faraday)
- Lenz's Law: opposing direction
- $P_{\text{loss}} = I^2 R$ (eddy current heating)
Useful vs Unwanted
- Useful: braking, cooking, detection
- Unwanted: core heating in transformers
- Fix: laminate the core
Critical Factors
- Closed conducting loop required
- Faster change = larger eddy current
- Slots/cuts reduce eddy currents
A fresh five-question set drawn from this lesson's bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
ApplyBand 4(3 marks) 1. A roller coaster train uses magnetic brakes to slow at the end of the ride. Explain, using Faraday's Law and Lenz's Law, why the braking force is greatest when the train is travelling fastest.
1 mark: identifies faster motion → greater rate of flux change · 1 mark: greater EMF → larger eddy currents · 1 mark: Lenz's Law — larger opposing force opposes greater motion
AnalyseBand 5(3 marks) 2. A solid copper pendulum swings through a magnetic field and stops quickly. An identical pendulum with slots cut through it swings much longer. Explain why cutting slots reduces the damping effect. In your answer, refer to eddy current loops and resistance.
1 mark: slots break up large eddy-current loops into smaller restricted paths · 1 mark: smaller loops have greater effective resistance · 1 mark: less current → smaller opposing force → less damping
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set drawn from the lesson bank.
Short Answer — Model Answers
Q1 (3 marks): When the train travels faster, the conducting plate or rail moves through the magnetic field at a greater rate, increasing the rate of change of magnetic flux through the conductor (1 mark). By Faraday's Law, the greater rate of change induces a larger EMF, driving larger eddy currents in the conductor (1 mark). By Lenz's Law, these larger eddy currents produce a greater opposing magnetic force — the braking force is therefore greatest when the train is fastest (1 mark). This makes magnetic braking self-regulating.
Q2 (3 marks): In the solid pendulum, eddy currents can flow in large loops spanning the whole plate, providing low-resistance paths for large currents (1 mark). Cutting slots interrupts these large loops, forcing any eddy currents into smaller, higher-resistance paths confined between adjacent slots (1 mark). The higher effective resistance means less eddy current flows ($I = \varepsilon/R$), producing a smaller opposing magnetic force and much less damping — the slotted pendulum swings much longer (1 mark).
Five timed questions on eddy currents and induction applications. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaAt the start you were asked about Westinghouse's 1971 induction cooktop and the copper pipe experiment — what single principle explains both?
The answer is Faraday's Law + Lenz's Law. In the copper pipe, the falling magnet continuously changes the flux through the copper walls, inducing eddy currents. Those currents create an opposing field (Lenz's Law) that slows the magnet to terminal velocity. Plastic has no free electrons — no eddy currents, no opposing force, free fall. In the induction cooktop, the rapidly alternating 20–40 kHz field changes flux through the steel pot base at enormous rates, inducing large eddy currents that heat the pot. The glass-ceramic surface is an insulator — no currents, no heating.
Cutting the copper pipe lengthwise removes the closed conducting loop, eliminating the eddy currents and restoring near-free-fall speed. The same principle explains why slotted transformer cores and laminated iron cores behave differently from solid ones.