Year 12 Physics Module 6 ⏱ ~40 min 5 MC · 2 Short Answer Lesson 16 of 21

Eddy Currents and Induction Applications

Changing magnetic flux drives circulating currents in conductors. Those eddy currents can produce useful braking and heating, or unwanted losses. This lesson connects the model to cooktops, detectors and laminated transformer cores.

Today's hook: An induction cooktop heats a conducting pan without a flame, while a magnet falls slowly through a copper pipe. What changing-flux principle explains both effects, and where does the energy go?
0/5TASKS
Learn0 of 6 steps complete
1
Retrieve changing flux, identify closed conducting paths and separate useful applications from losses.
Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application & data response Master Mastery tasks Exam Exam-style questions
Before you read, predict

A strong magnet is dropped down a vertical copper pipe and a vertical plastic pipe of the same dimensions.

  1. In which pipe does the magnet fall faster?
  2. What force acts on the magnet in the copper pipe that does not act in the plastic pipe?
  3. 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…

Learning Intentions
goals

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
Scan these before reading
vocab
Eddy currentsLoops of electric current induced within the bulk of a conductor by a changing magnetic field.
Magnetic brakingUsing eddy currents to create a drag force that slows a moving conductor without friction.
LaminationDividing a metal core into thin insulated layers to restrict eddy current paths and reduce energy losses.
Induction cooktopA cooking surface that uses rapidly changing magnetic fields to induce eddy currents, and hence heat, directly in metal cookware.
Misconceptions to fix
✗ Wrong: Copper is magnetic, so it attracts or repels a magnet directly.
✓ Right: Copper is not ferromagnetic — it is not attracted to a magnet, and is in fact very weakly diamagnetic — but it is an excellent conductor. When a magnet moves through it, changing flux induces eddy currents, and those currents produce an opposing magnetic field, not any magnetic property of copper itself.
✗ Wrong: Laminating a transformer core makes it magnetically weaker.
✓ Right: Lamination is aimed at the conducting paths, not the magnetism: it subdivides the core so eddy currents cannot flow in large loops. The magnetic behaviour is not completely untouched — the insulating layers mean slightly less iron in the same cross-section — but the flux path is essentially preserved, which is why the technique works.

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.

Cross-lesson links: L14 established the Lenz-law direction rule. L19 applies it to eddy currents as both a problem (power loss in transformer cores) and a solution (induction heating, magnetic braking). Eddy currents are Faraday's law (L13) applied to bulk conductors rather than discrete coils.

Core Content

2
Model eddy currents as induced loops that oppose the flux change.
1
Eddy Currents and Lenz's Law
+5 XP

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:

  1. The magnet's moving field induces eddy currents in the tube walls.
  2. These currents create an opposing magnetic field (Lenz's Law).
  3. The opposing field repels the falling magnet, slowing it to terminal velocity.
  4. If the tube is cut lengthwise, eddy currents cannot flow in complete circuits, the magnet falls nearly as fast as in free air.
Key insight

Eddy currents require a closed conducting loop. A cut, slot or insulating layer interrupts large paths and usually reduces the current; the amount depends on the geometry and impedance.

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, so a cut or slot interrupts the largest paths and greatly reduces them, though smaller loops can persist. 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…

3
Connect magnetic braking to conservation of energy and $I^2R$ heating.
2
Magnetic Braking and Energy Conservation
+5 XP

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.

Energy conservation argument

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.

Within a stated low-speed operating range, faster motion can increase the changing-flux rate and braking force. This is not a universal proportional law: resistance, inductive impedance, geometry, field profile and speed all affect the current and force.

Magnetic braking: mechanical energy → electrical energy in eddy-current loops → internal energy by $I^2R$ heating. Faster flux change often produces stronger braking within a stated operating range, but there is no universal $F\propto v$ law: geometry, field profile, resistance, inductive impedance and speed all matter. The method is contactless, so mechanical wear is low.

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…

4
Explain why laminated, high-resistivity cores reduce unwanted eddy-current losses.
3
Reducing Eddy Current Losses, Lamination
+5 XP

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.

Enrichment, induction cooktops and metal detectors

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.

5
Use the induction model to compare braking, heating and detection while stating operating limits.
Interactive Tool, Electromagnetic Induction Open fullscreen ↗

Using the induction interactive: which scenario produces the largest induced eddy current in a conductor?

Key relationships

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

Material and energy limits

Copper and aluminium are useful because they conduct, not because they are intrinsically magnetic. Lenz forces transfer mechanical energy into the induced-current circuit; $I^2R$ describes the subsequent heating. Laminations or slots interrupt large current loops, increasing effective path resistance and reducing unwanted loss. Braking strength depends on changing-flux rate and circuit impedance, so “fastest means greatest” is only a qualitative local trend.

Activity 1, Analyse Applications
ApplyBand 4

Apply Faraday's and Lenz's Laws to real-world scenarios

  1. A roller coaster uses magnetic brakes at the end of the ride. Within the stated operating range, explain why the braking force is generally larger when the coaster is moving faster, and state why this is not a universal proportional law.
  2. Transformer cores are laminated. Explain what would happen if a solid iron core were used instead, and why this would reduce efficiency.
  3. (Enrichment) An induction cooktop does not heat a glass or ceramic pot. Explain why only metal pans work.

Which statement is safest about speed and magnetic-braking force?

6
Compare solid and slotted conductors, then consolidate the useful-versus-unwanted picture.
Activity 2, Slotted Pendulum Concept Check
UnderstandBand 4

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 statement is correct?

Wrap-up, Summary and Copy-into-Books

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
Practice1 retrieval set + 2 SAQs
7
Check eddy-current applications with mixed retrieval and short-answer practice.
Quick recall, eddy currents and applications
+5 XP

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.

Short Answer, 6 marks
+5 XP

ApplyBand 4(3 marks) 1. A roller coaster train uses magnetic brakes to slow at the end of the ride. Within the stated operating range, explain why braking is generally stronger at higher speed and why the relationship is not universal.

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): Within the brake's stated operating range, faster motion can increase the rate of change of flux through the conductor (1 mark). Faraday's Law then gives a larger induced EMF and, subject to the circuit impedance, a larger eddy current (1 mark). Lenz's Law gives a larger opposing force in that regime (1 mark). The relationship is not universally proportional because geometry, field profile, resistance and inductive impedance also affect the force.

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).

Reviewrevisit + module checkpoint
8
Revisit the energy pathway and complete the module-level retrieval check.
Check what actually stuck
Take the full module quiz
quiz

A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.

Start the module quiz →
How did your thinking change?

At the start you were asked about an induction cooktop and the copper-pipe experiment, what changing-flux principle explains both?

The answer is Faraday's Law + Lenz's Law. In the copper pipe, the falling magnet changes flux through the copper walls, inducing eddy currents whose opposing field slows the magnet. Plastic does not provide the same conducting paths. In the cooktop, an alternating field induces currents in the conducting pan, where $I^2R$ heating transfers electrical energy to internal energy. The glass-ceramic surface is not directly heated by eddy currents, although it may warm by contact with the pan.

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.

← Previous: Lenz's Law and Direction Next: Transformers, Theory and Operation →

Module overview · Physics · Checkpoint 4 · Module quiz