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Eddy Currents

Key idea: Explain how eddy currents are induced by changing flux, why they cause heating and magnetic braking, and how lamination/slits reduce losses (A Level Physics).

  • GCE A-Level H2 Physics 2027
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Learning objectives

  • Explain simple applications of electromagnetic induction, including motional e.m.f. and eddy currents.

1. Definitions (Must Know)

A. Eddy currents

Eddy currents are circulating currents induced in a bulk conductor when the magnetic flux through it changes.

2. Key Ideas (What Earns Marks)

  • Eddy currents appear when flux linkage changes: ε = -d(NΦ)/dt.
  • By Lenz’s law, eddy currents produce magnetic effects that oppose the change that created them.
  • Eddy currents cause:
    • heating (energy dissipated as I²R),
    • magnetic braking / damping (retarding forces on motion).
  • Laminating a core reduces eddy current losses by breaking up large current loops.
Syllabus link (9478)

Eddy currents are a simple application of electromagnetic induction (18f) and connect directly to transformer core losses (hysteresis + eddy currents).

3. Detailed Explanations

A. Why a falling magnet slows down in a copper tube

As the magnet falls, the magnetic flux through the copper changes. This induces eddy currents in the copper. By Lenz’s law, the magnetic field from the eddy currents opposes the change, producing an upward retarding force on the magnet.

A north-up bar magnet moves downward through a conducting copper tube. Circular eddy-current paths form above and below it, producing an upward magnetic braking force. A cause-and-effect chain shows motion causing changing flux, eddy currents, slower fall and heating.
Changing flux induces eddy currents whose magnetic effect opposes the fall; the tube warms as energy is dissipated.

B. Why eddy currents cause heating

Eddy currents flow through the resistance of the conductor. Electrical energy is dissipated as thermal energy at a rate ∝ I²R, so the metal warms up.

4. Common Mistakes

  • Saying eddy currents happen just because “a magnet is near” (you need a change in flux linkage).
  • Mixing up “eddy currents oppose motion” with “eddy currents oppose the field” (they oppose the change).
  • Forgetting that heating is a real energy loss mechanism.

5. Exam Tips

  • For “explain” questions, use this chain:
    • changing flux → induced e.m.f. → eddy currents → magnetic effect opposes change → damping/braking + heating.
  • If the question asks how to reduce eddy currents: “use a laminated core” or “use slotted conductor”.

6. Worked Examples

Modelled example 1

Qualitative: why does lamination reduce losses?

Core

Problem

Explain why laminating a transformer core reduces energy loss.
Study the worked solution

  1. Identify the unwanted process

    Method

    Changing core flux induces eddy currents in the conducting core.

    Reason

    A bulk conductor provides closed paths in which induced currents can circulate.

    Working

    changing flux → induced e.m.f. → eddy currents

  2. State what lamination changes

    Method

    Thin insulated sheets break large circulating paths into smaller loops.

    Reason

    The insulation interrupts paths across the sheets, increasing their effective resistance and reducing enclosed loop area.

    Working

    bulk core → separated conducting sheets

  3. Complete the energy argument

    Method

    Smaller eddy currents cause less heating, so transformer energy loss decreases.

    Reason

    For a current path, thermal dissipation is associated with I²R.

    Working

    smaller I → less I²R heating

Guided practice 2

Copper pipe demo (qualitative)

About 5 min

Problem

A magnet falls slowly through a copper pipe but quickly through a plastic pipe. Explain the difference in terms of eddy currents.

Try this before viewing the solution

Hints

Hint 1: start with the changing quantity
As the magnet moves, consider the magnetic flux through nearby sections of pipe.
Hint 2: compare the materials
Explain both the retarding magnetic effect in copper and why a similar current cannot form in plastic.
View solution step by step

  1. Establish induction

    Method

    The moving magnet changes the magnetic flux through sections of the copper pipe, inducing eddy currents.

    Reason

    Faraday’s law requires a changing flux, not merely a nearby magnet.

    Working

    motion → changing flux → circulating current

  2. Apply Lenz’s law

    Method

    The eddy-current magnetic effect opposes the change, producing an upward retarding force.

    Reason

    The induced effect opposes the change that produced it, so it brakes the falling magnet.

    Working

    retarding force opposite the downward motion

  3. Account for material and energy

    Method

    Copper permits substantial currents and warms by I²R dissipation; plastic does not provide mobile charge carriers for such currents.

    Reason

    The magnet’s mechanical energy is transferred mainly to thermal energy in the pipe.

    Working

    copper: braking + heating; plastic: little braking

Common misconception 3

Slits and damping

Find and correct the mistake

Learner claim

A metal plate swinging through a magnetic field is strongly damped. After radial slits are cut in it, a learner predicts stronger damping because the plate has more edges. Diagnose and correct the claim.

Try this before viewing the solution

Effect of the slits

View solution step by step

  1. Reject the edge argument

    Method

    The damping decreases; the number of edges is not the controlling factor.

    Reason

    Eddy-current magnitude depends on usable closed conducting paths and their resistance.

    Working

    more edges ⇏ larger eddy currents

  2. Trace the path change

    Method

    The slits interrupt large circulating loops and leave smaller, higher-resistance paths.

    Reason

    That reduces the induced current for a given flux-change rate.

    Working

    slits → smaller loops → smaller current

  3. Infer the motion change

    Method

    A smaller opposing magnetic effect produces weaker damping and less energy dissipation.

    Reason

    Lenz’s-law braking is weaker when the eddy currents are smaller.

    Working

    smaller current → smaller retarding force

Examiner practice 4

Desirable vs undesirable eddy currents

4 marks

Examination question

Give one situation where eddy currents are useful and one where they are unwanted. Explain the relevant effect in each case. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Name a useful situation

    1 mark

    Method

    Eddy-current braking or damping is useful, for example in an eddy-current brake.

    Reason

    The question requires a valid application, not just the phrase “magnetic braking”.

    Working

    useful example: eddy-current brake

  2. Explain the useful effect

    1 mark

    Method

    Induced currents create a magnetic effect opposing the motion.

    Reason

    Lenz’s law supplies a non-contact retarding force.

    Working

    changing flux → opposing magnetic effect

  3. Name an unwanted situation

    1 mark

    Method

    Eddy currents are unwanted in a transformer core.

    Reason

    The core carries changing flux and can support circulating currents.

    Working

    unwanted example: transformer core

  4. Explain the unwanted effect

    1 mark

    Method

    The currents dissipate energy as heat and reduce transformer efficiency.

    Reason

    The conducting core has resistance, so eddy currents cause thermal loss.

    Working

    I²R heating

Challenge 5

Scaling idea (rates and heating)

Minimal support

Independent transfer

In an induction setup, the rate of change of flux linkage triples while the resistance of the eddy-current paths stays roughly constant. Determine the factors by which (i) induced e.m.f. magnitude, (ii) eddy current magnitude and (iii) heating power change.

Try this before viewing the solution

Hints

Hint 1: link the three relations
Use Faraday’s law, I = ε/R and P = I²R in sequence.
View solution step by step

  1. Scale induced e.m.f.

    Method

    The induced e.m.f. magnitude triples.

    Reason

    Faraday’s law makes |ε| proportional to the rate of change of flux linkage.

    Working

    |ε| ∝ |d(NΦ)/dt|

  2. Scale current

    Method

    The eddy current magnitude triples.

    Reason

    Resistance is stated to remain roughly constant, so I = ε/R.

    Working

    I_new/I_old = 3

  3. Scale heating

    Method

    The heating power increases by a factor of nine.

    Reason

    At constant resistance, power depends on the square of current.

    Working

    P_new/P_old = (I_new/I_old)² = 3² = 9

7. Mind Stretchers

Mind stretcher 1: Motion dependenceExtension

A metal plate swings through a magnetic field. Explain why the damping is strongest when the plate is moving fastest.

Show Answer

Faster motion causes a larger rate of change of flux linkage.

So the induced e.m.f. is larger, eddy currents are larger, and the opposing magnetic effect (and energy dissipation) is stronger.

Mind stretcher 2: Energy transferExtension

A conductor experiences eddy-current braking. Where does the kinetic energy go, and why must an external agent do work to keep it moving at constant speed?

Show Answer

Eddy currents dissipate energy as heat in the conductor (I²R), so kinetic energy is converted mainly to thermal energy.

By Lenz’s law, the induced effects oppose the motion, creating a retarding force. To maintain constant speed, an external agent must provide an equal forward force and do work, supplying the energy that ends up as heat.

Continue with the next resource in this course.

Course and syllabus information
Course
GCE A-Level H2 Physics
Edition
GCE A-Level H2 Physics 2027