Physics • Year 12 • Module 6 • Lesson 19
Eddy Currents and Induction Applications
Apply your understanding of eddy current physics to real scenarios, data interpretation, and diagram critique tasks.
1. Classify eddy current effects — useful or unwanted?
Each scenario below involves eddy currents. For each row: (i) state whether the eddy currents are useful or unwanted, (ii) identify the energy transformation involved, and (iii) name one design feature used to enhance or reduce the eddy currents. 9 marks (1 per cell)
| Scenario | Useful / Unwanted? | Energy transformation | Design feature |
|---|---|---|---|
| Eddy currents in the iron core of a power transformer operating at 50 Hz | |||
| Eddy currents induced in the aluminium fins of a roller-coaster magnetic brake | |||
| Eddy currents in the solid copper rotor of a large AC induction motor |
2. Interpret graph — magnetic braking speed vs time
A 0.50 kg aluminium plate is released from rest and swings as a pendulum through a uniform magnetic field region. A sensor records the plate’s speed every 0.10 s. The graph below shows the results for both a solid plate and an identical plate with six parallel slots cut through it. 8 marks
Figure 2. Speed of a 0.50 kg aluminium pendulum plate swinging through a uniform magnetic field region. Solid line: solid plate. Dashed line: identical plate with six parallel slots. Illustrative data.
2.1 Describe the difference in the speed-time behaviour between the solid plate and the slotted plate after the peak speed. 2 marks
2.2 Using Faraday’s Law and Lenz’s Law, explain why the solid plate slows down rapidly after it enters the magnetic field region. 3 marks
2.3 Explain how cutting slots into the plate reduces the braking force, linking your answer to the concept of conducting loop size. 3 marks
3. Compare solid and laminated transformer cores
Complete the two-column table for three key features. Write a concise description that contrasts the two core designs. 6 marks (1 per cell)
| Feature | Solid iron core | Laminated iron core |
|---|---|---|
| Size of eddy current loops | ||
| Resistance of eddy current path | ||
| Energy lost as heat (I²R loss) |
4. Predict and justify — the Lenz’s Law drop tower
An Australian theme park operates a free-fall tower ride. At the end of the drop, large permanent magnets fixed to the falling carriage pass over aluminium guide rails to brake the carriage to rest. The ride engineers test three guide rail configurations:
- Rail A: Solid aluminium slab, 30 mm thick.
- Rail B: Aluminium slab with vertical slots every 20 mm (like a comb), 30 mm thick.
- Rail C: Aluminium slab, 5 mm thick (same width as A).
6 marks
4.1 Rank Rails A, B, and C from strongest to weakest braking force at the same carriage speed. Justify your ranking with reference to eddy current physics. 3 marks
4.2 The engineers notice that the braking force is much greater when the carriage is moving fast than when it is nearly stopped. Explain this observation using Faraday’s Law. 3 marks
Q1 — Classify eddy current effects
Transformer core: Unwanted / magnetic energy → heat (I²R) / lamination of the iron core reduces loop size and increases resistance.
Roller-coaster magnetic brake: Useful / kinetic energy → heat / large solid aluminium fins maximise conducting loop area and conductivity to maximise braking force.
Induction motor solid copper rotor: Unwanted (reduces efficiency) / magnetic energy → heat / using a squirrel-cage rotor with discrete conductor bars limits eddy paths and improves efficiency. Accept also: using higher-resistance alloys to reduce eddy current magnitude.
Q2.1 — Describe difference in speed-time behaviour (2 marks)
After the peak, the solid plate decelerates rapidly and approaches zero speed by approximately 1.8 s [1]. The slotted plate decelerates much more slowly, retaining a speed above 1.0 m s⁻¹ even at 2.0 s [1].
Q2.2 — Why the solid plate stops quickly (3 marks)
As the solid aluminium plate moves through the magnetic field region, the magnetic flux through it changes (Faraday’s Law) [1]. This induces an emf that drives large eddy currents in closed loops across the full width of the solid plate [1]. By Lenz’s Law, these currents create a magnetic field that opposes the plate’s motion, producing a strong retarding (drag) force that rapidly reduces the plate’s kinetic energy to heat [1].
Q2.3 — Why slots reduce the braking force (3 marks)
The slots interrupt the conducting path across the width of the plate, preventing eddy currents from flowing in large loops [1]. Eddy currents are confined to narrow strips between each pair of slots, drastically reducing the cross-sectional area of each current loop [1]. Smaller loops have higher resistance and carry smaller currents (by Ohm’s Law), so the opposing magnetic force and the rate of energy dissipation are both greatly reduced [1].
Q3 — Compare solid and laminated transformer cores
Size of eddy current loops: Solid: large loops spanning the full cross-section of the core. Laminated: small loops confined to each thin lamination.
Resistance of eddy current path: Solid: low resistance (large cross-section, short path length). Laminated: high resistance (thin, insulated layers force high-resistance paths).
Energy lost as heat (I²R loss): Solid: large currents and low resistance still result in high power loss (P = I²R — but I dominates as it is squared). Laminated: much smaller currents flow, so I²R loss is dramatically reduced.
Q4.1 — Rank the rails (3 marks)
Strongest to weakest: A > C > B [1 for correct ranking]. Rail A (solid, 30 mm) has the largest uninterrupted cross-section for eddy current loops — largest loops, lowest resistance, largest currents → strongest braking [1]. Rail C (solid, 5 mm) has the same loop width as A but much less thickness, reducing the loop area and therefore the flux linkage and eddy current magnitude → weaker braking than A [accept C slightly weaker than A]. Rail B (slotted, 30 mm) has the cross-section interrupted by slots; eddy current loops are broken into narrow, high-resistance strips → smallest currents and weakest braking [1].
Q4.2 — Braking force stronger at higher speed (3 marks)
Faraday’s Law states that induced emf is proportional to the rate of change of magnetic flux: ε = −dΦ/dt [1]. When the carriage moves faster, the magnetic flux through the aluminium rail changes at a greater rate, inducing a larger emf [1]. A larger emf drives a larger eddy current (by Ohm’s Law), which in turn creates a stronger opposing magnetic force (Lenz’s Law). Consequently, the braking force is greatest at high speed and diminishes as the carriage slows, producing smooth, self-regulating deceleration [1].