Physics • Year 12 • Module 6 • Lesson 19
Eddy Currents and Induction Applications
Lock in core vocabulary, the definition of eddy currents, Lenz’s Law opposition, and the purpose of lamination before tackling harder problems.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: eddy current, magnetic braking, lamination, Lenz’s Law, electromagnetic induction, magnetic flux, conductor, terminal velocity. 8 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The process by which a changing magnetic field produces an emf in a nearby conductor. | |
| 1.2 | Loops of electric current induced within a bulk conductor by a changing magnetic flux. | |
| 1.3 | The principle that an induced current always flows in a direction that opposes the change in magnetic flux that caused it. | |
| 1.4 | The product of magnetic field strength and the area of the surface it passes through, measured in webers (Wb). | |
| 1.5 | Dividing a metal core into thin insulated sheets to break up eddy current loops and reduce resistive heating. | |
| 1.6 | A braking method using induced eddy currents to create a drag force on a moving conductor without physical contact. | |
| 1.7 | A material that allows electric current to flow freely through it. | |
| 1.8 | The constant speed reached by a falling object when the drag force equals the gravitational force. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Eddy currents are induced by a static (non-changing) magnetic field inside a conductor. T / F
2.2 By Lenz’s Law, eddy currents create a magnetic field that opposes the change in flux that produced them. T / F
2.3 A magnet falls faster through a copper pipe than through a plastic pipe of the same dimensions. T / F
2.4 Cutting a copper pipe lengthwise (along the axis) eliminates the eddy-current braking effect on a falling magnet. T / F
2.5 Magnetic braking produces smooth, wear-free deceleration because there is no physical contact between the braking components. T / F
2.6 Laminating a transformer core increases eddy current losses by increasing the surface area of iron exposed to the magnetic field. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
conductor · eddy currents · flux · heat · kinetic · lamination · opposing · terminal velocity
When a ___________ experiences a changing magnetic ___________, Faraday’s Law predicts that an emf will be induced, driving ___________ in closed loops within the bulk material. By Lenz’s Law, these currents create an ___________ magnetic field that slows the rate of change. As a result, a magnet falling through a copper tube reaches a ___________ much lower than free fall. The ___________ energy of the magnet is ultimately dissipated as ___________ in the pipe walls. To reduce unwanted losses in transformer cores, ___________ is used to break up the large current loops into smaller, high-resistance paths.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What physical condition is required to produce eddy currents in a conductor?
4.2 Why does cutting a copper pipe lengthwise almost eliminate eddy current braking on a falling magnet?
4.3 State two advantages of magnetic braking over friction-based braking systems.
4.4 Why are transformer cores always made from laminated iron rather than a single solid iron block?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is reduced by”, “opposes”, “produces”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: changing magnetic flux · eddy currents · Lenz’s Law · magnetic braking · heat · lamination.
Q1 — Term–definition match
1.1 electromagnetic induction • 1.2 eddy current • 1.3 Lenz’s Law • 1.4 magnetic flux • 1.5 lamination • 1.6 magnetic braking • 1.7 conductor • 1.8 terminal velocity.
Q2 — True / false with correction
2.1 False. Eddy currents require a changing magnetic flux. A static (constant) field through a stationary conductor induces no emf and therefore no eddy currents.
2.2 True.
2.3 False. The magnet falls slower through the copper pipe because eddy currents in the copper create an opposing magnetic field (Lenz’s Law) that exerts an upward braking force on the magnet. Plastic is a non-conductor, so no eddy currents form and the magnet falls with only gravity and air resistance acting on it.
2.4 True. A lengthwise cut prevents eddy currents from flowing in complete closed loops, so no significant opposing field is created and the magnet falls nearly as fast as in free air.
2.5 True.
2.6 False. Lamination reduces eddy current losses by breaking the conduction path into thin insulated layers. Each layer confines the current to a much smaller loop, dramatically increasing resistance and reducing the magnitude of the eddy currents.
Q3 — Cloze paragraph
In order: conductor / flux / eddy currents / opposing / terminal velocity / kinetic / heat / lamination.
Q4.1 — Condition for eddy currents
The magnetic flux through the conductor must be changing. A static field produces no induced emf. The change can result from a moving magnetic source, a changing current, or a moving conductor entering or leaving a magnetic field.
Q4.2 — Why a lengthwise cut eliminates braking
A longitudinal cut breaks the continuous conducting path around the circumference of the pipe. Eddy currents need closed loops to flow; without a complete circuit, only tiny and ineffective displacement currents can exist. The braking force drops to near zero because no significant opposing magnetic field is generated.
Q4.3 — Advantages of magnetic braking
Any two of: (1) No physical contact between components, so no wear or friction-related degradation. (2) Smooth, speed-proportional deceleration (braking force increases with speed). (3) No brake dust or particulate emissions. (4) Low maintenance — no pads or discs to replace.
Q4.4 — Why transformer cores are laminated
A solid iron block would allow large eddy current loops across its entire cross-section. These loops have low resistance, so large currents flow, dissipating substantial energy as heat (I²R losses) and causing overheating. Lamination forces currents into thin layers of high resistance, greatly reducing both the current magnitude and the power lost.
Q5 — Sample concept map
Correct maps should include arrows such as:
- changing magnetic flux — induces → eddy currents
- eddy currents — governed by → Lenz’s Law
- eddy currents — enable → magnetic braking
- eddy currents — are dissipated as → heat
- lamination — reduces → eddy currents
- Lenz’s Law — explains the opposition in → magnetic braking
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).