Physics • Year 12 • Module 6 • Lesson 14
Lenz's Law and Direction
Lock in the key vocabulary, the four-step method for finding induced current direction, and the conservation of energy link before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: Lenz's Law, induced magnetic field, conservation of energy, right-hand grip rule, magnetic flux, opposition principle, Faraday's Law, mechanical work, perpetual motion machine, negative sign. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The direction of an induced current is always such that its magnetic field opposes the change in magnetic flux that produced it. | |
| 1.2 | The magnetic field created by the induced current, which acts to oppose the original flux change. | |
| 1.3 | The principle that energy cannot be created or destroyed; Lenz's Law is a direct consequence of this. | |
| 1.4 | Curl the fingers of the right hand in the direction of current flow; the extended thumb points in the direction of the magnetic field inside the loop. | |
| 1.5 | The product of the magnetic field strength (component perpendicular to the area) and the area of a loop: Φ = B A cosθ. | |
| 1.6 | The idea that an induced effect always acts to resist the cause that produced it; the core concept in Lenz's Law. | |
| 1.7 | The law relating induced emf to the rate of change of magnetic flux; the negative sign in this law embodies Lenz's Law. | |
| 1.8 | The work done by an external agent against the opposing magnetic force when causing a flux change; this is the source of the electrical energy generated by induction. | |
| 1.9 | A hypothetical device that produces continuous work with no energy input; Lenz's Law shows this is impossible. | |
| 1.10 | The mathematical symbol in Faraday’s Law (ϵ = −NΔΦ/Δt) that represents Lenz’s Law; it indicates the induced emf opposes the flux change that produced it. |
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 When magnetic flux through a loop is increasing, the induced current creates a magnetic field that reinforces the external magnetic field. T / F
2.2 When a bar magnet is held stationary inside a loop of wire, the induced emf is at its maximum value. T / F
2.3 Lenz's Law is a consequence of the conservation of energy. T / F
2.4 A south pole approaching a loop from the left produces the same current direction as a north pole approaching from the left. T / F
2.5 The negative sign in Faraday's Law (ϵ = −N ΔΦ/Δt) mathematically represents Lenz's Law. T / F
2.6 If the induced current aided the change in flux instead of opposing it, the magnet would decelerate as it entered the loop. 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:
conservation · decreasing · increasing · mechanical · negative · opposes · right-hand grip · work
Lenz's Law states that the induced current always produces a magnetic field that ___________ the change in magnetic flux that caused it. When flux is ___________, the induced field points in the opposite direction to the external field. When flux is ___________, the induced field reinforces the external field. Use the ___________ rule to find the current direction from the induced field direction. The ___________ sign in Faraday's Law (ϵ = −N ΔΦ/Δt) encodes this opposition. Lenz's Law is required by the ___________ of energy: the ___________ done against the opposing force provides the electrical energy. If the law were reversed, ___________ work could be extracted indefinitely — an impossible perpetual motion machine.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 State Lenz's Law in your own words. What does it tell you about the direction of an induced current?
4.2 List the four steps used to determine the direction of an induced current using Lenz's Law.
4.3 Explain why Lenz's Law is required by conservation of energy. What would happen if the induced current aided the flux change instead of opposing it?
4.4 What is the source of the electrical energy generated by electromagnetic induction?
5. Complete the Lenz's Law summary table
Fill in every empty cell. 10 marks (1 each)
| Scenario | Flux change | Direction of induced B field (relative to external B) | Effect on magnet | Current direction (viewed from magnet side) |
|---|---|---|---|---|
| North pole approaching loop | Increasing (rightward) | |||
| North pole receding from loop | Same direction as external B | |||
| South pole approaching loop | Repels magnet | |||
| Magnet stationary inside loop | Zero |
Q1 — Term–definition match
1.1 Lenz's Law • 1.2 induced magnetic field • 1.3 conservation of energy • 1.4 right-hand grip rule • 1.5 magnetic flux • 1.6 opposition principle • 1.7 Faraday's Law • 1.8 mechanical work • 1.9 perpetual motion machine • 1.10 negative sign.
Q2 — True / false with correction
2.1 False. When flux is increasing, the induced field opposes (acts against) the external field — it does not reinforce it. The induced field points opposite to the external field to resist the increase.
2.2 False. When the magnet is stationary, there is no change in flux (ΔΦ/Δt = 0), so there is no induced emf and no induced current. Induction requires a changing flux.
2.3 True.
2.4 False. A south pole approaching from the left creates a field pointing to the right (from south to north, inside the magnet, but the external field from a south pole points toward the south pole, i.e. to the right away from the loop when the south pole is on the left side). Wait — the external B field lines point into a south pole. So flux through the loop is increasing to the left (field lines entering south pole point leftward). The induced field must oppose (point rightward), giving a clockwise current viewed from the left. For a north pole approaching from the left, flux increases to the right; induced field points left; current is anticlockwise. The two directions are therefore opposite, not the same.
2.5 True.
2.6 False. If the induced current aided the change in flux, the induced field would attract the approaching magnet, causing it to accelerate indefinitely — not decelerate. This is why such a scenario violates conservation of energy and is impossible.
Q3 — Cloze paragraph
In order: opposes / increasing / decreasing / right-hand grip / negative / conservation / work / mechanical.
Q4.1 — Lenz's Law stated
The direction of an induced current is always such that the magnetic field it produces opposes the change in magnetic flux that created it. It tells you the induced current will always act to resist the cause of its induction.
Q4.2 — Four-step method
(1) Identify the direction of the external magnetic field through the loop. (2) Determine whether the flux is increasing or decreasing. (3) The induced magnetic field must oppose this change: if flux increases, induced B points opposite to external B; if flux decreases, induced B points in the same direction as external B. (4) Apply the right-hand grip rule: thumb in the direction of induced B — fingers curl in the direction of the induced current.
Q4.3 — Lenz's Law and conservation of energy
If the induced current aided the flux change, the induced field would attract an approaching magnet, causing it to accelerate. This increased speed would increase the flux change, which would increase the current, accelerating the magnet further — generating electrical energy from nothing. This violates conservation of energy. Lenz's Law (opposition) ensures the work done by an external agent against the opposing magnetic force equals the electrical energy produced.
Q4.4 — Source of electrical energy
The electrical energy comes from the mechanical work done by the external agent that causes the flux change (e.g. the person pushing the magnet into the loop, or the engine driving a generator). Energy is converted from mechanical to electrical form; no energy is created from nothing.
Q5 — Lenz's Law summary table
Row 1 (N approaching): Flux change = Increasing (rightward) • Induced B = Opposite to external B (points left, repelling) • Effect on magnet = Repels approaching magnet • Current = Anticlockwise (viewed from magnet side / left).
Row 2 (N receding): Flux change = Decreasing (rightward) • Induced B = Same direction as external B (points right, given) • Effect on magnet = Attracts receding magnet • Current = Clockwise (viewed from left).
Row 3 (S approaching): Flux change = Increasing (leftward, since field lines point into south pole) • Induced B = Opposite to external B (points rightward) • Effect on magnet = Repels approaching magnet (given) • Current = Clockwise (viewed from left / magnet side).
Row 4 (Stationary): Flux change = Zero (no change) • Induced B = None • Effect on magnet = None • Current = Zero (given).