Year 12 Physics Module 6 ⏱ ~40 min 5 MC · 2 Short Answer Lesson 15 of 21

Lenz's Law and Direction

Push a north pole toward a wire loop connected to a galvanometer and the needle flicks one way. Pull the magnet away and the needle flicks the other way. Lenz's law is the direction rule that explains both observations: the induced current creates a magnetic field that opposes the change in flux, so energy is conserved.

Today's hook: A north pole approaches a loop from your side of the page. The flux through the loop is changing, so an emf is induced. Which way must the loop's own magnetic field point, and why would the opposite direction violate conservation of energy?
0/5TASKS
Learn 0 of 6 steps complete
1
Retrieve flux change, set the viewing convention and separate L14 direction reasoning from L19 applications.
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 bar magnet approaches a loop of wire with its north pole facing the loop.

  1. As the north pole approaches, does the magnetic flux through the loop increase or decrease?
  2. The induced current will create its own magnetic field. Should this induced field point toward the approaching magnet (helping it) or away from it (opposing it)?
  3. If the induced current helped the magnet instead of opposing it, what would happen to the magnet's speed?

Warm-up, electromagnetic induction occurs when…

Learning Intentions
goals

Know, Lenz's Law

  • The induced current opposes the change in magnetic flux that produced it
  • Lenz's Law gives the direction of the induced current
  • It is represented by the negative sign in Faraday's Law

Understand, Conservation of Energy

  • Lenz's Law ensures energy is conserved in electromagnetic induction
  • If the induced current aided the change, energy would be created from nothing
  • The work done against the opposing force provides the electrical energy

Can Do, Predict Direction

  • Determine the direction of induced current using the right-hand grip rule
  • Predict the direction of induced magnetic field for any flux change
  • Apply Lenz's Law to real situations
Scan these before reading
vocab
Lenz's LawThe direction of the induced current is such that it opposes the change in magnetic flux that produced it.
Induced magnetic fieldThe magnetic field created by an induced current, which opposes the original flux change.
Conservation of energyEnergy cannot be created or destroyed. Lenz's Law ensures electrical energy comes from mechanical work.
Right-hand grip ruleCurl fingers in the direction of current; thumb points in the direction of the magnetic field inside the loop.
Cross-lesson links: L13 gave the magnitude of induced EMF. L14 gives the direction, Lenz's law states that induced current always opposes the change that caused it. This is the hardest M6 conceptual question type: 'determine the direction of the induced current when the flux is increasing/decreasing.'

Core Content

2
State Lenz's law as opposition to the change in flux, then connect the minus sign to direction.
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Lenz's Law
+5 XP

Opposition is the key

Push the north pole of a bar magnet toward a coil of wire connected to a galvanometer. As the magnet approaches, you feel resistance, the coil is pushing back. Look at the galvanometer: it deflects, showing a current flowing. That current creates its own magnetic field, and by the right-hand grip rule, this field points away from the approaching north pole, repelling it. Pull the magnet away and the current reverses, now attracting the magnet back. In every case, the induced current acts to oppose whatever change is happening to the flux: this is Lenz's Law.

Faraday's Law with Lenz's Law

$\varepsilon = -N \dfrac{\Delta \Phi}{\Delta t}$

The negative sign represents Lenz's Law, it ensures the induced emf opposes the flux change.

How to apply Lenz's Law, four steps:

  1. Determine the direction of the external magnetic field.
  2. Determine whether the flux is increasing or decreasing.
  3. The induced magnetic field must oppose this change: if flux increases, the induced field opposes the external field; if flux decreases, the induced field reinforces the external field.
  4. Use the right-hand grip rule to find the direction of current that produces this induced field.
Stop & Check

A north pole of a magnet is pulled away from a loop. Is the induced current clockwise or anticlockwise when viewed from the magnet's side? Explain your reasoning using Lenz's Law.

Lenz's Law: the induced current opposes the change in flux that caused it (= the negative sign in $\varepsilon = -N\Delta\Phi/\Delta t$). 4-step method: (1) direction of $\vec{B}$; (2) flux increasing or decreasing; (3) induced $\vec{B}$ opposes the change; (4) right-hand grip rule → current direction.

Pause, copy the highlighted Lenz's Law statement and 4-step method into your book before moving on.

Lenz's Law states that the induced current:

3
Explain why the induced current cannot help the change without creating energy from nothing.
2
Lenz's Law and Conservation of Energy
+5 XP

Why the opposition is physically necessary

We just saw that Lenz's Law says the induced current opposes flux change. That raises a question: is this opposition just a convenient mnemonic, or is there a deeper physical reason for it? This card answers it → it is demanded by conservation of energy; without opposition, a perpetual motion machine would be possible.

Lenz's Law is not just a convenient rule, it is required by the conservation of energy. Imagine what would happen if the induced current aided the change instead of opposing it:

  • A magnet approaches a loop. The induced current creates a field that attracts the magnet, pulling it in faster.
  • The magnet speeds up, increasing the flux change, which increases the induced current, which pulls it even faster.
  • The magnet would accelerate indefinitely, generating infinite electrical energy from nothing.

This is a perpetual motion machine, impossible. Lenz's Law prevents this by ensuring the induced current opposes the motion. To push the magnet into the loop, you must do work against the repulsive force. This work is converted into the electrical energy of the induced current. Energy is conserved.

HSC Tip

In extended response questions, always link Lenz's Law to conservation of energy. Explain that the work done against the opposing magnetic force is the source of the electrical energy.

Lenz's Law is demanded by conservation of energy. If the induced current aided the change, the magnet would accelerate indefinitely, creating infinite energy, impossible. Work done against the opposing magnetic force = electrical energy of the induced current.

Add the highlighted energy argument to your notes before the check below.

If the induced current aided the approaching magnet instead of opposing it, energy would still be conserved.

The work done pushing a magnet into a coil against the magnetic opposition is converted into electrical energy in the circuit.

Lenz's Law is a consequence of conservation of energy.

4
Use the induction interactive to compare approaching, stationary and receding magnet cases.
Interactive Tool, Electromagnetic Induction Open fullscreen ↗

Use the interactive. A north pole approaches a loop. The induced current creates a field that:

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Apply the repeatable algorithm: external field, flux change, induced field, current direction.
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Worked Example, Applying Lenz's Law
+5 XP

Determine induced current direction in three scenarios

We just saw why Lenz's Law must hold (energy conservation). That raises a question: how do we actually apply the 4-step method to determine which direction a current flows in a specific diagram? This card answers it → work through: direction of B → increasing or decreasing flux → induced B opposes → right-hand grip rule.

A circular loop lies in the plane of the page. A bar magnet with its north pole pointing toward the loop is moved toward the loop from the left.

  • (a) Determine the direction of the induced current in the loop when the magnet approaches.
  • (b) The magnet is held stationary. What is the induced current?
  • (c) The magnet is pulled away. Determine the new direction of the induced current.
Part (a), Magnet approaching
  1. External field: The north pole creates field lines pointing to the right (toward the loop). So B points right through the loop.
  2. Flux change: As the magnet approaches, flux to the right is increasing.
  3. Induced field must oppose: It must point to the left (opposing the increase).
  4. Right-hand grip rule: Thumb points left (induced field). Fingers curl anticlockwise when viewed from the left.

Answer: Anticlockwise (viewed from the left).

Part (b), Magnet stationary

No flux change means no induced emf and no induced current. Answer: Zero current.

Part (c), Magnet receding
  1. External field still points right, but now flux is decreasing.
  2. Induced field must oppose the decrease: it points right (reinforcing the decreasing field).
  3. Right-hand grip rule: Thumb points right. Fingers curl clockwise when viewed from the left.

Answer: Clockwise (viewed from the left).

Flux increasing → induced field opposes external field (anticlockwise for N-pole approaching). Flux decreasing → induced field reinforces external field (clockwise for N-pole receding). Stationary magnet → zero flux change → zero current.

Pause, write the highlighted Lenz's Law direction results for the three scenarios into your book before moving on.

A south pole approaches a loop from the left. The induced current viewed from the left is:

Repeatable direction algorithm

1. State the viewing side and chosen loop normal. 2. Decide whether signed flux is increasing or decreasing. 3. Choose the induced field that opposes that change, not the original field. 4. Use the right-hand grip rule to convert induced-field direction to conventional-current direction as viewed from the stated side. 5. Only then infer any force or motion.

6
Consolidate direction and conservation reasoning, with eddy-current applications held as a preview of L19.
Activity 1, Simulator Exploration
ApplyBand 3

Use the interactive above and your knowledge to answer

  1. With the magnet approaching, observe the induced current direction. Use the right-hand grip rule to verify the induced field repels the approaching magnet.
  2. What changes when the magnet recedes? Explain why the induced field now attracts the receding magnet.
  3. A south pole approaches the loop. Predict the current direction and induced field direction before checking.
  4. Explain how each scenario conserves energy.

Three of these statements about Lenz's Law are correct. Pick the odd one out.

Activity 2, Falling Coil Challenge
UnderstandBand 5

Apply Lenz's Law to a more complex scenario

A coil is dropped from rest through the region between the poles of a strong magnet. As it enters the field, it experiences an upward magnetic force. As it leaves the field on the other side, it also experiences an upward magnetic force.

Explain both effects using Lenz's Law, and link each to conservation of energy.

Connect the ideas, Lenz's Law summary

Flux increasing → induced field opposes external field → induced current creates repulsive force

Flux decreasing → induced field reinforces external field → induced current creates attractive force

Flux unchanged → no induced emf, no induced current

In all cases: work done against the opposing force = electrical energy generated. Energy is always conserved.

Application preview, not the full L19 story

The same Lenz-law direction rule appears when currents are induced in a solid conductor. In a copper tube, a spinning copper disc or an electromagnetic brake, the conductor behaves like many tiny closed loops. Their induced currents create magnetic forces that oppose the motion or flux change that produced them.

For L14, keep the idea qualitative: induced currents oppose the change and conserve energy. L19 owns the detailed eddy-current applications, heating, braking design and losses.

A loop moves into a region of magnetic field. Which statement best applies Lenz's Law?

Practice 1 retrieval set + 2 SAQs
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Check direction, right-hand grip and conservation reasoning with MC and short-answer practice.
Quick recall, Lenz's Law and direction
+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(2 marks) 1. A bar magnet with its north pole facing downward is dropped toward a horizontal loop of wire. Determine the direction of the induced current in the loop as the magnet approaches (clockwise or anticlockwise when viewed from above). Show your reasoning using Lenz's Law.

1 mark: correct identification of flux change and opposing field direction · 1 mark: correct current direction with right-hand grip rule applied

AnalyseBand 5(4 marks) 2. Explain why Lenz's Law is a necessary consequence of conservation of energy. In your answer, describe what would happen if the induced current aided rather than opposed the change in flux, and explain what physical principle would be violated.

1 mark: if current aided the change, it would attract the magnet/reinforce motion · 1 mark: this would cause the magnet to accelerate indefinitely · 1 mark: this would create electrical energy from nothing (perpetual motion) · 1 mark: this violates conservation of energy; Lenz's Law ensures work done against opposition equals electrical energy produced

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 (2 marks): As the north pole approaches from above, the magnetic flux downward through the loop is increasing (flux pointing downward is increasing). By Lenz's Law, the induced magnetic field must oppose this increase, so it points upward (opposing the downward flux). Applying the right-hand grip rule: curl the fingers of the right hand so the thumb points upward (toward the induced field). The fingers curl anticlockwise when viewed from above. Answer: Anticlockwise (viewed from above) (2 marks for correct reasoning and direction).

Q2 (4 marks): If the induced current aided the change: as a north pole approached a loop, the induced current would create a field that attracted the magnet, pulling it in faster (1 mark). This would cause the magnet to accelerate, increasing the rate of flux change, which would increase the induced current, attracting the magnet even more strongly (1 mark). The magnet would accelerate indefinitely, and the loop would continuously generate ever-increasing electrical energy without any external energy input, a perpetual motion machine (1 mark). This violates conservation of energy, which states that energy cannot be created from nothing. Lenz's Law ensures the induced current always opposes the motion, so mechanical work must be done against the opposing force. This work is the source of the electrical energy. Energy is conserved (1 mark).

Review Final check
8
Review what transferred and revisit the opening conservation question.
Check what actually stuck
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quiz

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How did your thinking change?

At the start you were asked about a north pole approaching a loop from your side of the page: which way must the loop's own magnetic field point, and why would the opposite direction violate conservation of energy?

The answer: as the north pole approaches, magnetic flux through the loop from the magnet is increasing. The induced field must oppose that increase, so the face of the loop nearest the magnet behaves like a north pole and repels the approaching north pole. If the induced current instead attracted the magnet, the magnet would speed up, increasing the flux-change rate and generating still more current without an energy source. Lenz's law prevents that: mechanical work against the opposing magnetic force is converted into electrical energy and then into thermal energy in the circuit.

Extend your thinking: L19 uses this same principle for eddy-current applications such as braking and induction cooking. For L14, the key is the direction algorithm: state the viewing side, identify the flux change, choose the induced field that opposes that change, then use the right-hand grip rule.

← Previous: Faraday's Law of Induction Next: Eddy Currents and Induction Applications →

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