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D.4.4 (HL)—Lenz’s law

Syllabus
First assessment 2025
Objective
—
Level
HL

Apply Lenz’s Law

HL only

State the direction rule

Lenz’s law says the induced emf and induced current act in a direction that opposes the change in magnetic flux that produces them. It does not oppose the magnetic field itself; it opposes the change.

Use a four-step check

  1. Identify whether external flux through the coil increases or decreases.
  2. Determine the induced field needed to oppose that change.
  3. Use the right-hand rule to find current direction/polarity.
  4. Check that the induced current would require external work, consistent with energy conservation.

Interpret emf sign

A positive or negative emf is relative to the chosen loop direction and reference terminal. The sign records polarity/direction; it is not a negative amount of energy or a separate magnitude.

Common trap

Do not make the induced field reinforce an increasing flux. That would violate energy conservation and reverse the predicted current.

D.4.4 (HL) Exam Analysis

HL only

Assessment in practice

1–2 marks
How it is assessed

Questions predict induced polarity/current or explain why a loaded secondary voltage is reduced.

Command terms

State and explain / Which law

What earns marks

Identify the flux change, choose an induced field opposing that change, then use the right-hand rule and state the polarity convention.

Watch for

Opposing the field rather than the change, or applying the right-hand rule before identifying whether flux is increasing or decreasing.

Representative question

Question 1

[Maximum number: 1]

Which law is equivalent to the law of conservation of energy?

A

Coulomb's law

B

Ohm's Law

C

Newton's first law

D

Lenz's law

Retrieve the D.4 Induction Model

HL only

D.4 induction is secure when you connect geometry, rate of change and direction.

  • Magnetic flux: Φ=BA cosθ
  • Changing flux linkage induces emf by Faraday’s law
  • Motional emf: ε=BvL for a perpendicular moving conductor
  • Lenz’s law gives induced direction and reflects energy conservation
  • A rotating coil produces sinusoidal emf
  • Faster rotation shortens the period and increases peak emf when other variables are fixed