A small “push back” that explains a lot
If you have ever slid a magnet toward a coil in class and felt that weird, stubborn resistance, you have already met Lenz’s law. It’s the kind of moment IB Physics loves: simple setup, surprising result, and an explanation that connects formulas to the real world.
In IB Physics, Lenz’s law is the reason electromagnetic induction doesn’t turn into a perpetual-motion cheat code. Induced current always responds in a way that opposes the change that created it. That one idea shows up everywhere: in direction questions, in generator explanations, and in the famous negative sign inside Faraday’s law.
Quick exam checklist for IB Physics induction questions
Use this quick routine when you see coils, magnets, flux, or changing fields in IB Physics:
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Identify whether magnetic flux is increasing or decreasing (through the loop).
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Decide what magnetic field the loop would need to oppose that change.
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Use the right-hand grip rule to get the induced current direction.
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Only then reach for the math: (\varepsilon = -N,\Delta\Phi/\Delta t).
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Add an energy sentence: “External work becomes electrical energy because the induced effect resists the change.”
For structured practice, the D.4 Induction Questionbank is built to mirror the kind of reasoning IB markschemes reward.
Lenz’s law: the definition you should actually use
The clean definition is:
Lenz’s law states that the induced emf (and induced current) is in a direction that opposes the change in magnetic flux that produced it.
In IB Physics, you almost never earn full marks by quoting the definition alone. You earn marks by stating the change clearly.
Magnetic flux is:
And Faraday’s law is often written as:
That minus sign is not decoration. It is Lenz’s law encoded into the equation.
If you want a syllabus-aligned reference you can trust when wording matters, use the IB Physics Key Definitions glossary.
Why the minus sign matters in IB Physics (energy, not vibes)
Here’s the story IB Physics examiners are quietly looking for.
Imagine pushing a magnet into a coil. Flux through the coil increases. Faraday’s law says an emf is induced. Lenz’s law adds the direction: the induced current creates its own magnetic field that tries to stop the increase in flux. That means the coil behaves like it is pushing back.
That push back is energy conservation made visible. If the induced current supported the change instead of opposing it, the system would amplify itself: more motion induces more current, which strengthens the field, which encourages more motion. Free energy. Not happening.
RevisionDojo goes deeper on this reasoning (the kind that earns explanation marks) in How Does Induction Embody the Principle of Energy Conservation?.
A classic IB Physics direction example (without getting lost)
Scenario: A north pole moves toward a coil.
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What changes? Flux through the coil increases.
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What must the coil do? Create a magnetic field that opposes the increase. The easiest mental model: the coil produces a north pole facing the incoming north pole (repulsion).
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What current creates that? Use the right-hand grip rule: if your thumb points out of the coil face (toward the magnet) to represent a north pole, your curled fingers give the induced current direction. From the magnet’s side, this is typically counterclockwise.
If you want extra fluency, pair the direction logic with worked, syllabus-first notes in D.4.1 Principles of electromagnetic induction (HL only) Notes.
Where Lenz’s law shows up in real life (and in IB Physics marks)
Lenz’s law is not only “magnet into coil.” In IB Physics, it’s also how you explain devices:
Generators
A rotating coil changes flux, inducing emf. Under load, the induced currents create magnetic effects that oppose rotation. That is why generators require mechanical input power.
Transformers
Changing current in the primary coil changes flux in the core, inducing emf in the secondary. The induced effects oppose the change, linking neatly to energy transfer and losses.
Electromagnetic braking
Moving conductors through magnetic fields create eddy currents. Those eddy currents produce fields that oppose motion, creating a smooth braking force.
To connect the broader field ideas, see D.2 Electric and magnetic fields and the step-by-step Lessons for D.2 Electric and magnetic fields.
Common mistakes IB Physics students make with Lenz’s law
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Forgetting to state the change. “It opposes the change” is incomplete unless you name whether flux is increasing or decreasing.
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Mixing up field direction and current direction. First decide the induced magnetic field direction, then convert to current with the right-hand rule.
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Skipping the energy sentence. Many explanation questions are really conservation-of-energy questions in disguise.
If you want more intuition about the “nature resists change” framing, read What does Lenz's law reveal about nature's resistance to change.
Bring it home: Lenz’s law is your “reasoning engine” for IB Physics
Lenz’s law is not just another definition to store away. In IB Physics, it’s a decision-making tool: identify the flux change, oppose it, and the direction becomes inevitable. When you combine that with Faraday’s law, the negative sign stops feeling mysterious and starts feeling like a promise that energy will balance.
To turn this into exam confidence, use RevisionDojo’s full toolkit: the Questionbank for repetition, Study Notes and Cheatsheets for clarity, Flashcards for definitions, AI Chat for “check my reasoning,” Grading tools for exam-style feedback, Predicted Papers and Mock Exams for timing, and the Tutors when you want a human to spot the pattern you keep missing. Start with the D.4 Induction Questionbank and make Lenz’s law one of the easiest marks you collect.
