Lenz's Law states that an induced current flows in the direction that opposes the change in magnetic flux that produced it. On AP Physics 2, it's the directional rule for electromagnetic induction, the physical meaning of the negative sign in Faraday's Law, and a consequence of energy conservation.
Lenz's Law tells you which way an induced current flows. When the magnetic flux through a loop changes (because the field gets stronger or weaker, the loop changes area, or the loop rotates), the induced current runs in the direction that fights that change. Flux increasing into the page? The induced current makes its own magnetic field out of the page to push back. Flux decreasing? The induced current tries to prop it up.
Here's the intuitive version: nature hates a change in flux. The induced current is the loop's protest against whatever you're doing to it. That's not arbitrary stubbornness, it's energy conservation. If the induced current helped the change instead of opposing it, you'd get a runaway feedback loop creating energy from nothing. So Lenz's Law is really conservation of energy wearing a magnetism costume. Mathematically, it lives inside the negative sign in Faraday's Law (EMF = -dΦ/dt), and Faraday's Law gives the size of the induced EMF while Lenz's Law gives its direction.
Lenz's Law sits in the electromagnetic induction portion of AP Physics 2, alongside Faraday's Law, motional EMF, and eddy currents. Any time a question asks for the direction of an induced current, or whether a magnet falling toward a loop gets attracted or repelled, you're being tested on Lenz's Law. It also connects to the course's big conceptual theme of conservation laws, since the opposition built into Lenz's Law is exactly what keeps induced currents from violating energy conservation. Expect it to show up in qualitative reasoning prompts where you have to justify a direction with a cause-and-effect chain: flux changes, induced current opposes, here's the resulting force.
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Faraday's Law (Magnetism & Electromagnetic Induction)
Faraday's Law and Lenz's Law are two halves of one idea. Faraday tells you how big the induced EMF is (proportional to the rate of flux change), and Lenz tells you which way it points. The negative sign in EMF = -dΦ/dt is Lenz's Law written in math.
Eddy Currents (Magnetism & Electromagnetic Induction)
Eddy currents are Lenz's Law happening inside a solid conductor. When a metal sheet moves through a magnetic field, swirling currents form that oppose the motion, which is why a magnet falls slowly through a copper tube. Magnetic braking is Lenz's Law doing useful work.
Lorentz Force (Magnetism & Electromagnetic Induction)
Lenz's Law predicts the induced current's direction; the Lorentz force (F = qv × B) explains the resulting push or pull. In motional EMF setups, you can derive Lenz's prediction from scratch by tracking the magnetic force on charges in the moving conductor. They should always agree, which makes Lorentz force your best tool for double-checking a Lenz's Law answer.
Conservation of Energy (course-wide theme)
Lenz's Law isn't a separate rule of nature, it's energy conservation applied to induction. The work you do pushing a magnet against the induced field's opposition is exactly the electrical energy that shows up in the circuit. If the induced current reinforced the change instead, you'd get free energy, which physics doesn't allow.
Lenz's Law shows up most often in qualitative reasoning, where you predict and justify the direction of an induced current. The setup is usually a loop near a changing field, a magnet moving toward or away from a coil, or a conducting bar sliding on rails. Your job is to build the chain: state how the flux is changing, invoke Lenz's Law to get the induced current's direction (right-hand rule for the induced field), then determine any resulting force. The 2025 exam's FRQ 1 built on exactly this territory, with parallel current-carrying wires and the magnetic fields they create, the foundation you need before layering on induction. A reliable shortcut for MCQs: the induced effect always opposes the motion or the change, so a magnet approaching a loop gets repelled and a magnet leaving gets attracted. If your answer ever has the induced current speeding up the change, you've made a sign error somewhere.
Faraday's Law is quantitative: it says the induced EMF equals the rate of change of magnetic flux through the loop. Lenz's Law is directional: it says that induced EMF drives current in the direction that opposes the flux change. On the exam, use Faraday when the question asks 'how much EMF or current,' and use Lenz when it asks 'which way does the current flow' or 'is the force attractive or repulsive.' They're not competing laws; Lenz is the meaning of the minus sign in Faraday's equation.
Lenz's Law says an induced current always flows in the direction that opposes the change in magnetic flux that created it.
Lenz's Law gives direction and Faraday's Law gives magnitude; together they fully describe an induced EMF.
The opposition in Lenz's Law is required by conservation of energy, because a current that reinforced the change would create energy from nothing.
A magnet moving toward a conducting loop is repelled by the induced current, and a magnet moving away is attracted, since the loop always resists the change.
Eddy currents and magnetic braking are real-world applications of Lenz's Law, where induced currents in a conductor oppose its motion through a field.
On FRQs, justify direction with a full chain of reasoning: identify the flux change, apply Lenz's Law for the induced current, then use the right-hand rule for forces and fields.
Lenz's Law states that the current induced in a conductor flows in the direction that opposes the change in magnetic flux causing it. It's the rule you use to find the direction of any induced current on the exam.
Essentially yes. Faraday's Law, EMF = -dΦ/dt, gives the size of the induced EMF, and the negative sign encodes Lenz's Law, telling you the induced current opposes the flux change. Lenz's Law is the physical interpretation of that sign.
No, it's the opposite. Lenz's Law exists because of energy conservation. The induced current must oppose the change, since a current that reinforced it would amplify itself endlessly and generate energy from nothing. The work done against the opposition becomes the electrical energy in the circuit.
Lenz's Law is a directional rule about induced currents in loops with changing flux. The Lorentz force, F = qv × B, is the force on a moving charge in a magnetic field. In motional EMF problems they connect: the Lorentz force on charges in a moving conductor produces exactly the current direction Lenz's Law predicts.
The falling magnet changes the flux through each ring of the tube, inducing eddy currents. By Lenz's Law, those currents create magnetic fields that oppose the magnet's motion, so the tube effectively brakes the magnet. This is a classic conceptual question for testing Lenz's Law.