Physics C: E&M · Unit 5: Electromagnetism · 14 min read · Updated 2026-05-11
Lenz's Law — AP Physics C: Electricity and Magnetism
AP Physics C: Electricity and Magnetism · Unit 5: Electromagnetism · 14 min read
1. What Is Lenz's Law?★★☆☆☆⏱ 3 min
Lenz's Law is a fundamental rule of electromagnetic induction that predicts the direction of induced current and induced emf generated by a changing magnetic flux. It is often called the law of inertia for electromagnetism, because the induced effect always opposes the change that created it.
Within AP Physics C: E&M Unit 5, Lenz's Law is a core concept, accounting for 16-24% of total exam score. It appears in both multiple-choice and free-response questions, and is required for full credit in nearly all induction FRQ problems.
2. The 4-Step Method for Applying Lenz's Law★★☆☆☆⏱ 4 min
The most common mistake when applying Lenz's Law is confusing "opposing change" with "opposing the original magnetic field". Lenz's Law opposes the change in flux, not the original field, so the induced field can point in the same direction as the original field if flux is decreasing.
Identify the direction of the original external magnetic field $\vec{B}$ through the conducting loop.
Determine whether the total magnetic flux $\Phi_B = \vec{B} \cdot \vec{A}$ through the loop is increasing or decreasing.
Assign direction of induced magnetic field $\vec{B}_{ind}$: if flux is increasing, $\vec{B}_{ind}$ points opposite to $\vec{B}$; if flux is decreasing, $\vec{B}_{ind}$ points in the same direction as $\vec{B}$.
Use the right-hand rule for current loops: point your right thumb in the direction of $\vec{B}_{ind}$, and your fingers curl in the direction of the induced current.
Exam tip: Always specify your reference frame for direction (e.g., "as viewed from above the loop") on the AP exam, to avoid losing points for ambiguous answers.
3. Lenz's Law for Moving Conducting Loops★★★☆☆⏱ 3 min
One of the most common AP exam problem types is a conducting loop moving through a region of magnetic field, entering or exiting a uniform B region. Flux changes because the area of the loop inside the B region changes, so induction occurs. A useful shortcut: the net magnetic force on the loop always opposes the motion of the loop, because motion causes the flux change, so induced force resists that change.
4. Lenz's Law for Sliding Conducting Rail Problems★★★☆☆⏱ 4 min
Sliding conducting rail setups are the most common FRQ context for motional emf, and Lenz's Law is required to find the direction of current through the circuit's resistor and the force on the sliding bar. The setup forms a closed conducting loop with fixed rails connected by a resistor at one end, and a movable conducting bar that changes the area of the loop as it slides.
Exam tip: Always trace the current around the full loop when asked for direction through a specific component like the resistor — direction in the bar is opposite to direction in the resistor, so it is easy to mix up.
Common Pitfalls
Why: Students memorize the word "oppose" and apply it to the original field instead of the change in flux, which gives the wrong direction when flux is decreasing
Why: Students see a moving conductor in a magnetic field and automatically assume induction occurs, even when flux does not change
Why: Students assume the perspective is obvious, but AP exam graders require explicit direction to award full credit
Why: Students confuse the direction of the change, leading to a result that violates conservation of energy
Why: It is easy to forget the current flows around a closed loop, so direction reverses in different components
Why: Many FRQs require direction or sign for full credit, and missing the sign leads to wrong differential equations for circuits