An inductor is a circuit element, usually a coil of wire, that stores energy in its magnetic field and opposes changes in current through self-inductance, producing a back emf of magnitude L(dI/dt). In AP Physics C: E&M, inductors anchor RL and LC circuit analysis in Topic 5.2.
An inductor is a coil of wire (often wrapped around an iron or ferrite core) that stores energy in a magnetic field when current flows through it. Its defining property is self-inductance (L, measured in henries). When the current through the coil changes, the changing magnetic flux through its own loops induces an emf that fights the change. That's Faraday's law turned inward on the device itself, giving you the relationship emf = -L(dI/dt).
Here's the intuition that makes everything else click. An inductor is current's version of inertia. It doesn't care what the current is, only how fast it's changing. Steady current? The ideal inductor acts like a plain wire. Try to switch the current on or off suddenly? The inductor pushes back hard with a back emf. The energy it absorbs while current builds up gets parked in the magnetic field as U = ½LI², and that energy can be handed back to the circuit later, which is exactly what drives LC oscillations.
Inductors live in Topic 5.2 (Inductance) within the Electromagnetic Induction unit of AP Physics C: E&M. They're where the whole course comes full circle. Everything you learned about circuits in Unit 3 and magnetic fields and Faraday's law in Units 4-5 gets combined into one component. Topic 5.2 expects you to define inductance from flux (L = Φ/I for a single loop's worth of reasoning), compute the energy stored in an inductor, and analyze circuits containing inductors, including the exponential behavior of RL circuits and the oscillating behavior of LC circuits.
This is also one of the most reliable FRQ topics in the course. The College Board loves putting an inductor in a circuit and asking what happens at t = 0 (inductor blocks current change, acts like an open switch), at steady state (inductor acts like a bare wire), and everywhere in between (exponential or sinusoidal behavior governed by a differential equation you may have to set up yourself).
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Self-inductance (Unit 5)
Self-inductance is the property; the inductor is the physical device built to have a lot of it. Any loop of wire has some self-inductance, but coiling many turns concentrates the flux so L is large enough to matter in a circuit. When an exam question says 'inductor of known inductance L,' it's telling you the self-inductance value to plug into emf = -L(dI/dt).
Magnetic Field (Unit 4)
The inductor's stored energy literally lives in the magnetic field inside the coil. A solenoid's field B = μ₀nI from Ampère's law is what lets you derive an inductor's L from its geometry, and the energy density u = B²/(2μ₀) integrated over the coil's volume gives you back U = ½LI². Field physics and circuit physics are the same physics here.
Mutual Inductance (Unit 5)
Self-inductance is a coil inducing an emf in itself; mutual inductance is one coil inducing an emf in a neighboring coil. Same Faraday's law logic, different victim. Two inductors placed near each other can couple, which is the operating principle behind transformers.
Capacitors and LC Circuits (Units 3 and 5)
A capacitor stores energy in an electric field (½CV²); an inductor stores it in a magnetic field (½LI²). Connect them and energy sloshes back and forth between the two, producing simple harmonic oscillation of charge and current with angular frequency ω = 1/√(LC). This is the E&M twin of the spring-mass system from Mechanics.
Inductors show up heavily on FRQs, often inside experimental design or circuit analysis problems. The 2024 exam (FRQ Q2) put an inductor of known inductance L in series with resistors and a battery and asked how to determine an unknown resistance, which requires you to reason about the RL circuit's behavior over time. The 2026 exam (FRQ Q3) had an experiment using charged capacitors to determine an inductance L₁, classic LC circuit territory where the oscillation period depends on √(LC).
What you actually have to do with inductors on the exam: (1) state the limiting behaviors, meaning the inductor acts like an open circuit the instant a switch closes and like a plain wire at steady state; (2) write Kirchhoff's loop rule including the -L(dI/dt) term and solve or interpret the resulting differential equation; (3) compute stored energy with U = ½LI²; and (4) sketch or interpret graphs of current vs. time, which are exponential in RL circuits and sinusoidal in LC circuits. MCQs often test the limiting-case behavior and the energy formula directly.
Both store energy and both create time-dependent circuit behavior, so it's easy to blur them. The cleanest way to keep them straight is by what each one resists changing. A capacitor stores energy in an electric field (U = ½CV²) and resists sudden changes in voltage, so it acts like a wire at t = 0 and an open circuit at steady state. An inductor stores energy in a magnetic field (U = ½LI²) and resists sudden changes in current, so it does the exact opposite, acting like an open circuit at t = 0 and a wire at steady state. They're mirror images of each other, which is why combining them produces oscillation.
An inductor opposes changes in current by generating a back emf equal in magnitude to L(dI/dt), so current through an inductor can never jump instantaneously.
The instant a switch closes, an ideal inductor behaves like an open circuit; after a long time, it behaves like a plain wire with no potential difference across it.
Energy stored in an inductor is U = ½LI², and that energy physically resides in the magnetic field inside the coil.
In an RL circuit, current grows or decays exponentially with time constant τ = L/R.
In an LC circuit, energy oscillates between the capacitor's electric field and the inductor's magnetic field with angular frequency ω = 1/√(LC).
Inductance is measured in henries (H), where one henry means one volt of back emf per one ampere-per-second of current change.
An inductor is a coil of wire that stores energy in a magnetic field and opposes changes in current through self-inductance L, producing a back emf of magnitude L(dI/dt). It's the central device of Topic 5.2 (Inductance) in Unit 5.
No, not steady current. An ideal inductor only resists changes in current, so at the instant a switch closes it acts like an open circuit, but once the current reaches steady state it acts like an ordinary wire with zero potential difference across it.
A capacitor stores energy in an electric field (½CV²) and resists changes in voltage; an inductor stores energy in a magnetic field (½LI²) and resists changes in current. Their switch-closing behaviors are exactly opposite, with the capacitor starting as a wire and the inductor starting as an open circuit.
U = ½LI², where L is the inductance in henries and I is the current. It's the magnetic-field counterpart to a capacitor's U = ½CV².
Mostly in RL and LC circuit FRQs. The 2024 exam used an inductor in series with resistors and a battery in an experimental design question, and the 2026 exam asked students to determine an inductance using charged capacitors in an LC setup. Expect to analyze t = 0 and steady-state behavior, set up loop equations with the -L(dI/dt) term, and work with τ = L/R or ω = 1/√(LC).