You've already covered capacitors — now meet their electromagnetic counterpart. An inductor is a coil of wire that stores energy in a magnetic field when current flows through it. Where a capacitor resists changes in voltage, an inductor resists changes in current. This duality shapes nearly every power supply, filter, and radio circuit in existence.

How they work: When current through a coil changes, the resulting change in magnetic flux induces a voltage that opposes the change (Lenz's Law). This opposition is quantified by inductance (L), measured in Henrys (H). The governing equation is:

V = L × (dI/dt)

A 10 mH inductor with current changing at 1 A/ms produces V = 0.01 × 1000 = 10 V across it. This is why you never open-circuit an inductor carrying current — the sudden dI/dt spike generates a dangerously high voltage (the same principle that makes ignition coils produce 40 kV sparks).

Key parameters for selection:

• Inductance (L): nH for RF, µH for switching regulators, mH for filters and sensors
• Saturation current: The current at which the core's magnetic material saturates and inductance drops sharply — exceed this and your circuit misbehaves
• DC resistance (DCR): The wire has resistance, which causes I²R losses and heat
• Core material: Air (linear, low L), ferrite (high frequency), iron powder (high current), laminated steel (line frequency)

Real-world application — buck converter: Every USB charger, laptop power brick, and server PSU uses a switching regulator with an inductor at its heart. In a buck (step-down) converter, a transistor switches DC on and off at 100 kHz–2 MHz. The inductor smooths this pulsing current into steady DC output. The inductance value sets the ripple current: L = (V_in - V_out) × D / (f × ΔI), where D is duty cycle, f is switching frequency, and ΔI is the acceptable ripple. For a 12 V to 5 V converter at 500 kHz with 0.3 A ripple: L = (12 − 5) × 0.42 / (500000 × 0.3) ≈ 19.6 µH. You'd pick a standard 22 µH inductor rated above your max load current.

Rule of thumb: An inductor and capacitor together form an LC filter with resonant frequency f = 1 / (2π√(LC)). This is the basis of every tuned radio circuit and EMI filter. A 100 µH inductor paired with a 100 nF capacitor resonates at about 50 kHz — useful for blocking switching noise from reaching sensitive analog circuits.

Common gotcha: Inductors near saturation don't fail gracefully — inductance collapses, ripple current skyrockets, and components overheat. Always check the saturation current rating, not just the inductance value, when selecting parts.