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A
Stores energy in an electric field between plates
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B
Stores energy in a magnetic field around it when current flows; resists changes in current — passes DC easily but opposes AC, especially at high frequencies
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C
Allows current in only one direction
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D
Converts electrical energy to heat
Why this is the answer
Inductor: a coil of wire (often around a magnetic core) that stores energy in a magnetic field. Energy stored: E = ½LI², where L is inductance (Henries) and I is current. Key properties: (1) Resists CHANGES in current — voltage across inductor = L × (di/dt); when current is steady, voltage drop is ~0 (just the wire resistance); rapid changes in current produce large voltage spikes; (2) Passes DC easily — once current is steady, inductor looks like a wire (just its DC resistance); (3) Opposes AC — impedance increases with frequency: X_L = 2πfL (reactance, in ohms); at high frequencies, inductors become like 'open circuits'; (4) Stores energy in magnetic field — opposite of capacitor which stores in electric field. Applications: (1) FILTERS — inductors in series block high frequencies (low-pass filter); used in audio crossovers, power supply filtering; (2) TRANSFORMERS — magnetic coupling between two coils; (3) MOTORS, GENERATORS, SOLENOIDS — convert between electrical and mechanical energy via magnetic field; (4) RELAYS — coil energizes to operate switch contacts; (5) IGNITION COILS — automotive; pulse the primary, get high-voltage spark from secondary; (6) RADIO TUNING — LC circuit (inductor + capacitor) resonates at specific frequency; (7) ENERGY STORAGE — buck/boost switching converters store and release energy in inductor; (8) CHOKES — block AC noise from power lines while passing DC. Construction: wire wound around an air or magnetic core. Core materials: AIR (low inductance, used at high frequencies); FERRITE (intermediate, common in switching power supplies); IRON or LAMINATED STEEL (high inductance, used in transformers and 60 Hz applications). Inductance increases with: more turns, larger cross-sectional area, magnetic core material, closer winding. Symbol: looped or coiled line; sometimes with bars indicating iron core. Units: HENRY (H), millihenry (mH), microhenry (μH), nanohenry (nH). Typical values: μH range for RF circuits, mH for audio crossovers, H for power applications. Inductor and capacitor behave oppositely: capacitor — passes AC, blocks DC; inductor — passes DC, blocks AC (high frequency). LC circuits combining both can resonate at a specific frequency: f = 1/(2π√LC) — fundamental to radio receivers, oscillators, filters.
Source: ASVAB EI, Inductors