What is an inductor?

• A. A resistor
• B. A capacitor
• C. A component that has the ability to concentrate a magnetic field
• D. A diode

Answer: (C)

An inductor is a coil of wire that stores energy in a magnetic field created by the current through it. The magnetic field is concentrated around the coil (and within any core), and the energy stored grows with the current according to W = 1/2 L I^2, where L is the inductance. When the current changes, the inductor resists that change by generating an opposing voltage V = L di/dt. In AC, its impedance increases with frequency as X_L = 2π f L, making it useful for filtering and smoothing current. This behavior is distinct from a resistor (energy dissipated as heat), a capacitor (stores energy in an electric field between plates), and a diode (a nonlinear device that conducts in one direction). So the description that best fits is a component that has the ability to concentrate a magnetic field.