What is the function of an inductor in a circuit?

• A. It stores energy in an electrostatic field.
• B. It opposes a change in current.
• C. It acts as an open in an AC circuit.
• D. It opposes changes in voltage.

Answer: (B)

An inductor resists changes in current by storing energy in its magnetic field and generating a back electromotive force that opposes the current’s change. The governing relation is v = L di/dt, so when current tries to change quickly, the induced voltage acts to oppose that change. This is why inductors slow down changes in current and are used to smooth or filter currents. In DC, once the current is steady, the inductor behaves like a short circuit because di/dt is zero. In AC, the inductor presents impedance that increases with frequency, and the current lags the voltage by 90 degrees. The other options describe different components or behaviors: storing energy electrostatically is what a capacitor does; an inductor does not inherently open an AC circuit; opposing changes in voltage is also a behavior more associated with capacitors.