The working principle of inductors is based on the phenomenon of electromagnetic induction.

When the current passes through the wire, it creates a concentric magnetic field centered on the wire. If the wire is bent into a spring shape or wound into a coil, the magnetic flux inside the inductor will point in the same direction, thus enhancing the magnetic field. By adjusting the number of turns of the coil, a magnetic field proportional to the number of turns can be generated.

Specifically, a current passing through an inductor creates a magnetic field, and a change in the magnetic field creates a current, which follows the law of electromagnetic induction: $e = L \cdot \frac{di}{dt}$, where $e$represents the back electromotive force, $L$represents the self-inductance of the inductor, and $\frac{di}{dt}$represents the rate of change of current per unit time.

When a direct current continues to flow in the same direction, the inductor will not generate a back electromotive force. That is to say, the inductor is equivalent to a path for DC current, and has no blocking effect; When alternating current passes through, due to the periodic change in the size and direction of the current, the inductor will produce a counter-electromotive force, which obstructs the current of alternating current, usually called inductive reactance, and its expression is: $x_L = 2\pi fL$, where $x_L$is the inductive reactance of the inductor, $f$is the frequency of the alternating current flowing through the inductor, and $L$is the self-inductance of the inductor.

The impedance of the inductor is $z$(unit $\Omega$) : $z = \omega L = 2\pi fL$, where $f$is the AC frequency. Using this property of an inductor, it can be used as a resistance (impedance) in AC circuits. For example, in low-frequency circuits can be used to block low-frequency alternating current; In LC tuning circuit, the capacitor can be used for tuning, filtering, frequency selection, frequency division and so on.

In short, the inductor realizes the mutual conversion of electric energy and magnetic energy through electromagnetic induction, which shows an obstruction effect on alternating current, while the direct current is equivalent to a path, and its inductance, frequency and other factors will affect its working characteristics and its role in the circuit.