The RL Time Constant

An inductor and resistor in series form the magnetic mirror of the RC circuit. When voltage is applied, the inductor's back EMF initially blocks current; over time the current rises exponentially toward its steady value V/R with time constant τ = L/R.

Energy in the Magnetic Field

The inductor stores energy in its magnetic field, E = ½LI². When the circuit is interrupted, that energy must go somewhere — producing the flyback voltage spikes that make relay coils dangerous to switch without protection. This same principle makes inductive kickback the bane of switch designers.

RL Circuits in Frequency

The inductor's impedance, Z_L = 2πfL, grows with frequency, so an RL pair filters by passing or blocking based on f_c = R/(2πL). Combined with the RC filter's behavior, these two networks are the building blocks of every analog equalizer, crossover, and RF front end.