RL Cutoff Frequency

An RL filter corners where the inductive reactance $2\pi f L$ has grown to equal the resistance, giving $f_c = R/2\pi L$. Fifty ohms with a 10 mH choke corners at about 796 Hz. Above that the inductor dominates and current falls; below it the resistor dominates and the inductor is barely there. The behaviour mirrors the RC case exactly, with the roles of "rising" and "falling" reactance swapped.

Notice that raising the resistance raises the corner, which feels backwards until you see it as a race: the corner is where reactance catches up with resistance, so a bigger target takes a higher frequency to reach. The same reasoning gives the time-domain view, since $\tau = L/R$ and $f_c = 1/2\pi\tau$ again — a larger R makes the inductor's current settle faster, not slower.

In practice RC is preferred wherever it will do the job, because inductors are big, expensive, magnetically noisy and never ideal: a real coil has winding resistance, interwinding capacitance and a self-resonance above which it stops being an inductor at all. RL earns its place where high current makes capacitors impractical — mains filters, switching supply output chokes, motor drive dV/dt filters — and in the humbler case of a relay coil, where the same L/R that defines this corner is what decides how long the contacts take to drop out.