20.4 Self-Induction and RL Transients

If current in a coil changes, magnetic flux produced by that same coil changes. By Faraday-Lenz law, an induced emf appears in the coil itself. This self-induced emf opposes current change, which is why an inductor resists sudden current jumps.

In a series RL circuit connected to a DC source, current rises exponentially rather than instantly. Initially, dI/dt is high, so back emf is large and current is small. As current approaches steady value V/R, dI/dt falls, back emf decays, and the resistor takes most of the voltage drop.

Time constant gives an immediate scale check. Large L or small R means slower current response. This is useful in smoothing circuits, filtering, and current-limiting contexts where abrupt changes are undesirable.

The inductor is therefore an energy-storage element, not a pure energy sink. During current build-up it absorbs energy; during collapse it can release energy back into circuit elements. This is why inductive loads can create voltage spikes when switching is abrupt.