Inductance and RL circuits

An inductor is the electrical mirror image of a capacitor: where a capacitor opposes changes in voltage across it, an inductor opposes changes in current through it, via the electromagnetic induction of the changing magnetic field its own current creates. Try to change the current suddenly and the inductor generates a back-voltage fighting the change — not opposing current itself, since a steady current produces no changing flux and so no opposition at all.

Close a switch on a battery, resistor and inductor in series and this opposition governs the whole approach to steady state. The current cannot jump to its final value instantly, because that would require an infinite rate of change and therefore an infinite back-voltage. Instead it climbs exponentially, exactly the shape RC charging takes, with a time constant τ = L/R rather than RC. A larger inductance means a stronger opposition to change and so a slower climb; a larger resistance limits the final current but also, less obviously, makes the approach faster, since R appears in the denominator.

This is the precise inverse of the RC story: there, resistance delayed voltage; here, resistance (together with inductance) delays current. The inductor's stored energy sits in its magnetic field rather than an electric one, released as current decays when the source is removed rather than as voltage decays. Real inductors — solenoid windings, transformer coils, motor windings — all inherit this current-smoothing behaviour, which is exactly why inductors are used to suppress sudden current spikes in power supplies.