Self-inductance $L$ quantifies a coil's tendency to oppose changes in its own current. Flux linkage: $N\Phi = LI$, so $L = N\Phi/I$. Induced back-EMF: $\varepsilon = -L\frac{dI}{dt}$. Unit: henry (H); 1 H = 1 V·s/A. For a solenoid: $L = \mu_0 n^2 V = \mu_0 n^2 Al = \mu_0 N^2 A/l$, where $V = Al$ is volume. For a toroid: $L = \mu_0 N^2 A/(2\pi r)$. Self-inductance depends on geometry and the medium (increases by factor $\mu_r$ with a magnetic core). An inductor in a circuit acts like electrical inertia — it opposes rapid changes in current.