self-inductance
A coil resists changes to its own current. When you try to switch its current on or off, the coil's own changing magnetic field fights the change. Self-inductance measures how strongly it resists — a kind of electrical inertia. It answers the question: why can't the current in a coil change instantly?
Precisely, current I in a coil creates a magnetic flux; changing that current changes the flux, which by Faraday's law induces an EMF opposing the change: EMF = - L dI/dt. The constant L is the self-inductance, measured in henries (H), where 1 H = 1 V s / A. It depends only on the coil's geometry — for a long solenoid with N turns, length l, and cross-section A, L = mu_0 N^2 A / l, so more turns give much more inductance.
Self-inductance shapes every circuit that switches current: it smooths currents, causes sparks when switches open, and stores energy in a magnetic field, U = 1/2 L I^2. The honest analogy is that inductance is like mass for current — it does not stop current, it only opposes sudden CHANGES in it, so the current has to build up and die away gradually rather than jumping.
A 0.2 H coil whose current is changing at 3 A/s develops a back-voltage of magnitude EMF = L dI/dt = 0.2 * 3 = 0.6 V, opposing the change.
0.2 H with the current rising at 3 A/s pushes back with 0.6 V.
Inductance opposes the CHANGE in current, not the current itself. A steady DC current through an ideal inductor feels no opposition and produces no voltage across it at all.