$L = \mu_0 n^2 Al = \mu_0 N^2 A/l$. Factors affecting $L$: (1) $N^2$ — doubling turns quadruples $L$. (2) $A$ — larger cross-section increases $L$. (3) $1/l$— shorter solenoid (same$N$) increases$L$. (4) Core material — with a ferromagnetic core,$L = \mu_r \mu_0 N^2 A/l$(enhanced by$\mu_r$which can be 1000+). Self-inductance does not depend on the current — it's a geometric property. Analogy with capacitance:$C = \varepsilon_0 A/d$(geometry only),$L = \mu_0 n^2 V$(geometry only).$