14)
Thus, the self-induced emf always opposes any change  (increase or
decrease) of current in the coil It is possible to calculate the self-inductance for circuits with simple
geometries Let us calculate the self-inductance of a long solenoid of cross-
sectional area A and length l, having n turns per unit length The magnetic
field due to a current I flowing in the solenoid is B = m0 n I  (neglecting edge
effects, as before)