the coil also changes and an emf is induced in the coil. Using Eq. ( . ), the induced emf is given by ( B – N Φ ε = – I L ε = ( .
) 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 = μ n I (neglecting edge effects, as before).
The total flux linked with the solenoid is ( ) ( ) ( B N nl n I A Φ μ