Self inductance of a long solenoid
Let us consider a solenoid of N turns with length l and area of cross section A. It carries a current I. If B is the magnetic field at any point inside the solenoid, then
Magnetic flux per turn = B × area of each turn
Hence, the total magnetic flux (φ) linked with the solenoid is given by the product of flux through each turn and the total number of turns.