In field and wave electromagnetics 2nd edition (david k cheng) problem 6-6
figure 6-37 shows an infinitely long solenoid with air core having a radius b and n closely wound turns per unit length. the windings are slanted at an angle α and carry a curren i. determine the magnetic flux density both inside and outside the solenoid.
in solution this problem can be divided two small problems. magnetic flux density due to surface of the cylindrical tube b1
and magnetic flux density due to solenoid coil b2 and solution say surface current is 2πbnisinα.
but i cant understand why surface current is 2πbnisinα. why solenoid length l is ignored?