Question

In magnetic field of electric current ,magnetic induction along the axis of long straight solenoid

According to another circuit law

$B^​​​​.dl=u₀ (nLI)

Here current flowing through the rectangular path 'nl' times taken ,why

Answer 1

We are taking no because we need the total lenghl through which the current is flowing.
A solenoid is not a straight wire,but made up of number of turns.
So taking l won't give the total length.
For total length we need to multiply n,which is number of turns per unit lenght.
Remember that magnetic field is produced by each and every turn.so n is important here.