Given:
Number of turns per unit length of the solenoid = n
(a) Since the net magnetic field near the centre of the solenoid is 0,
For the solenoid,
...(2)
From (1) and (2), we get
(b) On putting the value of n in (2), we get
Now, are perpendicular to each other.
Thus, the net magnetic field near the centre of the solenoid is given by