Question

Current flowing through a long solenoid is varied. Then, magnetic flux
density of the magnetic field inside it varies:

Answer 1

Magnetic field (flux density) B due to a solenoid , inside the solenoid near the axis  is given by :

           B = μ  n i = μ (N/L) i

 n = number of turns per unit length, N = total number of turns, L = length
 i = current through the solenoid

So magnetic flux density varies directly proportional to the current i.

Answer 2

it is given by B = u i l/ 2r

where u is constant =