If the number of turns per unit length of a solenoid is doubled, what happens to the magnetic field in the solenoid?

What is an Amperian loop?

$A$ long solenoid has $200$ turns per cm and carries a current of $2.5 \, A$. The magnetic field at its centre is (given $\mu_0 = 4\pi \times 10^{-7} \, T \cdot m/A$):

Calculate the axial magnetic field of a finite solenoid.

Two coaxial solenoids of different radii carry current $I$ in the same direction. Let $\overrightarrow{F_1}$ be the magnetic force on the inner solenoid due to the outer one and $\overrightarrow{F_2}$ be the magnetic force on the outer solenoid due to the inner one. Then

What are the directions of the magnetic field between and outside a pair of two parallel large sheets carrying currents in the same directions,as illustrated in the figure?