What is a solenoid? And what is a long solenoid? Explain.

(N/A) solenoid is a long wire wound in a close-packed helix. When an electric current flows through it,it produces a magnetic field similar to that of a bar magnet.
$A$ long solenoid is defined as a solenoid whose length $(L)$ is much greater than its radius $(R)$,i.e.,$L \gg R$.
In a long solenoid,the magnetic field inside is nearly uniform and directed along the axis of the solenoid. The magnetic field outside the solenoid is negligible. For an ideal long solenoid,the magnetic field at any point inside is given by $B = \mu_0 n I$,where $\mu_0$ is the permeability of free space,$n$ is the number of turns per unit length,and $I$ is the current flowing through the solenoid.