A solenoid has a length 1 m and inner diameter 4 cm and it carries a current of 5 A. It consists of five close packed layers, each with 800 turns along its length. The magnitude of the magnetic field at the centre is nearly equal to
Number of turns per unit length, 5×8001=4000 turns per meter.
Length, L ≫ d
\Rightarrow The solenoid is long and can be treated as an ideal solenoid.
The magnetic field at the centre is given by