A solenoid has length L = 1.23m and inner diameter
d=3.55cm and it
carries a current i = 5.57 A, It
Consists of five close-packed
layers, each with 850 turns along
length L. what is B at its centre?​

The magnetic field is 24.2 mT.

Explanation:

The magnetic field can be calculated with the following equation:

Where:

i: is the current = 5.57 A

N: is the number of turns = 5*850 turns (it is five close-packed)

L: is the length = 1.23 m

: is the permeability = 4π*10⁻⁷ T*m/A          

Hence, the magnetic field is:

Therefore, the magnetic field is 24.2 mT.

I hope it helps you!