A solenoid with 2,781 turns has a radius of 74.7 mm and is 38.4 cm long. If this solenoid carries a current of 68.4 A, what is the magnitude of the magnetic field near the center of the solenoid

B = 0.62 T

Explanation:

Assuming that the solenoid can be treated as of infinite length (as its radius is much smaller than the length) we can consider that the magnetic field created by the current, near the center of it, is constant, and that it's almost zero outside it.Applying Ampere's law to a closed surface that encloses the wire carrying the current, we get:

       

where μ₀ = 4*π*10⁻⁷ T*m/A, n = turns per unit length = 2,781/0.384 m and I = 68.4 AReplacing by the givens in (1) we get:B= 4*π*10⁻⁷ T*m/A* 7242.2 (1 /m) * 68.4 A = 0.62 TB = 0.62 T