A cylindrical solenoid with length L and cross sectional area A = R 2 is formed by

A cylindrical solenoid with length L and cross sectional area A = πR² is formed by wrapping n turns per unit length of a wire that carries a current I . Estimate the magnitude of the magnetic field just outside the solenoid and far away from the ends when L >> R. Do this by integrating ∇ · B over a hemispherical volume of radius a → ∞oriented so the disk-like portion of its surface cuts perpendicularly through the solenoid at a point far away from both ends.