試題詳解

21.A solenoid consisting of n turns per unit length with length 4a, equal to four times its radius a, carries a steady current I. About the magnitude of magnetic B-field at the center (Bcenter) and at the point of each end (Bend-point) along its axis, which statement in the following is correct?

(A) B_end-point is larger than one half of B_center, i.e., B_end-point ＞ B_center/2
(B) B_end-point is equal to one half of B_center, i.e., B_end-point = B_center/2
(C) B_end-point is smaller than one half of B_center, i.e., B_end-point ＜ B_center/2
(D) Along the axis and in free space, B_center is maximal and equal to μ₀nI

(E) None of the above