試題詳解

17. The first figure shows a closed loop with current i. The loop consists of a half-circle of radius r and a half circle of radius R = 2r. The second figure shows the same half-circles but the small one has been flipped over. The net magnetic field at the common center of the circular sections is B₁ for the first figure and B₂ for the second figure. What is the magnitude of the ratio B₂ / B₁?

(A) 0.75,
(B) 3.0,
(C) 0.10,
(D) 1.5,
(E) 5.0.