B= Ho(cos0-cos8） 2 4πR 41 01=π/202=π 4πR B=B+B2+B3 x>R→ (3) B= 4o1R2 2(R2+x2)32 B≈4R3 = 461S m 2x3 元 2元x 定义 Pn ISn 磁矩 B=Lo Pu 2πxI (cos cos ) 4 1 2 0 3    −  = RI B I RO 1 2 R 3 I  = 40 B = B1 + B2 + B3 (3) x  R 2 2 3 / 2 2 0 2(R x ) IR B + =  3 2 02 x IR B   3 0 2 x IS  =    S n p ISn m  定义 = m p 3 0 2 xp B m    =  θ 1 =  2 θ 2 =  磁矩