uo ldl sing B=」dB 4兀 ·利用r=a/sine;1= acote;dl=ad/sin20,可得 e all B sin ede B1 62 B (cos61-c062) 4丌a ldh 特例:(1)对无限长直电流, P 61=0;2=π,有 dB 61 B 2丌a1 = 0 ;2 = ,有 ·利用r=a/sin;l=-a cot;dl=a d/sin2,可得 B = 0 I 4a (cos1 - cos2 ) ·特例:(1)对无限长直电流, · P a r Idl 2 1 dB o l B = dB = 0 4 Idl sin r 2 sin d a I B = 2 1 4 0 2 a I B 0 π μ =