2.如图 -2 所示,在竖直放置的长直导线AB附近,有一水平放置的有限长直导线CD-|||-C端到长直导线的距离为a,CD长为b,若AB中通以电流I1,CD中通以电流I2,则导线CD-|||-受的安培力的大小为 () -)。-|||-(A) =dfrac ({mu )_(0)(I)_(1)(I)_(2)}(2pi )ln dfrac (d+b)(a) (B) =dfrac (2pi {l)_(1)(l)_(2)}({mu )_(0)}ln dfrac (b)(a)-|||-(C) =dfrac (2pi {l)_(1)(I)_(2)}({u)_(0)}ldfrac (a+b)(a) . (D) =dfrac ({mu )_(0)(I)_(1)(I)_(2)}(2pi )ln dfrac (b)(a)-|||-Ai-|||-I1-|||-C I2 D-|||-a _ b--|||-l2-|||-B-|||-图 -2 图 5-3