B=4马 B= 4。L 2πa 2πa d。=1B,= 2πa d dF dl, B dF u1 12 4π×10×1×1 dl 2n a 2π×1 =2x10-7N.ma I1 I2 dF12 dF21 dl2 dl1 a I B o 2 2 2 a I B o 2 1 1 1 1 2 1 2 1 2 1 2 dl a I I dF I B dl o B1 B2 2 1 4 10 1 1 2 7 1 2 a I I dl dF o 7 1 2 10 N m