Find the magnitude of the magnetic field at a distance of 1.70 cm from a single long wire carrying a current of 23.3 A. (2) This part of the problem asks for something about an arbitrary long wire, rather than for something specifically described in this problem. To make a connection to the problem, the wire carries the same current as that in the rails in the problem statement. Notice also that a distance of 1.70 cm from one of the rails is the midpoint between the rails. Substitute numerical values into B = μ0I 2πr to find the magnitude of the magnetic field at the indicated distance from a long wire carrying the indicated current. Bwire = μ0I 2πr = (4π ✕ 10−7 T · m/A) A 2π m = ✕ 10−4 T