Halliday, Resnick & Walker — Magnetic Fields Due to Currents: Problem 5

5 In Fig. 29-26, four long straight wires are perpendicular to the page, and their cross sections form a square of edge length \(a = 40 \text{ cm}\). The currents are out of the page in wires 1 and 4 and into the page in wires 2 and 3, and each wire carries \(12 \text{ A}\). In unit-vector notation, what is the net magnetic field at the square’s center?