Answer
13 amperes, upward.
Work Step by Step
Use equation 20–7 to calculate the magnitude of the second current.
$$F=\frac{\mu_oI_1I_2}{2\pi d}\mathcal{l}_2$$
$$I_2=\frac{2\pi d}{\mu_oI_1}\frac{F}{\mathcal{l}_2}$$
$$=\frac{2\pi (9.0\times10^{-2}m)}{ (4\pi \times10^{-7} T\cdot m/A)(28A)}\frac{7.8\times10^{-4}N}{m}$$
$$=13A$$
The force is attractive, which tells us that the currents run in the same direction. The current in the second wire is also upward.
