일반물리학 2 — Magnetic Fields Due to Currents: Problem 18

18 Figure 29-34 shows an arrangement known as Helmholtz coil. It consists of two circular coaxial coils, each of 200 turns and radius $R = 20.0$ cm, separated by a distance $s = R$. The two coils carry equal currents $i = 20.2$ mA in the same direction. Find the magnitude of the net magnetic field at $P$, midway between the coils.