"The magnetic field at the center of a 50-turn coil of radius 15 cm is 0.60mT. find the current in the coil." N=50, r = .15m, B = 6.0 * 10^-4 T, magnetic permeability = 4 * pi * 10^-7. B = (mag.per. * N * I)/ L However, I don't have L, which is the length. I do have r, the radius, but it's not in the equation... How do i solve this?