a circular coil has a 18.0 cm radius and consists of 30.0 closely wound turns of wire. an externally produced magnetic field of 2.00 mt is perpendicular to the coil. a) if no current is in the coil, determine the magnetic flux through the coil. b) when the current in the coils is 3.30 a in a certain direction, the net flux through the coil is found to vanish. determine the inductance of the coil.