Chapter 24, Problem 53GP

Bats are capable of navigating using the earth’s field—a plus for an animal that may fly great distances from its roost at night, if, while sleeping during the day, hats are exposed to a field of a similar magnitude but different direction than the earth’s field, they are more likely to lose their way during their next lengthy night flight. Suppose you are a researcher doing such an experiment in a location where the earth's field is 50 μT at a 60° angle below horizontal. You make a 50-cm-diameter, 100-turn coil around a roosting box: the sleeping bats are at the center of the coil. You wish to pass a current through the coil to produce a field that, when combined with the earth’s field, creates a net field with the same strength and dip angle (60 below horizontal) as the earth’s field hut with a horizontal component that points south rather than north. What are the proper orientation of the coil and the necessary current?