Chapter 24, Problem 4P

For a particular scientific experiment, it is important to be com­pletely isolated from any magnetic field, including the earth’s field. The earth’s field is approximately 50 μT, bin at any particular location it may be a bit more or less than this. A 1.00-m-diarneter current loop with 200 turns of wire is adjusted to carry a current of 0.215 A; at this current, the coil’s field at the center is exactly equal to the earth’s field in magnitude but opposite in direction, so that the total field at the center of the coil is zero. What is the strength of the earth’s magnetic field at this location?