The U.S. Navy can detect a submarine by measuring the magnetic field produced by its met...

The U.S. Navy can detect a submarine by measuring the magnetic field produced by its metal hull (usually made of a magnetic metal). For the purpose of calculating its magnetic field, model a submarine as one large current loop located at the middle of the submarine carrying 8000 A, as calculated in Example 20.9 and with a diameter equal to the diameter of the submarine. Assume that far from the submarine the magnitude of the magnetic field varies as where is the field at the center of the current loop, is the radius of the loop, and r is the distance from the center of the loop (and also the center of the submarine). What is the approximate value of the magnetic field a distance of 1000 m from the submarine?

(reference example 20.9)