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 B₀(r/r₀)^-3, where B₀ is the field at the center of the current loop, r0 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?