The magnetic field created by an infinitely long wire carrying a current, which is given by Equation (28.9), will be used extensively. Of course, there is no such thing as an infinitely long wire! The field of a finite length of wire is derived -- Equation (28.8) -- and the infinite wire field is the result of taking the limit as the length of the wire becomes infinite. How long is "long enough" to be considered infinite? It's easy to say "when the length of the wire is much larger than the distance from it", but that doesn't give an intuitive feel for realistic dimensions. Consider a mechanical analogy: I can approximate the Earth as flat in the immediate region where I'm standing, because the radius of curvature of the Earth is very large compared to my dimensions. I can do the same on the Moon, but not on a basketball. However, bacteria or other single-cell organisms could probably approximate the basketball as flat at their location.