Identify the Wire Geometry First Before writing any formula, decide whether the conductor is finite, effectively infinite, semi-infinite, or a cylindrical current distribution. Most mistakes happen before algebra starts because the wrong case is chosen.

Track the Perpendicular Distance The field depends on the shortest perpendicular distance from the point to the wire, not simply on any labelled length in the figure. In NEET diagrams, mixing the actual perpendicular with some oblique segment is a standard trap.

Separate Inside and Outside Cylinder Results Outside a wire or cylinder, the field behaves like that of a long straight conductor carrying the total current. Inside a solid conductor, the enclosed current changes with radius; inside a hollow cylinder, the field can be zero in the hollow part.