A long wire of radius 2a is constructed as shown with a cylindrical cavity of radius a along its entire length. A current I flows in the positive z-direction (that is, out of the page). What is the current density J in the wire? What is the magnetic field at (0,0)? (Hint: Replace the cavity with a solid wire having current density J z and another having current density - j z.) What is the magnetic field at (a, 0)? Derive an expression for the magnetic field at all points along the x-axis. W hat is the magnetic field at (a, a)?