At the cylindrical
part of the surface, E is normal to the surface
at every point, and its magnitude is constant,
since it depends only on r The surface area
of the curved part is  2prl, where l is the length
of the cylinder FIGURE 1 26 (a) Electric field due to an
infinitely long thin straight wire is radial,
(b) The Gaussian surface for a long thin
wire of uniform linear charge density