How to Apply Gauss's Law for Symmetrical Electric Objects?

[intervoxel]

Homework Statement

I need a strategy to resolve electrostatic problems such as the infinite cylindrical shell of radius r1 and r2 of non conducting material. I'm confused in applying a gaussian surface in this and others cases. Is there a recipe for it?

Homework Equations

Gauss's law.

The Attempt at a Solution

 

[Redbelly98]

Usually, the faces of a Gaussian surface are either perpendicular or parallel to the electric field. The faces that are parallel contribute nothing to the sum (or integral) of E·A (or E·dA). Also, the field is usually of equal magnitude over the faces where it is perpendicular, so those have a contribution of simply E·A.