This question is regarding the application of Gauss's Law to find the electric field within given objects, etc. For example, the question asks you to find the electric field inside a charged sphere, but how do you know when to use little "r" and big "R" when it comes to the radius? Here are two problems that I just did and got the wrong answer because I had used the incorrect "r." Problem 1: Use Gauss's Law to find the electric field inside a uniformly charged sphere (charge density rho) Problem 2: Find the electric field inside a sphere which carries a charge density proportional to the distance from the origin, rho = kr, for some constant k. (Hint: This charge is not uniform, and you must integrate to get the enclosed charge). My question is on problem 1, they used little "r" to denote the radial distance to the Gaussian spherical surface within the sphere and used big "R" to denote the radial distance from the origin (center of sphere) to the outer surface of the sphere. However, on problem 2, they only used little "r" to denote the radial distance to from the origin to the surface of the Gaussian surface. In both scenarios, the Gaussian sphere is contained within the charged sphere. Thus, how do you determine when one "r" value is relevant or if you need both big and little r? Thanks.