1. The problem statement, all variables and given/known data Consider a hollow cylinder of radius 10 m., length 50 m., and charge 6 C., and a rod with radius 4 m., length 50 m., and charge -4 C. The rod is uniformly placed inside the center of the hollow cylinder. What is the electric field at radius = 15 m.? 2. Relevant equations [PLAIN]http://upload.wikimedia.org/math/0/3/5/035b153014908c0431f00b5ddb60c999.png[PLAIN]http://upload.wikimedia.org/wikipedia/commons/thumb/8/86/OiintLaTeX.svg/34px-OiintLaTeX.svg.png[PLAIN]http://upload.wikimedia.org/math/3/9/f/39f4ca19d0c263fd02c0e50cb8829239.png [Broken] 3. The attempt at a solution I am unsure where to place my Gaussian surface(s). Do I need more than one? Does the space between the cylinder and rod matter? It seems like the electric field outside of the cylinder will be independent of the rod's radius. It sounds like a trick question though, because the problem gave the radius of the rod and extraneous information is never given for these problems, although I might be wrong about this.