[SOLVED] Gauss' Law With an Infinite Cylinder of Charge 1. The problem statement, all variables and given/known data The test question was to find the potential difference between a point S above a cylinder of charger per length lambda, and a point on the surface of the cylinder having radius R and infinite length. 2. Relevant equations Stupid me tried to grind through it during the test. But looking at the prof's solution: http://www.phys.washington.edu/users/schick/321A/321-07exam1soln.pdf Here's the test if you need to know the question how he worded it: http://www.phys.washington.edu/users/schick/321A/321-07exam1.pdf Problem #1. I don't get how he got "E(s)*2pi*L*S I get the 2pi and L come from integrating over phi and the length of the cylinder, but I can't figure out where that S came from. Is it because you'd need a cosine(theta) for the direction that the cylinder is from S? Where cosine is S/distance? Then wouldn't he have to add in a bunch of other stuff? I'm just not seeing something here and I feel embarrassed.