I was looking for a derivation of E=(2kλ)/r for an infinite line of charge. I understood that you need to use Gauss's Law and a cylinder around the line. When looked it up, I found this: http://www.vizitsolutions.com/portfolio/gausslaw/lineCharge.html He starts out with ∫E⋅dA=4πq. I have never seen Gauss's Law shown like this and am not sure how it is equivalent. Can someone please show me how he got 4πq from Q/ε0?