I am in AP Physics E&M and have learned how to find electric field strengths using Gaussian surfaces for static charges, however I cannot understand why the field produced by an insulating plate is half of that produced by a conducting one. Also, in the example, we are only considering surface charge density, so in theory, it shouldn't matter whether or not the material is a conductor or insulator. However as the lesson goes, we have an infinite connecting plate (to ignore side effects) and we use both end-caps of a Gaussian Pillbox shape, corresponding to the 2A seen in the bottom equation (see option 2): However when we try the same experiment on an insulating plate, apparently we only consider one end-cap of the Gaussian surface and thus only use A in the bottom equation, leading to half the the field strength as a conducting plate with the same charge. Can someone explain to me why this happens?