Knowing that Gauss' law states that the closed integral of e * dA = q(enclosed)/e naut, how would you find exactly what A is in any given problem? I know it varies from situation to situation depending on the geometry of the charge. For instance, I know that for an infinite wire/line of charge, the area will be 2*pi*r*l. While I understand that this is the area of the lateral sides of the cylinder that serves as the Gaussian surface in this particular example, I don't understand why the ends don't count. I've heard explanations such as the ends are parallel and that the electric field would therefore never go through them. This explanation doesn't make sense to me, however, because (and correct me if I'm mistaken) the electric field given off of a charge is multi-directional. Since the electric field is multi-directional, wouldn't at least SOME of the electric field go through the ends? From my understand the "they're perpendicular so the electric field doesn't go through them" explanation would only make sense if the electric field were strictly 90% to the ends, otherwise some of the field should go through.