I'm reading the book "Electricity and Magnetism: Vol. II" by Edward M. Purcell, in which he describes Gauss' law as stating that the flux through a closed surface is 4π times the enclosed charge (he uses the Centimeter-Gram-Second system). Later, he refers to Gauss' law as stating that the change in field from one side of a layer to the other must be 4πσ, where σ is the charge density in the layer. I don't see the relation. Maybe it's an obvious corollary of Gauss' law, but in any case I don't understand how. The offending text can be found here: http://rapidshare.com/files/270469221/offending_passage.pdf The bad news actually only starts in section 1.14 (on the second page), but the first page is included because it get reffered to in the one of the two offending passage.