K, the "idealized surface current density" Hey, I don't quite understand that guy, K. I have an exam on Sunday in E&M, I'm studying from Jackson. I haven't found any definition of 'K'. If anyone could give me a rigurous definition and an integral form, if there's any, I'd appreciate it. Oh, and since we're at it, I stumped into that next statement: "Suppose that the upper half of space is filled with a permeable media, while the other half is empty space. If, in the x-y plane, K is in the x direction, it follows that A (vector potential) is also in that direction in the entire space". Huh?