I went through the book of engineering electromagnetics and I have several questions I dont understand. 1. If the line integral of electrical field is not zero at the existence of changing magnetic field, then how can KVL holds with AC current flowing in the circuit? I do not know what's wrong with my understanding. Isnt AC current generates changing magnetic field at the surface of the circuit, which generates non-zero integral of electrical field along the line path and contradict with KVL? 2. The book mentions about that conduction current density J= σE is the motion of charge in a region of zero net charge density and convection current density J=ρv is the motion of volume charge density. I couldnt understand because the formula of conduction current density J= σE is derived from the one of convection current density J=ρv. Ok, if they are different, would you please give me specific examples about what conduction current is and what convection current is. Personally I thought they were just conduction current. 3. I really couldnt understand the boundary conditions Etangential1=Etangential2 and Htangential1=Htangential2 of time-varying fields. Where do partialB/partialT and partialD/partialT go? Isnt it time-varying fields and shouldnt these two terms be zero? The book says that they should be zero between any two real physical media but also mentions that surface charge density is physical possible for either dielectrics, perfect conductors or imperfect conductors and surface current density for perfect conductors. I am confused. If surface charge density and surface current density are possible, why should these two terms be zero? Many thanks for your help!