content(warnings): Add W17-W28, X8-X12, N7 — E&M misconceptions, cross-refs, notes

concepts/em/u11/e11-1-current-density.tex

@@ -14,6 +14,8 @@ dI=\vec{J}\cdot d\vec{A}.
 \]
 Thus $\vec{J}$ links the local flow of charge to the total current through a cross section.}
 
+\wc{Electrons flow opposite to conventional current}{The \emph{conventional current} direction is defined as the direction positive charges would move (from higher to lower potential). In metal wires, the actual charge carriers are electrons, which move \emph{opposite} to the conventional current direction. This historical convention does not affect circuit analysis results.}
+
 \nt{Conventional current is defined to point in the direction that positive charges would move. Therefore $\vec{J}$ points in the conventional-current direction. In a metal wire, the mobile charge carriers are electrons, so $q=-e$ and the electron drift velocity $\vec{v}_d$ points opposite to $\vec{J}$. If the carriers were positive instead, then $\vec{v}_d$ and $\vec{J}$ would point in the same direction.}
 
 \mprop{Microscopic-macroscopic current relations}{Let $n$ denote the carrier number density, let $q$ denote the charge of each carrier, let $\vec{v}_d$ denote the drift velocity, and let $S$ be a surface with oriented area element $d\vec{A}$. Then the current density is