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

concepts/em/u11/e11-2-resistance-ohm.tex

@@ -8,6 +8,8 @@ R = \frac{V}{I}.
 \]
 The SI unit of resistance is the \emph{ohm}, denoted $\Omega$, where $1\,\Omega = 1\,\mathrm{V/A}$.}
 
+\wc{Current is not ``used up'' by resistors}{Current $I$ is the same through every element in a series circuit. A resistor does not ``consume'' current --- it drops voltage (potential energy per charge). Think of the circuit as a closed loop: whatever current leaves the battery returns to it. What is ``used up'' is electrical potential energy (converted to heat), not charge.}
+
 \dfn{Resistivity, conductivity}{The \emph{resistivity} $\rho$ of a material is an intrinsic property that quantifies how strongly that material opposes the flow of electric current. The \emph{conductivity} $\sigma$ is the reciprocal of resistivity:
 \[
 \sigma = \frac{1}{\rho}.