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

concepts/em/u13/e13-2-faraday.tex

@@ -18,6 +18,8 @@ where $d\vec{A}$ is the oriented area element (direction given by the right-hand
 \]
 The minus sign encodes Lenz's law: the induced current produces a magnetic field that opposes the change in flux that created it.}
 
+\wc{Faraday's law creates EMF, not necessarily current}{A changing magnetic flux induces an \emph{electromotive force} $\mathcal{E}=-d\Phi_B/dt$. Whether current flows depends on whether the loop is conducting and whether there is a complete circuit. A changing flux through an open loop or a broken ring creates EMF (a potential difference) but no current flows.}
+
 \pf{Derivation from Faraday's law}{
 The EMF is the work per unit charge by the induced non-conservative field: $\mathcal{E}=\oint_C\vec{E}\cdot d\vec{\ell}$. When the magnetic flux $\Phi_B=\int_S\vec{B}\cdot d\vec{A}$ through the loop changes in time, a non-conservative electric field is induced with non-zero circulation. Energy conservation requires this circulation to equal the rate of flux change (with the minus sign from Lenz's law):
 \[