\[{V_p \over V_s}={N_p \over N_s}={I_s \over I_p} \tag{xfmr coil ratios} \]
\textbf{Lenz's law:} $I_{\operatorname{emf}}$ opposes $\Delta \Phi$ \\
- (emf creates $I$ with associated field that opposes $\Delta \phi$)
+ (emf creates $I$ with associated field that opposes $\Delta \Phi$)
\textbf{Eddy currents:} counter movement within a field