Update textbook.en.md

10.brainstorming-innovative-courses/intercambio-curso-electromagnetismo/textbook.en.md

@@ -791,7 +791,10 @@ $`\displaystyle\iint_{S\,orient.} \overrightarrow{rot} \,\overrightarrow{E}\cdot
 
 $`\mathcal{C}_E = fem = \mathcal{E}`$ : circulación del campo eléctrico = *fuerza electromotriz = voltaje inducido*
 
-$`fem = \mathcal{C}_E = \displaystyle \oint_{\Gamma\,orient.\leftrightarrow S} \overrightarrow{E}\cdot\overrightarrow{dl}`$ 
+$`fem = \mathcal{C}_E = \mathcal{E} 
+= \displaystyle \oint_{\Gamma\,orient.\leftrightarrow S} \overrightarrow{E}\cdot\overrightarrow{dl}
+= - \dfrac{\partial}{\partial t} \left( \displaystyle\iint_S \overrightarrow{B}\cdot \overrightarrow{dS}\right)
+= - \dfrac{\partial \Phi_B}{\partial t}`$