diff --git a/06_cicruits_electriques.md b/06_cicruits_electriques.md
index 2855d6791e406f139e7244fa6d8066ccb1536448..405f7a4e88bf38eaa46801401de7dcdf43be7673 100644
--- a/06_cicruits_electriques.md
+++ b/06_cicruits_electriques.md
@@ -5,4 +5,52 @@ allons décrire les circuits les plus simples ici sans aller dans des cas trop a
 
 ## Les résistances en série et en parallèle
 
-Quand deux ou plus 
\ No newline at end of file
+Quand deux résistances ou plus sont connectées bouts à bouts sur un seul chemin
+comme sur la @fig:three_res on dit qu'elles sont branchées **en série**.
+
+![Les résistances $R_1$, $R_2$, $R_3$ sont connectées bout à bout à la source $V$ et traversées par le courant $I$. La chute de voltage au travers des trois résistances est respectivement de $V_1$, $V_2$, $V_3$.](figs/three_res.svg){#fig:three_res width=50%}
+
+Toutes les charges passant par $R_1$ passera aussi par $R_2$ et $R_3$. On
+sait donc que le courant sera le même au travers de chaque résistance,
+sinon cela impliquerait que les résistances créeraient ou stockeraient des charges
+ce qui n'est pas observé dans les circuits
+(ou que la conservation de la charge ne serait pas respectée).
+Le voltage $V$ (également appelée tension) et on suppose que les fils ont une résistance négligeable (nulle). On a que $V_1$, $V_2$, et $V_3$ sont les différences
+de potentiels au travers de chaque résistance. On sait de la loi d'Ohm que
+\begin{equation*}
+V=RI,
+\end{equation*}
+et donc 
+\begin{equation*}
+V_1=R_1 I,\quad
+V_2=R_2 I,\quad
+V_3=R_3 I.
+\end{equation*}
+
+---
+
+Question (Conservation) #
+
+Quelle est la relation entre $V$ et $V_1$, $V_2$, $V_3$?
+
+---
+
+---
+
+Réponse (Conservation) #
+
+L'énergie étant conservée, on a naturellement que
+$$
+V=V_1+V_2+V_3.
+$$
+
+---
+
+On déduit de cette relation que
+$$
+V=R_1I+R_2I+R_3I=(R_1+R_2+R_3)I,
+$$
+et donc qu'on peut remplacer les trois résistances par une résistance équivalente (ou nette), où
+$$
+R_\mathrm{eq}=R_1+R_2+R_3.
+$$
\ No newline at end of file
diff --git a/figs/three_res.svg b/figs/three_res.svg
new file mode 100644
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