From 16252c9aee130d6e224eb89f4bf1c5daef5f89e7 Mon Sep 17 00:00:00 2001
From: Orestis <orestis.malaspinas@pm.me>
Date: Thu, 27 Apr 2023 11:27:55 +0200
Subject: [PATCH] added some exercises

---
 exercices/circuits.md        | 127 +++++++++++++++++++++
 exercices/figs/rc.svg        | 205 ++++++++++++++++++++++++++++++++++
 exercices/figs/three_cap.svg | 206 +++++++++++++++++++++++++++++++++++
 exercices/figs/three_res.svg | 186 +++++++++++++++++++++++++++++++
 4 files changed, 724 insertions(+)
 create mode 100644 exercices/figs/rc.svg
 create mode 100644 exercices/figs/three_cap.svg
 create mode 100644 exercices/figs/three_res.svg

diff --git a/exercices/circuits.md b/exercices/circuits.md
index 63027d1..9de6412 100644
--- a/exercices/circuits.md
+++ b/exercices/circuits.md
@@ -46,3 +46,130 @@ Corrigé (Luminosité d'ampoules) #
 1. Quand l'interrupteur est fermé, le courant passant dans $C$ est séparé en deux parties égales pour passer dans $A$ et $B$, car leurs résistances sont égales. Ainsi, $A$ et $B$ auront la même luminosité, mais brilleront moins que $C$ qui a un courant plus important.
 2. Quand on ouvre l'interrupteur, l'ampoule $A$ ne sera plus alimentée en courant. On a donc que le courant passera uniquement par $B$ et le circuit sera équivalent à avoir uniquement deux ampoules en série. La résistance équivalente de ce circuit ($=2R$) sera plus grande que celle du circuit avec l'interrupteur fermé ($=3R/2$). Ainsi en fermant l'interrupteur on diminue le courant sortant de la batterie passant par $C$ qui sera moins lumineuse. En revanche, $B$ va recevoir plus de courant que dans le cas où l'interrupteur est fermé. Elle brillera donc plus. Aussi comme $C$ et $B$ ont la même résistance, elles auront la même luminosité.
 
+---
+
+Exercice (Résistances équivalentes) #
+
+Soit le circuit de la @fig:three_res. Quel est le courant sortant de la batterie?
+
+![Un circuit électrique avec trois résistances.](figs/three_res.svg){#fig:three_res width=50%}
+
+Corrigé (Résistances équivalentes) #
+
+Les résistances de $8\Omega$ et $4\Omega$ sont en parallèle et donc leur résistance équivalente se calcule comme
+$$
+\frac{1}{R_\mathrm{eq1}}=\frac{1}{8}+\frac{1}{4}=\frac{3}{8},\Leftrightarrow R_\mathrm{eq1}=\frac{8}{3}\Omega.
+$$
+Ensuite il nous reste deux résistances en série, et il vient
+$$
+R_\mathrm{eq}=6+\frac{8}{3}=\frac{26}{3}\Omega.
+$$
+Finalement, on utilise la loi d'Ohm
+$$
+V=R_\mathrm{eq}I\Leftrightarrow I=\frac{V}{R_\mathrm{eq}}=3\mathrm{A}.
+$$
+
+---
+
+Exercice (Capacités équivalentes) #
+
+1. Déterminer la capacité d'un condensateur unique qui aurait le même effet que la combinaison de la @fig:three_cap. Prendre $C_1=C_2=C_3=C$.
+2. Si on suppose $V$ et $C$ connus. Que vaut la charge dans chaque condensateur?
+3. Si on suppose $V$ et $C$ connus. Que vaut le voltage dans chaque condensateur?
+
+![Un circuit électrique avec trois condensateurs.](figs/three_cap.svg){#fig:three_cap width=50%}
+
+Corrigé (Capacités équivalentes) #
+
+1. On commence par calculer la capacité équivalente de $C_2$ et $C_3$ qui sont en parallèle et on obtient
+$$
+C_{23}=C_2+C_3=2C.
+$$
+Ensuite $C_{23}$ est en série avec $C_1$ en on as
+$$
+\frac{1}{C_\mathrm{eq}}=\frac{1}{C_1}+\frac{1}{C_{23}}=\frac{3}{2C},\Leftrightarrow C_\mathrm{eq}=\frac{2}{3}C.
+$$
+2. La charge totale quittant la batterie est donnée par la relation 
+$$
+Q=C_\mathrm{eq}V=\frac{2CV}{3}.
+$$
+Les capacités $C_2$, et $C_3$ étant les mêmes, on a que $Q_2=Q_3$ et que ces charges sont la moitié de $Q$ (la charge est séparée de manière égale entre les deux condensateurs car ils ont la même capacité). La conservation de la charge fait que la charge dans $C_1$ est $Q$. On a donc à la fin que $Q_1=Q$, et $Q_2=Q_3=Q/2$.
+
+En utilisant l'équaiton $Q=C\cdot V$ sur chacune des charges on a donc
+\begin{align}
+V_1&=\frac{Q_1}{C_1}=\frac{Q}{C}=\frac{2V}{3},\\
+V_2&=\frac{Q_2}{C_2}=\frac{Q}{2C}=\frac{V}{3},\\
+V_3&=\frac{Q_3}{C_3}=\frac{Q}{2C}=\frac{V}{3}.
+\end{align}
+
+---
+
+Exercice (Décharge de circuit RC) #
+
+Soit un condensateur a une capacité $C$ chargée et est connecté à une résistance $R$ comme dans la @fig:rc qui a une tension $V_0$.
+
+![Un circuit RC.](figs/rc.svg){#fig:rc width=50%}
+
+Quelle est la durée $t$ nécessaire pour que le condensateur voit sa tension baisser à $10\%$ de la tension initiale?
+
+Corrigé (Décharge de circuit RC) #
+
+La décroissance de la tension dans le condensateur est donnée par
+$$
+V_C(t)=V_0e^{-\frac{t}{RC}}.
+$$
+On cherche $t$ tel que
+$$
+V_C(t)=0.1V_0.
+$$
+On obtient
+\begin{align}
+&V_0e^{-\frac{t}{RC}}=0.1V_0,\nonumber\\
+&e^{-\frac{t}{RC}}=0.1,\nonumber\\
+&-\frac{t}{RC}=\ln{0.1}=-2.3,\nonumber\\
+&t=-2.3RC.
+\end{align}
+
+---
+
+Exercice (Sèche-linge) #
+
+Soit un sèche linge dont le la résistance est de $10\Omega$.
+
+1. Quel est le courant passant dans la résistance, si la machine à laver est branchée à une tension de $230\mathrm{V}$?
+2. Combien de charges passent en $60\mathrm{min}$?
+
+Corrigé (Sèche-linge) #
+
+De la loi d'Ohm on a que
+$$
+I=\frac{V}{R}=\frac{230}{10}=23\mathrm{A}.
+$$
+On sait que $I=\Delta Q/\Delta t$, avec $\Delta t=3600\mathrm{s}$. On a donc
+$$
+\Delta Q=23\cdot 3600\mathrm{C}.
+$$
+
+---
+
+Exercice (Chauffage) #
+
+Un chauffage est connecté à une source de tension alternative de $V_\mathrm{rms}=240\mathrm{V}$. Sa résistance est de $60\Omega$.
+
+1. Quelle est la puissance moyenne utilisée?
+2. Quelles sont les valeurs de puissance minimale et maximale?
+
+Corrigé (Chauffage) #
+
+En utilisant les différentes formules du cours, on a
+
+1. La moyenne de la puissance est donnée par
+$$
+\overline{P}=\frac{V_\mathrm{rms}^2}{R}=\frac{240^2}{60}\mathrm{W}.
+$$
+2. La puissance minimale est nulle (en effet il y a des moments où la tension est nulle). La puissance maximale étant donnée par
+$$
+P_\mathrm{max}=\frac{V_0^2}{R}=\frac{(\sqrt{2}V_\mathrm{rms})^2}{R}=2\frac{V_\mathrm{rms}^2}{R}=2\overline{P}.
+$$
+
+---
diff --git a/exercices/figs/rc.svg b/exercices/figs/rc.svg
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diff --git a/exercices/figs/three_cap.svg b/exercices/figs/three_cap.svg
new file mode 100644
index 0000000..af41b8e
--- /dev/null
+++ b/exercices/figs/three_cap.svg
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