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| 1 | +\section{Charge Sharing} |
| 2 | + |
| 3 | +\begin{frame}{Charge Sharing} |
| 4 | + Capacitors can be first charged up, then \textit{reconfigured into a different circuit}, usually via switches. \\[5pt] |
| 5 | + \underline{States to Analyze}: |
| 6 | + \begin{enumerate} |
| 7 | + \item \textbf{Initial state}: after charging up |
| 8 | + \item \textbf{Final state}: after charges redistribute in new configuration |
| 9 | + \end{enumerate} |
| 10 | + |
| 11 | + \textbf{NOTE!} \textit{Charges are *always* conserved from phase 1 to 2.} |
| 12 | +\end{frame} |
| 13 | + |
| 14 | +\begin{frame}{Charge Sharing: Steps to Solve} |
| 15 | + \begin{itemize} |
| 16 | + \item\textbf{ Note how many states} you will have to find charges for, and \textbf{draw their equivalent circuits}. Generally, there are two: an \textit{initial state and a final state}. |
| 17 | + \begin{itemize} |
| 18 | + \item Sometimes, there will be intermediate stages--but you \textit{solve those much like you will a two-state problem}. |
| 19 | + \end{itemize} |
| 20 | + \item Find the \textbf{charges on all capacitors in your initial state} in terms of your knowns. |
| 21 | + \begin{itemize} |
| 22 | + \item Most often, you’ll be given an \textit{initial charge on one cap, or a voltage source, and the capacitances of all caps}. |
| 23 | + \item For 16A, this is generally taken once the charges stabilize. |
| 24 | + \end{itemize} |
| 25 | + \item Find the \textbf{charges on all capacitors in your second state}, knowing that $Q_{final, total} = Q_{init, total}$ in your system because \textbf{charge is conserved}, in terms of your knowns. |
| 26 | + \end{itemize} |
| 27 | +\end{frame} |
| 28 | + |
| 29 | +\begin{frame}{Practice: Charge Sharing} |
| 30 | + \begin{tabular}{m{0.55\textwidth} m{0.35\textwidth}} |
| 31 | + & \multirow{2}{*}{ |
| 32 | + \begin{circuitikz}[scale=0.7, transform shape] |
| 33 | + \draw (0, 0) to[C=$C$, v_>=$\,\,V_i$] (0, 3) |
| 34 | + (0, 0) to[short] (3, 0) |
| 35 | + (0, 3) to[nos] (3, 3) |
| 36 | + (3, 0) to[C=$C$, v_>=$\,\,0\,V$] (3, 3); |
| 37 | + \end{circuitikz} |
| 38 | + } \\[5pt] |
| 39 | + Suppose the left capacitor has an initial voltage of $V_i$ & \\[30pt] |
| 40 | + \end{tabular} |
| 41 | + \textbf{What happens when we close the switch?} |
| 42 | +\end{frame} |
| 43 | + |
| 44 | +\begin{frame}{Practice: Charge Sharing [Solution]} |
| 45 | + \color{blue} |
| 46 | + \begin{tabular}{m{0.7\textwidth} m{0.2\textwidth}} |
| 47 | + Assuming ideal switch connection, \textbf{total charge must be conserved}. & \multirow{2}{*}{ |
| 48 | + \color{black} |
| 49 | + \begin{circuitikz}[scale=0.6, transform shape] |
| 50 | + \draw (0, 0) to[C=$C$, v_>=$\,\,V_i$] (0, 3) |
| 51 | + (0, 0) to[short] (3, 0) |
| 52 | + (0, 3) to[nos] (3, 3) |
| 53 | + (3, 0) to[C=$C$, v_>=$\,\,0\,V$] (3, 3); |
| 54 | + \end{circuitikz} |
| 55 | + } \\[15pt] |
| 56 | + Phase 1 is when switch is open and Phase 2 is after we \textit{close the switch and reach steady state}. & \\[20pt] |
| 57 | + \end{tabular} |
| 58 | + $Q_{1, final} = Q_1 / 2$ because each capacitor has the same voltage and therefore the same charge in Phase 2. \\[10pt] |
| 59 | + However, energy at Phase 2 = $\frac{CV^2}{2} = \frac{CQ^2}{2C^2} = \frac{Q^2}{2C}$. This is half the energy in Phase 1, so energy was dissipated between Phase 1 nd Phase 2! |
| 60 | +\end{frame} |
| 61 | + |
| 62 | +\begin{frame}{Challenge Practice: Charge Sharing} |
| 63 | + Phase 1: all $\phi_1$ switches are \textbf{closed}, and all $\phi_2$ switches are \textbf{open}. \\[5pt] |
| 64 | + Phase 2: all $\phi_1$ switches are \textbf{open}, and all $\phi_2$ switches are \textbf{closed}. \\[5pt] |
| 65 | + What is $V_x$ during phase 2 if $C_1 = C$ and $C_2 = 9C$? |
| 66 | + \begin{center} |
| 67 | + \begin{circuitikz}[scale=0.75, transform shape] |
| 68 | + \draw (0, 3) to[V=$V_{source}$] (0, 0.5) node[ground] {} |
| 69 | + (0, 3) to[short] (2, 3) |
| 70 | + (2, 3) to[ospst, l=$\phi_1$] (4, 3) |
| 71 | + (2, 3) to[short] (2, 1) |
| 72 | + (2, 1) to[cspst, l=$\phi_2$] (4, 1) |
| 73 | + (4, 3) to[C=$C_1$] (4, 1) |
| 74 | + (4, 3) to[cspst, l=$\phi_2$] (6, 3) |
| 75 | + (4, 1) to[ospst, l=$\phi_1$] (6, 1) |
| 76 | + (6, 3) to[C=$C_2$] (6, 1) node[ground] {} |
| 77 | + (6, 3) to[short] (8, 3) node[label={[font=\footnotesize] above:$V_x$}] {} |
| 78 | + (8, 3) to[ospst, l=$\phi_1$] (8, 1) node[ground] {}; |
| 79 | + \end{circuitikz} |
| 80 | + \end{center} |
| 81 | +\end{frame} |
| 82 | + |
| 83 | +\begin{frame}{Challenge Practice: Charge Sharing [Solution]} |
| 84 | + \color{blue} |
| 85 | + \underline{Phase 1} |
| 86 | + \begin{align*} |
| 87 | + Q_1 = C_1 V_1 = C_1(V_S - 0) = C_1 V_S \\ |
| 88 | + Q_2 = C_2 V_2 = (9C)(0 - 0) = 0 |
| 89 | + \end{align*} |
| 90 | + \begin{center} |
| 91 | + \color{black} |
| 92 | + \begin{circuitikz}[scale=0.75, transform shape] |
| 93 | + \draw (0, 3) to[V=$V_{source}$] (0, 0.5) node[ground] {} |
| 94 | + (0, 3) to[short] (2, 3) |
| 95 | + (2, 3) to[short] (4, 3) |
| 96 | + (4, 3) to[C=$C_1$, v=$\,$] (4, 1) |
| 97 | + (4, 1) to[short] (6, 1) |
| 98 | + (6, 3) to[C=$C_2$, v=$\,$] (6, 1) node[ground] {} |
| 99 | + (6, 3) to[short] (8, 3) node[label={[font=\footnotesize] above:$V_x$}] {} |
| 100 | + (8, 3) to[short] (8, 1) node[ground] {}; |
| 101 | + \end{circuitikz} |
| 102 | + \end{center} |
| 103 | +\end{frame} |
| 104 | + |
| 105 | +\begin{frame}{Challenge Practice: Charge Sharing [Solution]} |
| 106 | + \color{blue} |
| 107 | + \underline{Phase 2} |
| 108 | + \begin{align*} |
| 109 | + V_1 = V_x - V_S \,\, \rightarrow \,\, Q_1 = C_1(V_x - V_S) = C(V_X - V_S) \\ |
| 110 | + V_2 = V_X \,\, \rightarrow \,\, Q_2 = C_2 V_X = 9CV_X \qquad \qquad \qquad |
| 111 | + \end{align*} |
| 112 | + \begin{center} |
| 113 | + \color{black} |
| 114 | + \begin{circuitikz}[scale=0.75, transform shape] |
| 115 | + \draw (0, 3) to[V=$V_{source}$] (0, 0.5) node[ground] {} |
| 116 | + (0, 3) to[short] (2, 3) |
| 117 | + (2, 3) to[short] (2, 1) |
| 118 | + (2, 1) to[short] (4, 1) |
| 119 | + (4, 3) to[C=$C_1$, v=$\,$] (4, 1) |
| 120 | + (4, 3) to[short] (6, 3) |
| 121 | + (6, 3) to[C=$C_2$, v=$\,$] (6, 1) node[ground] {} |
| 122 | + (6, 3) to[short] (8, 3) node[label={[font=\footnotesize] above:$V_x$}] {}; |
| 123 | + \end{circuitikz} |
| 124 | + \end{center} |
| 125 | +\end{frame} |
| 126 | + |
| 127 | +\begin{frame}{Challenge Practice: Charge Sharing [Solution]} |
| 128 | + \color{blue} |
| 129 | + Total charge in Phase 1 = Total charge in Phase 2 |
| 130 | + \begin{align*} |
| 131 | + Q_{1, phase1} + Q_{2, phase1} = Q_{1, phase2} + Q_{2, phase2} \\ |
| 132 | + CV_ + 0 = C(V_X - V_S) + 9CV_X \\ |
| 133 | + 2CV_S = 10CV_X \,\, \rightarrow \,\, V_X = V_S / 5 |
| 134 | + \end{align*} |
| 135 | + |
| 136 | + \begin{center} |
| 137 | + \color{black} |
| 138 | + \begin{circuitikz}[scale=0.75, transform shape] |
| 139 | + \draw (0, 3) to[V=$V_{source}$] (0, 0.5) node[ground] {} |
| 140 | + (0, 3) to[short] (2, 3) |
| 141 | + (2, 3) to[short] (2, 1) |
| 142 | + (2, 1) to[short] (4, 1) |
| 143 | + (4, 3) to[C=$C_1$, v=$\,$] (4, 1) |
| 144 | + (4, 3) to[short] (6, 3) |
| 145 | + (6, 3) to[C=$C_2$, v=$\,$] (6, 1) node[ground] {} |
| 146 | + (6, 3) to[short] (8, 3) node[label={[font=\footnotesize] above:$V_x$}] {}; |
| 147 | + \end{circuitikz} |
| 148 | + \end{center} |
| 149 | +\end{frame} |
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