1 \begin{problem*}{26.67}
2 Capacitors $C_1=6.00\U{$\mu$F}$ and $C_2=2.00\U{$\mu$F}$ are charged
3 as a parallel combination across a $250\U{V}$ battery. The capacitors
4 are disconnected from the battery and from each other. Then they are
5 connected positive plate to negative plate and negative plate to
6 positive plate. Calculate the resulting charge on each capacitor.
17 TwoTerminal B = battery("$V$");
18 TwoTerminal C1 = capacitor("$C_1$", draw=false);
19 TwoTerminal C2 = capacitor("$C_2$", draw=false);
20 centerto(B, C1, offset=2u); C1.draw();
21 centerto(C1, C2, offset=2u); C2.draw();
22 wire(B.end, C1.end, rlsq, dist=u/2);
23 wire(B.end, C2.end, rlsq, dist=u/2);
24 wire(B.beg, C1.beg, rlsq, dist=-u/2);
25 wire(B.beg, C2.beg, rlsq, dist=-u/2);
26 label("$+Q_1$", C1.end, align=dir(70));
27 label("$-Q_1$", C1.beg, align=dir(110));
28 label("$+Q_2$", C2.end, align=dir(70));
29 label("$-Q_2$", C2.beg, align=dir(110));
31 pair c = C1.mid + (s/2, 0);
32 draw((c-(u,0)) -- (c+(u,0)), kirchhoff_pen, Arrow);
34 C1.centerto(B.mid, C2.mid, -s); C1.draw();
35 centerto(C1, C2, offset=u, reverse=true); C2.draw();
36 label("$+Q_1$", C1.end, align=NW);
37 label("$-Q_1$", C1.beg, align=SW);
38 label("$+Q_2$", C2.end, align=SE);
39 label("$-Q_2$", C2.beg, align=NE);
41 c = C2.mid + (s/2, 0);
42 draw((c-(u,0)) -- (c+(u,0)), kirchhoff_pen, Arrow);
44 C1.shift(s+u); C1.draw();
45 C2.shift(s+u); C2.draw();
48 label("$+Q_1'$", C1.end, align=NW);
49 label("$-Q_1'$", C1.beg, align=SW);
50 label("$-Q_2'$", C2.end, align=SE);
51 label("$+Q_2'$", C2.beg, align=NE);
55 After charging, the charges on the capacitors are
57 Q_1 &= C_1 V = 1.50\U{mC} \\
58 Q_2 &= C_2 V = 500\U{$\mu$C} \;.
61 After disconnecting the battery, flipping $C_2$, and reconnecting, the
62 total charge on one side is $Q_t=Q_2+(-Q_2)=1.00\U{mC}$. This charge
63 is divided into $Q_1'$ and $Q_2'$ such that
66 V_1' = \frac{Q_1'}{C_1} &= V_2' = \frac{Q_2'}{C_2} \\
67 Q_2' &= Q_1'\frac{C_2}{C_1} \\
68 Q_1' + Q_1'\frac{C_2}{C_1} &= Q_t \\
69 Q_1' &= \frac{Q_t}{1+\frac{C_2}{C_1}} = \ans{750.0\U{$\mu$C}} \\
70 Q_2' &= Q_1'\frac{C_2}{C_1}
71 = \frac{Q_t}{\frac{C_1}{C_2} + 1} = \ans{250.0\U{$\mu$C}}