1 \begin{problem*}{21.38}
2 The following equations describe an electric circuit:
4 -(220\Omega)I_1 + 5.80\U{V} - (370\Omega)I_2 &= 0 \label{eq.2_L1}\\
5 (370\Omega)I_2 + (150\Omega)I_3 - 3.10\U{V} &= 0 \label{eq.2_L2}\\
6 I_1 + I_3 - I_2 &= 0 \label{eq.2_J}
8 \Part{a} Draw a diagram of the circuit.
9 \Part{b} Calculate the unknowns and identify the physical meaning of
11 \end{problem*} % problem 21.38
15 Looking at the three equations, we see that the only unknowns are
16 $I_1$, $I_2$, and $I_3$. That looks like a circuit with current in
24 % draw dashed branches (with single CCW spiral)
25 draw (0,0)--(0,dy)--(-dx,dy)--(-dx,0)--(dx,0)--(dx,dy)--(0,dy) dashed evenly;
27 draw (0,0) withpen pencircle scaled 3pt;
28 draw (0,dy) withpen pencircle scaled 3pt;
32 By looking at Eqn.~\ref{eq.2_J} and identifying it with Kirchhoff's
33 junction rule on junction $A$, we can get current directions.
41 draw (0,dy) withpen pencircle scaled 3pt;
42 draw (0,0) withpen pencircle scaled 3pt;
43 puttext.bot("$A$", (0,0));
44 % draw dashed branches (with single CCW spiral)
45 draw (0,0)--(0,dy)--(-dx,dy)--(-dx,0)--(dx,0)--(dx,dy)--(0,dy) dashed evenly;
46 centreof.i((-dx,0), (0,0), cur);
47 current.A(c.i, phi.i, "", "$I_1$");
48 centreof.I((0,0), (0,dy), cur);
49 current.B(c.I, phi.I, "I_2", "");
50 centreof.j((dx,0), (0,0), cur);
51 current.C(c.j, phi.j, "I_3", "");
55 Eqn.~\ref{eq.2_L1} looks like a Kirchhoff's loop rule involving only
56 branches 1 and 2. The first term $-(220\Omega)I_1$ looks like a
57 $V=IR$ resistor drop in the direction of the current on branch 1, so
58 let's add a $220\Omega$ resistor to branch 1. Because the voltage
59 drops in our loop equation, we must be moving in the direction of the
60 current. Continuing through the Eqn.~\ref{eq.2_L1}, we see a constant
61 voltage increase, which looks like we crossed a battery from the
62 negative to positive side, so we'll add that onto branch 1 too.
63 Finally, there is a $-(370\Omega)I_2$ drop which looks like crossing a
64 resistor in the direction of the current on branch 2, so let's add a
65 $370\Omega$ resistor to branch 2.
72 % draw the left branch components
73 resistor.A(origin, normal, -90, "", "220\ohm");
74 battery.A(R.A.r, -90, "", "5.80 V");
77 N[2] := N[0] + (dx,0);
78 centreof.i(N[0], N[2], cur);
79 current.A(c.i, phi.i, "", "$I_1$");
80 % draw the middle branch components
81 N[3] := (xpart N[2], ypart N[1]);
82 centerto.B(R.A.r, R.A.l, dx, res);
83 resistor.B(B, normal, 90, "", "370\ohm");
84 centreof.I(N[2], R.B.l, cur);
85 current.B(c.I, phi.I, "I_2", "");
86 % draw the right branch components
88 N[5] := (xpart N[4], ypart N[3]);
89 centreof.j(N[4], N[2], cur);
90 current.C(c.j, phi.j, "I_3", "");
92 wire(N[0], N[2], rlsq);
93 wire(N[2], R.B.l, rlsq);
94 wire(N[1], R.B.r, nsq);
95 draw N[2]--N[4]--N[5]--N[3] dashed evenly;
97 draw N[2] withpen pencircle scaled 3pt;
98 draw N[3] withpen pencircle scaled 3pt;
99 % draw the loop direction
100 imesh((N[0]+N[3])/2, ypart (N[1]-N[0])/4, dx/4, ccw, 90, "");
105 Eqn.~\ref{eq.2_L2} looks like another Kirchhoff's loop rule, this time
106 involving only branches 2 and 3. The first term $-(370\Omega)I_2$
107 looks like a resistor gain \emph{against} the direction of the current
108 on branch 2. We already have a $370\Omega$ resistor to branch 2, so
109 this term just tells us we're moving upstream against $I_2$.
110 Continuing through the Eqn.~\ref{eq.2_L2}, we see another voltage
111 \emph{gain} $(150\Omega)I_3$. If we're moving upstream on $I_2$,
112 we'll also be moving upstream on $I_3$, so this voltage gain must be a
113 $150\Omega$ resistor on branch 3. The last term is a constant votage
114 \emph{drop}, which looks like we crossed a battery from the positive
115 to negative side, so we'll add that onto branch 3 too.
122 % draw the left branch components
123 resistor.A(origin, normal, -90, "", "220\ohm");
124 battery.A(R.A.r, -90, "", "5.80 V");
127 N[2] := N[0] + (dx,0);
128 centreof.i(N[0], N[2], cur);
129 current.A(c.i, phi.i, "", "$I_1$");
130 % draw the middle branch components
131 N[3] := (xpart N[2], ypart N[1]);
132 centerto.B(R.A.r, R.A.l, dx, res);
133 resistor.B(B, normal, 90, "", "370\ohm");
134 centreof.I(N[2], R.B.l, cur);
135 current.B(c.I, phi.I, "I_2", "");
136 % draw the right branch components
138 N[5] := (xpart N[4], ypart N[3]);
139 centreof.j(N[4], N[2], cur);
140 current.C(c.j, phi.j, "I_3", "");
141 resistor.C(N[4], normal, 90, "", "150\ohm");
142 battery.C(N[5], -90, "\mbox{3.10 V}", "");
144 wire(N[0], N[2], rlsq);
145 wire(N[2], R.B.l, rlsq);
146 wire(N[1], R.B.r, nsq);
147 wire(N[2], N[4], nsq);
148 wire(N[3], N[5], nsq);
150 draw N[2] withpen pencircle scaled 3pt;
151 draw N[3] withpen pencircle scaled 3pt;
152 % draw the loop direction
153 imesh((N[2]+N[5])/2, ypart (N[3]-N[2])/4, dx/4, ccw, 90, "");
159 Solve using your method of choice. With linear algebra:
168 220\Omega & 370\Omega & 0 \\
169 0 & 370\Omega & 150\Omega \\
179 Inverting the 3x3 matrix,
189 0.0031 & -0.0022 & 0.3267 \\
190 0.0009 & 0.0013 & -0.1942 \\
191 -0.0022 & 0.0035 & 0.4791
209 With regular algebra, we can save ourselves a bit of work by noticing
210 that this problem is the same as the one we just did (35)! Well, now
211 we have a battery on the first branch and none on the second, and the
212 batteries are facing down\ldots If we flip the picture over and swap
213 the first and second branches\ldots
220 % draw the left branch components (now middle)
221 resistor.A(origin, normal, 90, "", "220\ohm");
222 battery.A(R.A.r, 90, "", "5.80 V");
225 N[2] := N[0] - (dx,0);
226 centreof.i(N[1], N[0], cur);
227 current.A(c.i, phi.i, "", "$I_1$");
228 % draw the middle branch components (now left)
229 N[3] := (xpart N[2], ypart N[1]); % (now bottom)
230 centerto.B(R.A.r, R.A.l, -dx, res);
231 resistor.B(B, normal, 90, "", "370\ohm");
232 centreof.I(N[2], R.B.r, cur);
233 current.B(c.I, phi.I, "I_2", "");
234 % draw the right branch components
236 N[5] := (xpart N[4], ypart N[3]); % (now bottom)
237 centreof.j(N[5], N[4], cur);
238 current.C(c.j, phi.j, "I_3", "");
239 resistor.C(N[4], normal, -90, "\mbox{150\ohm}", "");
240 battery.C(N[5], 90, "", "3.10 V");
242 wire(N[0], N[2], rlsq);
243 wire(N[1], R.B.l, rlsq);
244 wire(N[2], R.B.r, nsq);
245 wire(N[2], N[4], nsq);
246 wire(N[3], N[5], nsq);
248 draw N[0] withpen pencircle scaled 3pt;
249 draw N[1] withpen pencircle scaled 3pt;
253 Alright, now it looks like the figure in Problem 35, except that the
254 things labeled $X_1$ and $X_2$ are reversed. We can take our analytic
255 solution to 35 (see the linear algebra notes) and exchange $1
256 \leftrightarrow 2$ giving
259 \frac{\frac{\epsilon_3}{R_3}+\frac{\epsilon_1}{R_1}}{\frac{R_2}{R_3}+\frac{R_2}{R_1}+1} &= I_2 = \ans{9.13\U{mA}} \\
260 \frac{\epsilon_1}{R_1} - \frac{1}{R_1}\frac{\frac{\epsilon_3}{R_3}+\frac{\epsilon_1}{R_1}}{\frac{1}{R_3}+\frac{1}{R_1}+\frac{1}{R_2}} &= I_1 = \ans{11.0\U{mA}} \\
261 \frac{\epsilon_3}{R_3} - \frac{1}{R_3}\frac{\frac{\epsilon_3}{R_3}+\frac{\epsilon_1}{R_1}}{\frac{1}{R_3}+\frac{1}{R_1}+\frac{1}{R_2}} &= I_3 = \ans{-1.87\U{mA}}