-\begin{problem*}{58}
- A $45\U{V}$ battery of negligable internal resistance is connected
- to a $38\U{k\Ohm}$ and a $27\U{k\Ohm}$ resistor in series. What
- reading will a voltmeter, of internal resistance $95\U{k\Ohm}$,
- give when used to measure the voltage across each resistor? What is
- the percent inaccuracy due to meter resistance for each case?
+\begin{problem*}{19.58} % internal resistance
+A $45\U{V}$ battery of negligable internal resistance is connected to
+a $38\U{k\Ohm}$ and a $27\U{k\Ohm}$ resistor in series. What reading
+will a voltmeter, of internal resistance $95\U{k\Ohm}$, give when used
+to measure the voltage across each resistor? What is the percent
+inaccuracy due to meter resistance for each case?
\end{problem*}
\begin{solution}
The original situation looks like
\begin{center}
\begin{asy}
- import Circ;
- real u = 0.5cm;
- TwoTerminal B = source((0,0), DC, 90, "$45\U{V}$", "$V$");
- pair a = B.end+(0,u);
- pair b = B.beg-(0,u);
- TwoTerminal Ra = resistor(a, normal, 0, "$38\U{k\Ohm}$", "$R_1$");
- TwoTerminal Rb = resistor(Ra.end, normal, 0, "$27\U{k\Ohm}$", "$R_2$");
- TwoTerminal I = current((Rb.end.x, (a.y+b.y)/2), -90, "", "$I$");
- wire(Rb.end, I.beg, nsq);
- wire(I.end, b, udsq);
- wire(b, B.beg, nsq);
- wire(a, B.end, nsq);
+import Circ;
+real u = 0.5cm;
+TwoTerminal B = source((0,0), DC, 90, "$45\U{V}$", "$V$");
+pair a = B.end+(0,u);
+pair b = B.beg-(0,u);
+TwoTerminal Ra = resistor(a, normal, 0, "$38\U{k\Ohm}$", "$R_1$");
+TwoTerminal Rb = resistor(Ra.end, normal, 0, "$27\U{k\Ohm}$", "$R_2$");
+TwoTerminal I = current((Rb.end.x, (a.y+b.y)/2), -90, "", "$I$");
+wire(Rb.end, I.beg, nsq);
+wire(I.end, b, udsq);
+wire(b, B.beg, nsq);
+wire(a, B.end, nsq);
\end{asy}
\end{center}
Using Kirchoff's loop rule
With the voltmeter across $R_1$ we have
\begin{center}
\begin{asy}
- import Circ;
- real u = 0.5cm;
- TwoTerminal B = source((0,0), DC, 90, "$45\U{V}$", "$V$");
- pair a = B.end+(0,u);
- pair b = B.beg-(0,u);
- TwoTerminal Ra = resistor(a, normal, 0, "$38\U{k\Ohm}$", "$R_1$");
- TwoTerminal Ia = current(Ra.end, 0, "", "$I_1$");
- TwoTerminal Rv = resistor(a+(0,4u), normal, 0, "$95\U{k\Ohm}$", "$R_v$");
- TwoTerminal Iv = current(Rv.end, 0, "", "$I_v$");
- TwoTerminal Rb = resistor(Ia.end, normal, 0, "$27\U{k\Ohm}$", "$R_2$");
- TwoTerminal I = current((Rb.end.x, (a.y+b.y)/2), -90, "", "$I_T$");
- wire(Rb.end, I.beg, nsq);
- wire(I.end, b, udsq);
- wire(b, B.beg, nsq);
- wire(a, B.end, nsq);
- wire(a, Rv.beg, nsq);
- wire(Iv.end, Ia.end, rlsq);
+import Circ;
+real u = 0.5cm;
+TwoTerminal B = source((0,0), DC, 90, "$45\U{V}$", "$V$");
+pair a = B.end+(0,u);
+pair b = B.beg-(0,u);
+TwoTerminal Ra = resistor(a, normal, 0, "$38\U{k\Ohm}$", "$R_1$");
+TwoTerminal Ia = current(Ra.end, 0, "", "$I_1$");
+TwoTerminal Rv = resistor(a+(0,4u), normal, 0, "$95\U{k\Ohm}$", "$R_v$");
+TwoTerminal Iv = current(Rv.end, 0, "", "$I_v$");
+TwoTerminal Rb = resistor(Ia.end, normal, 0, "$27\U{k\Ohm}$", "$R_2$");
+TwoTerminal I = current((Rb.end.x, (a.y+b.y)/2), -90, "", "$I_T$");
+wire(Rb.end, I.beg, nsq);
+wire(I.end, b, udsq);
+wire(b, B.beg, nsq);
+wire(a, B.end, nsq);
+wire(a, Rv.beg, nsq);
+wire(Iv.end, Ia.end, rlsq);
\end{asy}
\end{center}
Using our formula for resistors in parallel, we can bundle $R_v$ and $R_1$ into a single resistor $R_1'$, where