A voltmeter is used to determine the voltage across a resistive element in the circuit of Figure P2.77. The instrument is modeled by an ideal voltmeter in parallel with a 120-k? resistor, as shown. The meter is placed to measure the voltage across R4. Assume R1 = 8 k?, R2 = 22 k?, R3 = 50 k?, RS = 125 k?, and iS = 120 mA. Find the voltage across R4 with and without the voltmeter in the circuit for the following values:
a. R4 = 100?
b. R4 = 1 k?
c. R4 = 10 k?
d. R4 = 100 k?
Known quantities:
Schematic of the circuit shown in Figure P2.77, values of the components.
Find:
The voltage across with and without the voltmeter for the following values:
a)
b)
c)
d) .
Assumptions:
The voltmeter behavior is modeled as that of an ideal voltmeter in parallel with a 120- k? resistor.
Analysis:
We develop first an expression for in terms of . Next, using current division:
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