Lab 4 Kirchhoff’s Rules
Introduction
This lab examines and verifies both Kirchhoff’s rules pertaining to resistor circuits.
Equipment
Power supply, DMM, resistors, assorted wires and clips.
Background
Kirchhoff’s rules are circuit-related statements about the conservation of charge (the junction
rules) and the conservation of energy (the loop rule). The junction rule state that the total current
entering any point in a circuit is equal to the total current exiting that point.
I in total = I out total
The loop rule state that the total change in the potential energy (and thus the potential) for a
charge must be zero when a charge travels around a closed loop.
Σ
closed loop
ΔV = 0
Experiment: Building the Circuit
Build the following circuit using the power supply set at 10.0 V. Use three different resistors as
shown.
R2=500 Ω
R1=1000 Ω
R3=2000 Ω
Experiment: Junction Rule
Measure the four currents as shown in the diagram below. The DMM measures a positive current
if the current flows from the “mA” plug to the “COM” plug.
mA COM
A2
A3
COM
mA
A1
mA
A4
mA COM
COM
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Indicate the directions and label values of the currents in each segment of the circuit using the
diagram below.
J1
J2
There are two meaningful junctions here, J1 and J2.
Questions
For junction J1, calculate the total current entering and exiting it. Show work.
For junction J2, calculate the total current entering and exiting it. Show work.
How would you know which current (A2 or A3) is larger by looking only at the resistors R2 and R3.
Experiment: Loop Rule
Measure the four voltages as shown in the diagram below. The voltage is the potential at the V
side minus the potential at the COM side of the DMM. The voltages go up or down depending on
the direction toward which you traverse the circuit. We will traverse the circuit clockwise.
COM
COM
V1
V
COM
V
V0
V2
V3
V
V
COM
Start at the “–” side of the power supply. Let this voltage be zero volt. Indicate the voltages at
each point based on your measurements.
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0V
Questions
Does the voltage go up or down as you traverse the power supply clockwise?
Does the voltage go up or down as you traverse resistor 1 clockwise?
Does the voltage go up or down as you traverse resistor 2 clockwise?
Does the voltage go up or down as you traverse resistor 3 clockwise?
There are three possible loops in this circuit. Start at the point marked and go around clockwise.
loop 3
loop 1
loop 2
For loop 1, add up the clockwise voltages. What is the sum?
For loop 2, add up the clockwise voltages. What is the sum?
For loop 3, add up the clockwise voltages. What is the sum?
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