DC Circuit Analysis
Object: The purpose of this lab is to understand how current distributes in a DC circuit of
resistors.
Apparatus: AC/DC electronics lab board, resistors, and multimeter.
Introduction: When a circuit composed of resistors is connected to an emf device
(battery), there are some simple rules that can be applied to determine the current through
each circuit element (each resistor).
Loop Rule: The algebraic sum of the changes in potential encountered in a complete
traversal of any loop of a circuit must be zero.
Resistance Rule: For a move through a resistance in the direction of the current, the
change in potential is –iR; in the opposite direction it is iR.
EMF Rule: For a move through an ideal emf device in the direction of the emf arrow, the
change in potential is +emf; in the opposite direction it is –emf.
Junction Rule: The sum of the currents entering any junction must be equal to the sum of
the currents leaving that junction.
With these rules in hand, one can solve for the current in any branch of a circuit, if one
knows the value of the emfs in the circuit and resistance of the resistors.
Note: To measure currents in a circuit one must “break the circuit” and insert the
ammeter into the branch of the circuit where one is trying to measure the current.
Procedure: Choose two resistors that have different values, but are within a factor of a
100 of each other. To determine the resistance of each resistor, use the color coded
printed on the outside surface.
Figure 1: Diagram used to read color coded resistances.
Enter the information on the two chosen resistors below. In addition, use the mulitmeter
as an ohmmeter to measure the resistance of each resistor and check whether the color
coded value is within tolerance.
Table I
st
1
Colors
2nd
3
rd
Coded
Resistance
Tolerance
Measured
Resistance
Discrepancy
Significant?
#1
#2
Now check that the two batteries that are in the electronics lab board are properly
charged. Fill in the information below.
Table II
Labeled emf (Voltage)
Tolerance
#1
5%
#2
5%
Measured
Voltage
If the discrepancy is significant, ask your instructor for new batteries.
Discrepancy
Significant?
Part A. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 2: Circuit A.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table III
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
Part B. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 3: Circuit B.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table IV
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
Part C. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 4: Circuit C.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table V
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
Part D. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 5: Circuit D.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table VI
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
Part E. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 6: Circuit E.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table VII
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
Part F. In the circuit shown below, predict the values for the currents based upon the
rules listed in the introduction. Use the labeled emf and the color coded resistance in your
calculations. Note the direction of the batteries and the direction of the arrows for the
current. Write your predictions in the table below.
Figure 7: Circuit F.
Now build the circuit and measure the current using the ammeter to check your
predictions. The current should enter in the red probe and exit the black probe. Recording
your measurements in the table below and determine whether or not the discrepancy from
your predicted value is significant.
Table VIII
Prediction
Tolerance
i1
~5%
i2
~5%
i3
~5%
Measured
Current
Discrepancy
Significant?
The Laboratory Report: There is no lab report to turn in for this lab. Instead, please
turn in this lab manual, one per group.
LAB GROUP MEMBERS [List names below]