PHYS 223 General Physics
Laboratory Session #4
Resistance Networks
a
b
?
c
d
By now you are familiar with resistors and series/parallel reduction of
resistor networks. This lab will remind you of the resistor color code and
reinforce your knowledge of resistor networks.
Activity 1: Color Code - A Review
Resistors ordinarily used in electronic circuits have more or less linear resistance. The resistance is indicated by numbers (for high-precision parts), or
by painted stripes. The color code is given in the table below.
Color
Value Multiplier
Black
0
100
Brown
1
101
Red
2
102
Orange
3
103
Yellow
4
104
Green
5
105
Blue
6
106
Violet
7
107
Gray
8
108
White
9
109
Gold
10−1
Silver
10−2
None
1
Tolerance
5%
10%
20%
Tolerance
Read from
this end
Example: Yellow−violet−orange−silver
3
47 x 10
5%
47,000 ohms +− 5%
47K +− 5%
First digit
Multiplier
Second digit
Individual Resistors
You are given five resistors R1 . . . R5 . Record the color code on each resistor
and “decode” the values. Identify each resistor with a bit of masking tape.
Using a digital ohmmeter, measure the resistance of each resistor and record
this, as well. A table is convenient:
Resistor
number
1
2
3
Color Code
Value
Ohmmeter
Value
ETC.
Equivalent Resistance
Examine the resistor networks below. In each case, predict what the equivalent resistance of the network will be. Then, using the circuit board provided
and perhaps some alligator clip leads, construct the networks and measure
the equivalent resistance of each circuit. (Again, use the digital ohmmeter.)
Make sure that you have good connections between resistors and that you
press the ohmmeter leads tightly against the resistor wires when measuring
resistance.
2
Circuit 3
Circuit 1
R1
R2
R3
R1
R5
R3
R4
Circuit 2
R1
Circuit 4
R1
R3
R3
R4
R4
R5
R2
Activity 2: Black Boxes
Several “black boxes” are available in the lab. (Some may be different in
color.) They contain simple resistor networks connected to the four terminals.
Your job is to deduce the network contained in two of the black boxes.
Identify each box you work on by its number. Measure the resistance
between every pair of terminals (a-b, a-c, etc.). You will have six resistance
measurements for each box, then. From this information, draw the simplest
resistor network which could possibly be in the black box.
a
b
?
c
d
Each group should analyze 1 different resistor boxes for each person in the
group (minimum of 3 boxes).
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Activity 3: Resistance networks and voltage
drops
I. Set up the circuit below with 3 resistors of equal value. The power source
should be a power supply operating at 5 V. Using a digital voltmeter, measure
the voltage drop across the power supply. Now, before doing the measurement, predict what the voltage drops will be across each resistor. You should
not have to calculate any currents to do this! Do the measurements with a
voltmeter and compare to your predictions.
R1
R2
+
V
−
R3
R1 = R2 = R3
II. Now set up this circuit, with 4 resistors of equal value. Predict the
voltage drop across each resistor. Measure the voltage drops and compare
with your prediction.
R1
+
V
−
R2
R3
R4
R1 = R2 = R3 = R4
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Activity 4: Resistor Cube
Get a resistor cube from your instructor.
Do the following:
1. Use the resistor color code to determine the resistance of a each resistor
in the resistor cube.
2. Assuming the value you determined is correct for each resistor, use
symmetry to determine the resistance of the cube from one corner to
the opposite corner.
3. Measure the resistance of the cube.
4. Compare and Contrast. Is your measurement within tolerance?
5. Assuming the value you determined is correct for each resistor, use
symmetry to determine the resistance of the cube from one corner to
an adjacent corner.
6. Measure the resistance of the cube.
7. Compare and Contrast. Is your measurement within tolerance?
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Activity 5: Conceptual Questions
Serway, 1-15.
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