Dr. W. Pezzaglia
Las Positas College
Physics 2B Lab, Spring 2010
Lab #2 Ohms Law
Page 9
2010Feb01
Lab 2: Ohm’s Law
Feb 1, Monday: Lab 2 (today)
• Lab #1 due
• Video: Mechanical Universe # 33 “Electric Circuits”
• Discuss: Graphing techniques using Excel, elementary statistics of comparison.
• Activity: Ohm’s Law & Resistivity, described below, based upon LPC documents:
o http://lpc1.clpccd.cc.ca.us/lpc/physics/pdf/phys2/OhmsLaw2B.pdf
o http://lpc1.clpccd.cc.ca.us/lpc/physics/pdf/phys2/resist.pdf
Feb 8, Monday: Lab 3 (next week)
• Video: Mechanical Universe # 30 “Potential and Capacitance”
• Activity: Capacitors and RC Circuits, based upon
• http://lpc1.clpccd.cc.ca.us/lpc/physics/pdf/phys2/rcseries.pdf
==================================================================
Theory:
Ohm’s law: proposed in 1827 by German physicist Georg
Ohm. The law states that the current that will flow through a
conductor is proportional to the applied voltage. The modern
form of the formula is:
V = IR
Voltage “V” is measured in volts.
Current “I” is measured in amps.
Resistance “R” is measured in ohms.
For an “ohmic device”, the resistance is a constant. Generally however, resistance
changes with temperature (note that temperature will usually increase with the amount
of current flowing through the device). Resistance usually increases with temperature,
but for some devices, called “semiconductors”, the opposite is true.
The resistance of a wire of length “L”, cross section area “A” would be theoretically
given as:
R=
ρL
A
where “ρ” is the “resistivity” of the material. For example:
Copper: 1.72x10-8 Ω•m
Carbon: 3.5x10-5 Ω•m
9
Dr. W. Pezzaglia
Las Positas College
Physics 2B Lab, Spring 2010
Lab #2 Ohms Law
Page 10
2010Feb01
Experiment:
Part A: Ohm Meter
•
•
•
An “unknown resistor will be provided
Use the “resistor code” to read its “theoretical” resistance (in ohms)
Use an “ohmmeter” (multimeter on “ohm” setting) to measure the resistance.
Question 1 Report the results of your experiment
a) What is the color code sequence on the resistor? (draw it)
b) What is the “translated” resistance value (and uncertainty)?
c) Compare this to your measured resistance. Are you within uncertainty?
Part B: Ohm’s Law
•
•
•
•
•
An “unknown resistor will be provided
Use the “resistor code” to read its “theoretical” resistance (in ohms)
Measure current as a function of voltage (at least 5 points)
Plot Current vs Voltage
Fit plot with best line, determine slope
Question 2 Report the results of your experiment
a) Is Ohm’s law valid? (i.e. is your plot a line? Is your R-squared value close to 1?)
b) From the inverse of the slope, determine the resistance.
c) Compare this to the known resistance. Are you within uncertainty?
10
Dr. W. Pezzaglia
Las Positas College
Physics 2B Lab, Spring 2010
Lab #2 Ohms Law
Page 11
2010Feb01
Part C: Non-linear Resistance (non-ohmic device)
•
•
•
Use a non-ohmic device (e.g. light bulb or diode)
Measure current as a function of voltage (5 to 10 points)
Plot Current vs Voltage
Question 3 Report the results of your experiment
a) Is Ohm’s law valid? (i.e. is your plot a line?)
b) From the plot, is the resistance higher or lower for a higher voltage?
c) Interpret.
Part D: Resistivity
•
•
•
•
•
Measure the length of one chromel (nichrome) wire to 0.001m with the meter stick and the
diameter to 0.01mm with the micrometer)
Tape the wire to a meter stick, leaving spaces where leads may be clipped
Measure the resistance for at least 10 lengths along the wire.
Plot Resistance vs length.
Fit with best line and determine slope
Question 4 Report the results of your experiment
a) Is the resistance of the wire proportional to its length? (i.e. is the plot a line?)
b) What is the slope of the plot?
c) What is the cross section area of the wire? (what is its diameter?)
d) From slope and cross section area, determine the resistivity of the material.
e) Compare your measured resistivity to the known value for the material.
Part E: Resistor Networks
•
•
Measure the resistance of two resisters in series. Verify that the total resistance is the sum
of the resistors.
Measure the resistance of two resistors in parallel. Verify that the total resistance is equal
to:
R1 R2
R1 + R2
Question 5 Report the results of your experiment
========================================================================
EQUIPMENT LIST
• Micrometer
• 2 Chromel (Nichrome) wires of different diameter, both approximately 0.95 m long
• 3 Resistors, 100Ω – 1000Ω
• 2 DMMs
• DIGI 35 A Power Supply
• Patch Cords, Alligator Clips
• Multimeter Accuracy List (include in lab write-up)
• CRC Handbook
• Color Code for Resistors
• Meter Stick
• Non-linear device (diode or light bulb)
11
Dr. W. Pezzaglia
Las Positas College
Physics 2B Lab, Spring 2010
Lab #2 Ohms Law
Page 12
2010Feb01
Resistor Code:
The “politically correct” mnemonic to remember the color code is:
Better be right or your great big venture goes west.
(Be advised that the one that everyone uses in the real world is not “PC”, completely sexist and
racist, and hence cannot be quoted here).
st
3rd band
nd
Color 1 band 2 band
(multiplier)
0
0
×100
Brown 1
1
×101
Red
2
2
×102
Orange 3
3
×103
Yellow 4
4
×104
Green 5
5
×105
Blue
6
6
×106
Violet 7
7
×107
Gray
8
8
×108
White 9
9
×109
Black
Gold
×10-1
Silver
×10-2
None
The 4th band (D) gives the tolerance (uncertainty).
Blank: 20%
Silver: 10%
Gold: 5%
Red:
2%
Brown: 1%
Green: 0.5%
Blue: 0.25%
Violet: 0.1%
Gray: 0.05%
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