OHM'S LAW w/ CBL
Gunn Physics
When a voltage is applied to a complete circuit containing resistance,
current begins to flow. In an electrical circuit, the function
of a resistor generally is to limit the flow of current in the
circuit.
When current flows through a resistor, energy is converted from
electrical energy into heat energy, and a potential difference
appears across the leads of the resistor. This voltage will be
studied as a function of the current flow in the circuit.
Ohm's Law describes the relationship between three important electrical
quantities: I, the current, V, the potential difference, and R,
the resistance. I = V / R, Ohm's Law, holds for a wide range of
electrical devices and electrical phenomena.
In this lab, you will vary the amount of voltage being applied
to two devices in series, one a standard value resistance and
the second your "sample". The voltage across the standard
value, 1000 W in the diagram below, can be
converted into
current by dividing by the resistance of the standard. The voltage across
the sample can then be related to the current flow.
PURPOSE
 Observe Ohm's Law in a dynamic situation
 Compare the behavior of different electronic devices
MATERIALS
 Dcell batteries (2)
 Potentiometer (25 W)
 Resistors, diode, LED's
 Wires
 Clips to hold wires
 Voltage probes (2)
 CBL
 TI82/83
BEFORE YOU BEGIN
 If Ohm's Law holds true, how should a graph of Voltage vs.
Current look for a given resistance? Explain.
 If Ohm's Law holds true, how should graphs of Voltage vs.
Current compare for different resistances? Explain.
 If a Voltage vs. Current graph differed from your prediction
in question (1), how might you interpret your results? Try to
develop multiple interpretations.
PREPARE THE EQUIPMENT FOR DATA COLLECTION
 Collect the necessary electrical components, wires, clips
and batteries. Construct the circuit as shown in the diagram,
but don't connect both wires from the batteries.
 Connect the voltage probes to the CBL, and connect them to
the circuit carefully, respecting the colors of the lead wires
as shown in the diagram. This is important in order for the data
to be interpreted properly.
 Prepare the CBL for data collection by launching "OHMSLAW"
under [PRGM]. The time has been set for 10 seconds.
 During the collection of data, you will rotate the potentiometer
from completely counterclockwise to fully clockwise and back
once. With practice this can take the full time allotted on the
graph. Move the shaft of the potentiometer smoothly in each direction.
PROCEDURE
 Record the value for your "standard" resistor, and
what your "sample" is in the data table.
 Connect the batteries to the circuit. Rotate the shaft of
the potentiometer fully counterclockwise in preparation for collecting
data.
 Start data collection. Follow the process described in the
previous step (4). At the end of 10 seconds, data collection halts.
 Obtain the graph of Voltage vs. Time. Sketch this graph for
future reference.
 Obtain the graph of Current vs. Time. Compare the shape of
this graph with the previous one.
 Now obtain the graph of Real V vs.Real I. Complete step (2)
or (3) in the Analysis.
 Change the sample to other choices given by your instructor.
Repeat steps (16) for each. Make a total of 5 data runs and analyses.
 When finished with the data collection, disconnect the batteries
from the circuit and clean up your work area.
SUGGESTED SETTINGS
Sample  Standard Resistance

470 W Resistor  1000 W 
2,200 W Resistor  1000 W 
Diode  1N4002  470 W 
LED  470 W 
DATA TABLE
trial  Sample
 Standard
Resistance
 Slope of V vs. I Graph 
1  
 
2  
 
3  
 
4  
 
5  
 
6  
 
ANALYSIS
 What is the shape of the Voltage vs. Current graph for a resistor?
What does this indicate about the nature of the relationship between
these two quantities?
 If the graph is a straight line, determine the equation. What
is the slope, both value and units? Is this value related in any
way to the size of the sample resistance?
 Do all of your samples obey straight line Voltage vs. Current
relationships? If not, describe what the shape of the graph indicates
for each. Some outside reading may be necessary to interpret the
shape.
 Be prepared to discuss the results of your experimentation
or answer questions about your results in a quiz format at the
next class meeting.
NOTE: The same setup will be used with the ULI or PASCO
interfaces. Replace the "CBL" in the diagram with the appropriate
computer interface.
Uploaded 3/11/98