# Discovering Ohm's Law - DOC by 8yn4uU4

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Alabama Science in Motion                                                 Electricity: Discovering Ohms Law

Discovering Ohm’s Law
Prediction (sketch your prediction on the Student Response sheet)
How does the voltage across the resistor change as the current through a resistor is
increased? In the student response section, sketch your prediction for the shape of a
graph of the Voltage vs Current for a resistor.

Qty            Equipment and Materials
1              GLX with setup file: ohms law
1              Circuit Trainer Kit
1              Wire Kit
2              Multimeters
2              Banana to Banana test leads (preferably one red and one black)
1              Printer

Safety Precaution
     Follow all directions for using the equipment.
     Do not plug in or turn on the project board circuit trainer until your
     The resistor can get very hot. Do not to touch the resistor when
the circuit is closed.
Background
In the early 1800’s, Georg Simon Ohm discovered a fundamental
relationship between the current through a resistor and the voltage                 Georg Simon Ohm
across the resistor.
Overview
Use the variable power supply on the
Project Board circuit trainer and an
ammeter to step the current through a
resistor while measuring the voltage
across a resistor. After collecting the
data, use the GLX to graph the voltage
measured versus the set current. A
curve fit of your data should reveal
Ohm’s Law.

Revised 1/09                                                                                    Page 1 of 5
Alabama Science in Motion                                           Electricity: Discovering Ohm’s Law

Procedure
Equipment Setup: (Do not turn on your circuit trainer or meters until your
1. Select the 330 ohm resistor (orange, orange, brown) from your equipment.
2. Use the blue multimeter as a volt/ohm meter to test the experimental value of the 330 ohm
resistor. Record this experimental value in your data table.
3. Build a simple series circuit with a 330 ohm (330 Ω) resistor on the circuit trainer using the
variable (0 to +15)V supply.
   Use the yellow multimeter as current meter (wired in series with the resistor). Set the
ammeter to 200mA DC.
   You will not use the pointed probes with the ammeter in this experiment. Instead, use a red
banana to alligator clip wire as the positive test lead on the ammeter. Connect the alligator
clip to the resistor and place the banana plug of the red lead into the mA slot on the
ammeter. Use the black banana to banana test lead to connect the com port on the
ammeter to the common post on the circuit project board.
   Use the blue multimeter as a voltmeter (parallel with the resistor). After your circuit has
been checked by the instructor, set the voltmeter to 20V DC. Note: the voltmeter uses the
pointed probes to touch across the resistor once the current has been set to the
predetermined value. The voltmeter should only be connected during the measurement of
the voltage across the resistor. DO NOT insert the voltmeter probes into the breadboard or
you will damage the board.
Data Collection
During the data collection, you will use an ammeter set to milliamps due to the scale of
the experiment. However, Ohm’s Law is typically expressed in terms of volts and amps.
Therefore, your data table displays these current settings in Amps while the instructions
below refer to the milliamps settings you will use on the ammeter.
1. Once your instructor has checked your circuit, turn on your circuit trainer and both of your
multimeters.
2. Adjust the variable supply until the ammeter reads 0.0 mA. Note: the ammeter may fluctuate
in the tenths place, for example between -0.1 and 0.1 mA. Try, but do not spend too much
time trying to get the exact current value.
3. Measure and record the voltage for the 0.0ma setting in the data table.
4. Increase the current by 5 mA, measure the voltage and record the new voltage
measurement in the data table.
5. Continue increasing the current in 5 mA steps while recording the voltage for each step in
current until you complete the data table. (see step 6 before turning off the equipment).
6. In addition to measuring the voltage across the resistor for the 40mA setting, measure and
record the voltage across the power supply (+15V terminal to com). You will use this data
7. Turn off your multimeters and circuit trainers to save power, but DO NOT put away the
equipment until you instructor tells you. You may be asked to take additional data.

Revised 1/09                                                                               Page 2 of 5
Alabama Science in Motion                                       Electricity: Discovering Ohm’s Law

Data Analysis (GLX Setup)
Turn on the Xplorer GLX ( ) and open the GLX setup
file labeled Ohms Law.
1. In the Home screen, select Data Files and press
.
2. In the Data Files screen, use the arrow keys to
navigate up and over to Flash.
3. Arrow down to highlight Ohms Law.
4. Press    to open the file. (Note, the screen will blink and the word “Open” should
appear next to the file name.)
6. Press F2 (Table) to view the data table.
     The setup file has a Table with columns for ‘current (amps)’ and ‘voltage (v)’.
Note, the pre-set current values have been converted from milliamps to amps in
the GLX file.
7. Use the arrow keys to select the first cell under ‘voltage (v)’. Press F2 to make the
cell active for editing. Use the alpha-numeric keys to enter your first value of Voltage
from your Data Table. Press         to activate your entry (and automatically select the
next cell in the column).
8. Enter the rest of your values for Voltage and press        to activate each entry (and
automatically select the next cell).
Note: To make a correction to an entry after pressing    , press ESC and a dashed box
appears around the active cell. Use the arrow keys to move to the cell with the error.
Select F2 (Edit Cell), Correct the entry and press    to activate the change.
10. Press F1 to open the Graph screen. The graph has ‘voltage (v)’ on the vertical axis
and ‘current (amps)’ on the horizontal axis.
11. Press F3 (     ) to open the ‘Tools’ menu and select ‘Linear Fit’. Record the value of
the slope in the Data Table.
12. If a printer is available, print your graph. Otherwise, ADD a sketch of your graph of
voltage versus current to the same axis used to make your prediction in the Student
Response section of the lab.
13. In your data table, calculate the ratio of voltage to current for each data point.
14. Determine the average value of the ratio, voltage/current. Note, do not include the
(0Amps, 0Volts) reference point in your average calculation.

Revised 1/09                                                                           Page 3 of 5
Alabama Science in Motion                                                        Electricity: Discovering Ohm’s Law

Student Response Section
Name _________________________                                           Partner’s Name
Period                                                                   Date ___________

Prediction: How would the voltage across a
resistor change as the current through the
resistor is increased? Sketch a graph of voltage
versus current on the axis provided. Include
units and labels for your axes.

Data
Experimental value of your 330 ohm resistor.
R-Theoretical              R-Experimental (using the ohm-meter)
330 Ohm
*Note current has been converted to Amps.
Run #         Current          Voltage Voltage/Current                              Slope from the
(converted to amps)                                                         Graph Curve Fit
(Amps)            (Volts)    Volts/Amps                               Note the Units
1              0.0000                          NA*
2              0.0050
3              0.0100
4              0.0150
5              0.0200
6              0.0250
7              0.0300
8              0.0350
9              0.0400
*Average (Voltage/Current) =
*Do not include the (0 Amps, 0 Volts) reference point in your ratio and average calculations.
Power supply Voltage reading from step 6 in the data collection section:
Voltage(s) at 0.040 Amps                                       (Volts)

Revised 1/09                                                                                            Page 4 of 5
Alabama Science in Motion                                           Electricity: Discovering Ohm’s Law

Questions and Problems:
1.    Based on your graph, write an equation that relates voltage- V and current-I. (Hint: the
equation of a line is y=mx+b. What did you plot on the y-axis?...the x-axis?...)

2.    From the equation of a line, m represents the slope, (∆y/∆x). What symbol is used in
Ohm’s Law to represent the slope of a Voltage vs Current graph and what does this
symbol represent?

3.    According to Ohm’s Law, the ratio of voltage to current is the resistance. Compare the
Ohm-meter value for resistance to the slope of the Linear Fit of your graph. Calculate the
percent difference. (difference/average)x100%

4.    Describe in detail how you would use your graph to approximate the voltage when the
current is 0.0225 A. What value does your graph indicate for 0.0225 A?

5.    Using your graph, approximate the current when the voltage is 10V?

6.    You have probably heard that you can’t get something for nothing. In step 6 of the data
collection process you were asked to measure and record the power supply voltage in
addition to the voltage across the resistor when the current is .040 A. The purpose of this
measurement is to demonstrate that not all of the voltage of the supply is consumed by
the resistor. There is some loss of voltage (Ir) by the power supply in delivering current(I)
to the circuit resistor(R). Use the following expression to determine the voltage lost
internally to the supply (Ir). Finally, calculate the internal resistance (r) of the supply.
Eq: V(supply) = IR + I(r)

Revised 1/09                                                                               Page 5 of 5

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