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Ohm's Law
Purpose: to perform an experimental check of Ohm's Law.
Discussion: (The discussion is adapted from the LabPro lab – Ohm’s Law)
The fundamental relationship among the three important electrical quantities current, voltage, and
resistance was discovered by Georg Simon Ohm.
V  IR
The relationship and the unit of electrical resistance were both named for him to commemorate this
contribution to physics. One statement of Ohm’s law is that the resistance is equal to the ratio of the
voltage across the resistor and the current through the resistor.
R=V/I
In this experiment you will see if Ohm’s law is applicable to several different resistors using a Power
supply with a voltage readout and a current readout.
Current and voltage can be difficult to understand, because they cannot be observed directly. To clarify
these terms, some people make the comparison between electrical circuits and water flowing in pipes.
Here is a chart of the three electrical units we will study in this experiment.
Electrical Quantity
Description
Unit
Water Analogy
Voltage or Potential
Difference
A measure of the Energy
difference per unit charge
between two points in a
circuit.
Volt (V)
Water Pressure
Current
A measure of the flow of
charge in a circuit.
Ampere (A)
Amount of water
flowing
Resistance
A measure of how
Ohm ()
difficult it is for current to
flow in a circuit.
A measure of how
difficult it is for water
to flow through a pipe.
In this lab, you will construct a simple circuit using a single unknown resistance, R. Then you will use
an ammeter to measure the current, I, through the resistance and a voltmeter to measure the potential
difference, V, across the resistance. With this data, you can check the validity of Ohm's Law (V = IR)
in the circuit. We will also be looking at how the resistance is affected by the length of the wire, the
cross-sectional area (a bigger gauge number indicates a thinner wire), and the type of material the wire
is made of.
Materials
Multiple Resistor Device
Power supply with Ammeter and Voltmeter
Red and black alligator/banana plugs
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Procedure:
Before Experimenting: Use the resistor device to fill in the data table of resistor information. You
will complete the table by calculating the resistance of each resistor as you work through the
experiment.
Procedure: The effect of length of a wire and resistance.
1. Turn the current knob all the way to the right. (DO NOT TURN THIS KNOB AGAIN FOR
THE ENTIRE EXPERIMENT)
2. Turn the course voltage knob to the left.
3. Take a red wire that has a banana plug and alligator clip. Plug in the banana end to the power
supply to the red port on the power supply. Plug the black wire into the black port.
4. Attach the alligator clips to the ends of the no. 1 resistor. (Red on one, Black on one)
5. Turn on the power supply and notice how you can adjust the voltage a large amount with the
“course” knob and you can make small adjustments with the “fine” knob. Adjust the voltage to
take 5 readings of voltage and current between 0 and 2 V. Record your data in the table.
6. Use the 2nd statement of Ohm’s Law from the Introduction to calculate the resistance for each
reading. Calculate the average of all the resistance readings and record that value in the
“Resistor information” table.
7. Repeat step 5 and 6 for the no. 3 resistor.
8. Calculate your percent error.
Resistor information
Resistor Number
Stated
Resistance
(ohms)
Measured
Resistance
(ohms)
Percent Error
1
2
3
Circuit Diagrams: Below is the circuit diagram that would be used to represent each of these simple
circuits. Notice that the voltage source has one line shorter than the other. By convention, the shorter
line represents the negative terminal of the voltage source.
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Data Tables for Procedure 1
Voltage
Resistor 1
Current
Resistance
Resistor 2
Current
Resistance
Resistor 3
Current
Resistance
Average Resistance
Voltage
Average Resistance
Voltage
Average Resistance
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Analysis and Conclusion:
1. From the data in procedure 1, create a graph that illustrates the
relationship between voltage and current. Use voltage for the
vertical axis and current on the horizontal axis. (One graph, 3
lines).
Do not “connect the dots” for each line. After you have
plotted the data for a resistor, try to “eye” a best-fit straight line
through the points.
2. Calculate the slope of each line. Label each with the slope. What
does the slope represent? Does it agree with the calculated
measurements?
3. Write out Ohm’s Law. Do all the resistors obey Ohm’s law conceptually? Explain.
4. What other factor may be affecting the data that may not make our results perfect?
To turn in: Write a formal lab report detailing the experiment. This
report must be typed. The report should include separate sections for the
following: Purpose, Hypothesis, Materials, Procedure, Safety Precautions,
Data, Analysis, Conclusions and Error Analysis. Each section should be
identified by a bolded heading.