Download Lab Writeup Ohm`s Law

Ohm’s Law
LBS 267L
Purpose
The purpose of this experiment is to investigate the relationship between current and voltage in direct current circuits.
Theory
Ohm discovered that when the voltage across a resistor changes, the current through the resistor changes. He expressed this as I
= V/R (current is directly proportional to voltage and inversely proportional to resistance). In other words, as the voltage increases, so
does the current. The proportionality constant is the value of the resistance. The current is INVERSELY proportional to the resistance.
As the resistance increases, the current decreases.
If the voltage across a resistor is increased, the graph of voltage versus current shows a straight line (if the resistance remains
constant). The slope of the line is the value of the resistance. However, if the resistance CHANGES, the graph of voltage versus
current will not be a straight line. Instead, it will show a curve with a changing slope.
For a light bulb, the resistance of the filament will change as it heats up and cools down. At high AC frequencies, the filament
doesn’t have time to cool down, so it remains at a nearly constant temperature and the resistance stays relatively constant. At low AC
frequencies (e.g., less than one Hertz), the filament has time to change temperature. As a consequence, the resistance of the filament
changes dramatically.
Equipment Needed
Macintosh computer
Signal Interface II (CI-6552)
Science Workshop
Power Amplifier (CI-6552)
RLC Circuit (CI-6512)
two banana plug patch cords
Ohm’s Law Lab Write-up
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Setup
1. Make sure the power switches for the Signal Interface and Power Amplifier are on. Make sure that the Power Amplifier is
plugged into Analog Channel A.
2. Connect banana plug patch cords from the output of the Power Amplifier to the jacks on either side of the 10Ω resistor on the RLC
circuit board.
Data Recording
1. Open the Science Workshop document “Ohm’s Law Scope Only”. A Scope display will open along with the Signal Generator
window. Note that the Signal Generator is set to produce triangular pulses with an amplitude of ±5 volts and a rate of 1 Hz.
2. Click on the "MONitor" button. Observe the signal. Click on the “STOP” button while the full wave form is visible. Export the
Scope display to a file called “Scope Display”. Note that this file is a graphics file.
3. Open the Science Workshop document “Ohm’s Law All”. A Scope display will open along with a Graph window and a Table
window.
4. Click on the “RECord” button. The computer will take data for 2 seconds and stop.
5. Select the Graph window and click on the “Autoscale” button to observe the results. Select the Table window and export the data
to a text file called “10 ohms Data”. Delete the recorded data set.
6. Connect the banana cords to the 33Ω resistor and repeat the experiment. Save the data to a file called “33 ohms Data”. Delete the
recorded data set.
7. Now connect the banana cords to either end of the light bulb and repeat the experiment. Save the data to a file called “Light Bulb
Data”. Now click on the clock icon of the Table. This step will add time to the information recorded. Save this file as “Light
Bulb Time Data”. Delete the recorded data set. Close the program Science Workshop.
Data Analysis
1. Start the program KaleidaGraph and open the data files “10 Ohms Data”, “33 Ohms Data”, and “Light Bulb Data”. Make sure that
the lines skipped and delimiter are set correctly. There should be two lines skipped and 1 tab as a delimiter.
2. Make a plot of the data file “10 Ohms Data”. Under the Curve Fit menu select General and fit1 to produce a linear fit to the data.
Display the equation on the plot. The parameter m2 is the slope which corresponds to the resistance. Save the graph as a file
called “10 Ohm Graph”.
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3. Repeat the above procedure for the file “33 Ohm Data”. Save this plot as “33 Ohm Graph”.
4. Make a plot of the data file “Light Bulb Data”. Note that the data are not a straight line. Do not fit these data. Save this plot as
“Light Bulb Graph”.
5. Start the program Excel and open the data file “Light Bulb Time Data”. Make a new column entitled “Resistance (Ohms)”. In this
column calculate the ratio of voltage to current. Save the file as a tab delimited text file with the name “Light Bulb Resistance
Data”. Close the program Excel.
6. Make a plot of the data file “Light Bulb Resistance Data” using KaleidaGraph. Plot the time on the x axis and resistance on the y
axis.
Questions
1. What is the measured resistance of the two resistors. Report your results with proper significant figures and error bars.
2. Why is the plot of voltage versus resistance not a straight line for the light bulb?
3. What is the range of resistance for the light bulb? What is the temperature rise of the filament assuming it is made of tungsten?
(Hint, look on page 772 and 773 in Halliday, Resnick, and Walker). Estimate the uncertainty in your answer.
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