|PH112L - Physics II Lab|
Winter Term 2014
Instructor: Galen C. Duree Jr., PhD
Office: Room DL102 Phone: 872-6025 Box: 182
Office Hours: 9:30 AM - 11:00 AM T F & 4:20 PM - 5:15 PM M F
The third series of experiments for the Physics II labs will be conducted in BL114 next week.
The title of the experiments to be performed is "Capacitance". A description of the procedure is given in the lab manual, beginning on page 112. The lab reports are due in the green bin outside my office by 5:30 PM one week after you perform the experiments.
Section 61 will meet at 8:05 AM on Monday, Jan. 26.
Section 58 will meet at 8:05 AM on Thursday, Jan. 29.
For the Capacitance lab, you are to determine the capacitance of the three given capacitors (using two different techniques) and evaluate the model for a parallel connection of two or more capacitors.
Using the constant current source and the appropriate circuit, find the slope of the line of the voltage vs. time data. The slope should equal the current (i) divided by the capacitance (C). Solve for C. Since it is relatively easy to get the data, do at least three trials for each capacitor and your parallel arrangement. When you compute the final capacitance value, compute the average and standard deviation of the set (which will have 3 values, at a minimum).
Using the constant voltage source to charge the capacitors, record the voltage vs. time as the capacitor discharges. Take data 3 times (at a minimum) for each capacitor and the parallel arrangement you used in the first part. In the Analysis Section, fit a natural exponent to the curve. The coefficient in front of the t in the curve fit is equal to 1/RC where R is the resistance of the resistor (in ohms) and C is the capacitance of the capacitor. When you look at the fit parameters in Logger Pro, the coefficient in front of t is labeled C. This C is NOT the capacitance; the Logger Pro C is just the fit parameter and is equal to 1/RC. Sorry about the confusing situation, but that is just a fluke of coincidence with this lab - at least you know about it BEFORE you do your analysis. Solve for the capacitance for each trial and compute the average and standard deviation for each capacitor and the same parallel arrangement you used in the first part. The values from the first part should agree, within uncertainty, with the values obtained in this part. If they don't, you should think of a reason why this happened.
The last thing to do is to evaluate the model for capacitors connected in parallel. Using the values for the individual capacitors that you determined previously, add the values together for the capacitors that you used in making a parallel connection (this should have any two OR all three of the capacitors). Remember to calculate the uncertainty in the sum. Compare this theoretical value to the value you determined in the two experiments (charging and discharging) performed with the capacitors wired in parallel. Simply determine if the two (theoretical and experimental) values overlap within uncertainty. If they do, the model is acceptable for your experimental situation. If they do not, the model was not acceptable for your experimental situation.
In the Analysis Section, you should list the filename and then the values that you pull from that file with the Logger Pro fit uncertainty. Do NOT just list the averages in your lab book - this is not acceptable procedure for the lab book analysis so you will be graded down if you do not list the raw data contained in the files you analyze!
In your Procedure Section, there should be two sample graphs, one sample (without curve fits) for the charging case and one sample (without curve fits) for the discharging case. You do NOT need to include a sample for each capacitor; just one example for charging a capacitor and one for discharging a capacitor.
In the Analysis Section, there should be two sample graphs, one sample, with curve fits, for the charging case and one sample for the discharging case.
Remember: Departmental policy is that there should be one graph per page and it should be the only thing on that page. Also, make sure that you label the axes and include appropriate units!
Lab #2 Description:
For the "Resonance in Strings" experiment, you will be evaluating two models for waves on strings and determining the wave speed for a particular tension. The first part of the experiment will involve finding at least five (5) resonance frequencies for a particular tension in a string. Make sure that you mention in your procedure section how you determined the correct resonance frequency for a particular mode of vibration. Next, you will need to choose a single mode of vibration that was easy for you to work with in the first experiment (say, the n = 3 mode), and then change the weight hanging off the end and find the new frequency for the new tension. Since you have done one tension already in the first experiment, you need to do four more (at a minimum, but you can do more since it is rather easy) so that you have at least five different tensions.
In the Analysis section, make a graph of frequency vs. mode number (n) WITH ERROR BARS!. Fit a line to this and evaluate the model by seeing if a majority of the error bars for the points cross the fit line or not. If they do, the model is appropriate for your situation. If not, the model is not appropriate for your situation. The slope of this line is equal to the fundamental frequency for the string. Use this value (with its uncertainty) to determine the speed of waves on the string for the particular tension (this is the experimental wave speed!). Compare the experimental value to the theoretical value of SQRT(Tension/mass per unit length) (again, make sure you include uncertainties!). The second graph you should make is a plot of fn vs. SQRT(Tension), with error bars. If a majority of the error bars for the points cross the fit line, the model is appropriate for your situation. If not, the model is not appropriate for your situation.
Download a copy of the Error Analysis Worksheet
The last series of experiments will be conducted during the week of February 9, 2015.
Lab Report Format
A complete lab report must have the following sections:
Do not forget to update your Table of Contents and to number the pages in your lab notebook!
The first page of your report must have a title, the name of the P.I. (explicitly indicate who was the P.I.), the name of the lab assistant, and the date that the report was turned in (not the date that you performed the experiment!).
*Abstract - a BRIEF description of what you were investigating, how you conducted the experiment and your conclusions based on your experience. The abstract should not be a discussion of what you are going to do in the lab. The abstract cannot be written before you have analyzed your data. In the lab notebook, skip a page or two so that you can write this in at the beginning of the report after you have analyzed your data.
Introduction - a discussion about any details that you think would help someone perform the experiment. This should include a discussion about the model (equations that you are using) or the method selected for performing the tests. If you use models (equations) in your analysis, list the models in this section and describe what each of the variables represent.
**Procedure - a detailed description of what you did in the lab. At the beginning of this section, place the date and the time that you began the experiment. This section must include a schematic, detailing how things were connected, where appropriate. The raw data must appear in this section immediately following the description about how the data was taken. A person must be able to read your procedure section and be able to duplicate your results without having the lab manual present. Do not do any calculations in this section, just record how you performed the experiments and record the raw data. If the data is taken by computer, you must specify the path and filename where the data is stored. This information must be listed in this section right after the description of how the data was taken. Do not simply list all of the files generated at the end of the section.
This section be signed by the instructor before you leave the lab. If you turn in a report without the signature, it will not be accepted!
Analysis - a sample of the calculations made in the lab. This section should include a sample of the error calculations and propagation of errors used in your analysis. The final data that you are analyzing to generate conclusions, the values with appropriate uncertainties, must be shown in this section. The actual calculations for each one does not have to be included, as long as you show an example for one, but you may include them all if you wish. The calculations may be done by the computer, but include printouts of the worksheet in your lab book. Any graphs or printouts that are placed in your notebook must occupy one whole page and be trimmed to fit within the page and not hang outside of the notebook.
*Conclusion - this section must have a conclusion that is based on your experiments and analysis. If your conclusions do not following logically from your analysis, your grade will be deducted significantly. This section must also contain a brief description of significant factors that you think affected your data, in particular, the uncertainties in your data (factors that contributed to the error in your experiment). A good thing to keep in mind is to think of this report as a report you are submitting as part of your job responsibilities. If you do not think your boss would accept what you have to say, it is a safe bet that I will not like it either.
The asterisks indicate the sections that I will pay closest attention to.
Modified January 23, 2015 by Galen C. Duree Jr.