Physics 101L General Physics - Laboratory

Fall 2000

Sections:

Lab Manual:

Introduction

Scientific Skills

For example, suppose you are trying to measure the heat loss of an object by recording a change in its temperature. After careful thinking about the physics of this experiment, you decide that you expect a temperature change no larger than 1.0° C. Based on this estimate, you conclude that you need to choose a thermometer that can accurately measure temperature in increments of 0.1° C or smaller. Also, you will need to make sure that no outside processes such as changes in the room temperature, holding the object in your hand, air conditioner blowing on the object, etc. will affect the temperature of the object during the measurement. If you have paid attention to all of these details, and have a rough idea of what to expect, you will be able to judge whether or not your measurement makes sense. If you are careless, don't pay attention to experimental details, and don't have any idea what result to expect, you would probably not be concerned if you measured a temperature change of 10° C, and would probably only discover the problem much later, when you begin to write your lab report.

The second skill necessary for good scientific research is really an outgrowth of the first skill presented above. When an experiment is finished, the usefulness of the results depend entirely on the uncertainty associated with the measurement. This means that you must constantly pay attention to details of the experiment and measuring apparatus that could give you erroneous results. It also means that you must accurately estimate these results when you present your final data. For example, if you are trying to measure the temperature of an object, and your thermometer seems to consistently fluctuate during any measurement by ± 2° C, you must include this error in your results. For example, you might say ``We measured a temperature of 33 ± 2° C.'' By paying attention to possible sources of error, and properly including these uncertainties in your final results, you give the reader a true sense of the significance of your results.

Lab Report

A good lab report will always have certain qualities which make it useful to outside readers. First, the report should be written in such a way that a non-expert (someone not in your lab class) could read it and learn what principles you were trying to test, how you did the measurement, what data you obtained, whether or not you confirmed the theoretical prediction, and what errors were associated with the measurement. Also, it goes without saying that the report needs to be readable, with complete sentences and proper grammar. A report with too little information will not be useful, and a report which is filled with unnecessary text and equations will often confuse the reader. Graphs need to be properly labeled and numbers should always be quoted with the appropriate units. Finally, the document needs to be readable. It is not a prerequisite that the report be typed, but handwritten reports are often messy and difficult to read. Word-processors such as Microsoft Word and Word Perfect have equation editors and work well for reports, and spreadsheet software such as Microsoft Excel do a great job at graphing and making concise tables of data.

To help guide you through your first lab report, here is an outline with suggestions for the type of information you might need to include in each section.

1. Introduction: In this section, you should briefly describe the motivation for the experiment. This would typically be a few sentences and might include an important equation. For example, ``In this experiment, we will attempt to confirm the ideal gas law which is written as PV=nRT. This law gives us a relationship between the pressure, volume, number of molecules, and temperature of a gas. The law will be tested by varying one quantity (such as temperature) and confirming that the changes in the other quantities (pressure, volume, and number of molecules) follow the behavior dictated by the ideal gas law.''
2. Procedure: In this section you will give a brief description of the apparatus you will use and the procedure you will follow to accomplish the measurement. For example, ``In step one, we will fill a container of volume V with a known number of gas molecules n. We will then slowly heat the container while measuring the temperature and pressure of the gas. If the ideal gas law is obeyed, a graph of the pressure versus temperature should follow the equation P =(nR/V)T, with a slope of nR/V.''
3. Data and Analysis: This section should contain a clear presentation of the data obtained. A table of data is usually a good idea, and graphs are highly encouraged (and often required in many labs). It is important that you include all data, even if you suspect it is in error. Just point out the problem and offer a reasonable explanation for why the result might be inaccurate. Also remember to label all graphs, and include units for all numbers listed. This section of the report is also the place where you do calculations, make comments or statements about the results, and report any uncertainties, problems or surprises that you encountered during the experiment. After your data and graphs are presented, you might write: ``The plot of pressure versus temperature shown above has a slope of 10.1 Pascal/Kelvin. The expected value from the ideal gas law is 11.0 Pascal/Kelvin. Note that data point number three is very far from the rest of the points on the graph which might indicate a measurement error or problem with the apparatus...., etc.'' This section is also the appropriate place to answer questions that are asked in the lab manual or additional questions that your TA might have for you.
4. Conclusion and Error Analysis: This is the section where you state whether or not you confirmed the law being tested and present a thorough description of any relevant errors. For example, ``In this experiment we measured the relationship between pressure and temperature for a gas with a fixed volume and number of molecules. A plot of pressure versus temperature yielded a slope of 10.1 Pa/K and the ideal gas law predicts a value of 11.0 Pa/K. However, the pressure gauge was not stable and typically had an error of ± 20%. With this measurement error, our slope is 10.1 ± 2.0 Pa/K and is therefore consistent with the ideal gas law prediction.''

Grading/Attendance policy

Labs reports are due at the beginning of the lab, one week after they are completed. Your lowest lab grade will be dropped at the end of the semester and the remaining grades will be averaged together for a final lab grade. Also, it will not be possible to pass the lab class unless at least seven (for 101 and 107) or eight (102 and 108) of the labs are attended, and a completed report is handed in. Completing this minimum number of labs does not guarantee you will pass, but completing less than this minimum guarantees that you will not pass. We realize that each TA will grade a little differently from the others, so grades from all lab sections will be normalized to a common grading scale before they are added to your final course grade.

To earn a successful grade for your lab each week requires that you successfully complete all steps of the experiment, write a lab report that presents the experiment, data, analysis, and error analysis in a clear and concise manner, and correctly answer all of the questions in the lab manual, and those presented by your TA. Additional creativity and useful input beyond what is asked for in the lab manual is encouraged, and will often earn you that extra point. Poorly executed experiments, poorly written reports (this includes the scientific quality as well as grammar and neatness), failure to clearly analyze experimental errors, incorrect answers to questions, unlabeled graphs, and unit-less numbers will cost you points. To summarize, a successful experimentalist is one who understands the scientific goals and principles behind the experiment, pays clear attention to the details and potential errors, and presents the results in a clear and accurate report.