Modern Physics Lab: Getting Started

Physics 108 – Introduction to Modern Physics

Why do we have lab, anyway? You might think it’s just to illustrate class material. That’s a pretty good reason, but not very related to the role laboratories play in the real world. We are more interested in showing you how laboratory observations lead to a description of nature, and in letting you experience how the inherent limitations of laboratory work lead to uncertainty in that description. If you stay focused on these goals as you work through these labs, you are likely to begin wondering how the supposedly “exact” sciences could be so heavily based on laboratory work! Then we will have succeeded, because you will be more skeptical of science in general and more appreciative of the accomplishments of history’s great experimentalists.

We try to keep the focus as much as possible on the lab itself, and less so on the “publication” aspect of professional (especially academic) science. We emphasize the initial recording of experimental data in a laboratory notebook, and the preliminary analysis of that data to determine experimental results. We ask you to develop a style of record-keeping that is practiced in real research laboratories, where it may be as important to establish the chronology of events as it is to describe those events accurately. You may sometimes feel as if we are imposing an arbitrary set of picky rules, but if you make this note taking style a habit, it will serve you well, even if you don’t become a professional scientist.

To keep you aware of the limits of laboratory observation, we ask you to calculate the “uncertainty” in your experimental results. We don’t call it “error analysis”, because that implies you did something wrong. You shouldn’t make errors, but you should be uncertain! We intend that uncertainty analysis be a significant but minor part of your work for each lab. In the past, students have done much more than we now require for this part of the lab, so don’t listen to what they tell you. Just follow the instructions that follow! If you aren’t sure how they apply in a given situation, ask us to make it clear before you start calculating. Briefly, we ask you to

1) estimate the “measurement uncertainty” once only for each variable that you measure, justifying your estimate (see “3XAP below”)
2) do a “partial uncertainty analysis” for a single data set
3) use the “standard deviation technique” when instructed to do so. We also require that you do all your analysis in Excel. The physics 108 Excel Tutorial describes this spreadsheet program. There is a link to this tutorial from the physics 108 webpage of from Lyceum.

We intend the lab to be interactive. We tried to write the lab handouts in a way that keeps you thinking about what you’re doing, but we also want you to consult freely with lab instructors, the student assistant, and other students. The questions in the lab handouts are meant to stimulate such conversation. In some cases there is no single answer, or the answer is beyond the scope of class material. We ask that you answer the questions as you come to them in lab, so it will be easier for you to discuss them with us.

To prepare for lab, you will be asked to complete a short pre-lab exercise prior to lab.  The pre-lab, worth 10% of your lab grade, should be turned in at the beginning of you lab period.  Pre-lab exercises for each lab are found in Lyceum.

The Lab Notebook

The most important thing to know is that it’s your notebook. You are writing notes in your own words, for you to read and understand later. It’s not a report that you’re writing for the instructor. It’s more like a diary. To keep the notebook organized, we ask that you reserve the first page of the notebook for a Table of Contents that you update for each lab with its title and the page number on which it starts.

Each experiment should begin with a title, purpose and your partner’s name. The notebook is supposed to be an accurate record of what happened when you did the lab. That means you write down what you do when you are doing it, not the night before or a few days later. It must be sufficiently detailed to “stand alone”; that is, you or the grader should be able to understand what happened without also having to look at the lab handouts. Diagrams are especially helpful in accomplishing this. The notebook is supposed to indicate the order in which things happened. That means it’s strictly chronological. Never leave blank spaces with the idea of coming back later to fill them in unless you are leaving room for printed or other externally produced documents that are to be taped into the notebook.

Thus, during lab you write only in the notebook, never on “scrap paper”. It also means that if you make a mistake, you don’t erase, obliterate, use white-out, or tear out the page – just draw a single line through the mistake or a single cross through the page or section of page that should be ignored. You should still be able to read the supposedly mistaken information – it can happen that the mistake actually tells you something valuable that you’ll want to know later. The notebook is supposed to indicate when things happened, so each page must be dated. Because your notes are strictly chronological, you often need to refer back to something written earlier. That means the pages must be numbered like the pages of a book–if your first experiment ends on page 17, the second begins on page 18, and the last page in the book is page 160 or so.  Sometimes you need to refer forward. If you break off an analysis on page 30 and pick it up again five pages later, label the break off point with something like “continued on page 35” (O.K., in this case you are allowed to write something out of strict chronological order!), and on page 35 write “continued from page 30”.

One of the best ways to establish uncertainty for your lab measurements is to perform multiple trials of the same measurement.  We are going to use a method called 3XAP, or “three-times-alternating-partners”.  For each new measurement type, performing it “3XAP” means that each member of a 3-person lab group should perform the measurement once without sharing the values until everyone has completed the measurement.  For a lab group of 2 people, one of the members should perform a second trial, trying to reinitialize the measurement device when possible (for example, if measuring a length with a ruler, the person performing two trials should remove the ruler from the object being measured and realign the ruler on the object a second time).  It is important that lab group members do not “agree” on measurements as each measurement is taken: the point of uncertainty measurement is to establish a reasonable degree to which a measurement could differ rather than to try to compromise on a “best” value.  Make sure always to record all uncertainty data in your lab notebook.

One note about 3XAP measurements: if you use the same measurement tool more than once to make measurements, you don’t need to do 3XAP measurements each time.  As an example, if you had a set of five cubes whose dimensions you had to measure with a ruler, you would not have to measure the dimensions of all five cubes using 3XAP for each one.  Performing one measurement with 3XAP will establish the uncertainty in using the ruler, and you can assume that uncertainty for the remaining measurements will be the same as the uncertainty you established by the one 3XAP measurement.

Your conclusion should always restate the purpose of the lab, the final results and whether they agree with each other and/or an accepted value, greatest sources of uncertainty, and suggestions for improvement. Finally, the notebook is supposed to be a permanent record, so use pen only, and never insert loose papers. Use the tape provided in lab to immediately fasten individual computer plots or printouts to separate notebook pages.  Also, be sure to tape data analysis spreadsheets/graphs in your notebook before turning it in. The pre-lab sheet, when returned to you, may also be taped directly into your lab notebook at the beginning of the lab write-up; if you choose to do so, you are advised to leave space in your notebook to attach the pre-lab sheet at the beginning of the lab write-up when it is returned to you.  Also, any additional pre-lab information (notes, equations . . .) you feel may be helpful can be added into the notebook at the beginning of the lab writeup.

Labs are due exactly one week from the ending time of your lab period.  For example, if your lab meets on Tuesday, Week 1, your lab is due the following Tuesday; morning labs are due at 11:00am and afternoon labs are due at 4:00pm. A lab turned in less than 19 hours late will receive a 2-point penalty. If the lab is more late it will receive a 5-point penalty for each 24 hour period after the due date and time until the lab is turned in. The maximum penalty for a late lab is 20 points. Labs turned in beyond 4 days late will receive a maximum grade of 20/40, however must still be completed in full (in-lab and take home analysis must be completed). Because labs are an important part of an introductory course and there are only 5 lab experiments, you must complete all 5 labs to pass the course!