The Laboratory Experience

Introduction

Laboratory experiences are an important part of most science courses. Scientists and engineers are trained to keep records of their laboratory work in bound notebooks, so anyone considering a career in one of these fields should learn how to correctly keep such a notebook. Also, the College Board states that "Most college placement policies assume that students have had laboratory experience, and students should be prepared to show evidence of their laboratory work in case the college asks for it." For further information on the importance of labs in the AP Physics course, please read the appropriate section in the College Board's AP Physics Course Description (PDF download).

The Lab Record

A record of the work that you perform in the lab—your experimental procedure, qualitative observations, quantitative measurements, and even preliminary calculations—is typically recorded in a laboratory notebook. For a variety of reasons, you will not be required to keep a full laboratory notebook in this course. Instead, each weekly activity in the lab will be guided by a protocol available from this website, and recorded and reported in an individual assignments. Most of these assignments will provide you with an opportunity to collect data, apply the concepts learned in class in a lab setting, practice the theoretical calculations covered in class, and wrestle with the ever present demon of "experimental error."

Several times in the year you will be asked to complete a more comprehensive write-up of your work.

Some comments on the general style of the writeup: in your Engineer's Computation Pad you are attempting to keep an accurate record of the experimental work you perform in the lab. The document you turn in will not contain a perfect lab report of your experiment—that's something entirely different—but rather a record of what you did, how you did it, and what your results were. Although the lab record may reflect your personal style somewhat, your records need to be formal, neat, and complete enough so that any reasonably knowledgable person familiar with the subject of your experiment—another student, or the instructor of this course—can read the protocol and your record, understand exactly what you did, and repeat your experiment if necessary.

You need to write in black or blue ink. Pencil is unacceptable in a lab—the marks are easily smeared, and may accidentally be erased. If you make a mistake when recording data, draw a line through the mistake or cross it out, and add a small note indicating why the information you crossed out is incorrect.

Please read the lab protocol in advance, and set up lab record well in advance (at least the night before) before the day you perform the experiment—some teachers refer to this as the "PreLab." When you come into the laboratory to perform an experiment, it is presumed that you will have already read the lab procedure, understood the details of the experiment, and prepared your lab record with the following information, or at least spaces where you can record the following information. There is no need to cut out the protocol and tape it into your lab record—you may simply attach a copy of the protocol to the back of your lab report when you turn it in.

On the day of the experiment, I will be coming around to check that you have correctly prepared your lab notebook with entries for the following items:

Outline of a Lab Record

1. Title of Experiment
   Make the title, and all of the headings for each section in your notebook, descriptive enough that the reader can find information quickly and easily.
2. Date and Time of Record
3. The Name(s) of Lab Partners
4. Objective
   This should simply be an elaboration of the title, possible giving the method used to conduct the experiment. (1-2 sentences)
5. Table of Observed Data
   These tables, like everything else in the notebook, should be constructed before you perform the lab. Tables for recording observed data should be clearly labeled with descriptive terms and units, logically organized, with plenty of room in the margins for corrections or notes.
   
   The day of your lab, data will be entered directly into the table, in blue or black ink, as the experiment is performed. Incorrect data should be neatly lined out, with an explanation of why the data has been lined out written somewhere nearby. The correct data should be written in the table, if possible. If a large number of mistakes have been made, or an entire data table even is found to be incorrect, neatly draw a large "X" over the table, then write a note in the margin explaining the error and referring them to the end of the experiment entry. You can draw a new data table there for entering the correct information.
   
   Often, we'll be using computers to observe and record data collected in the lab. For these labs only, you may cut out the personalized computer printouts collected in the lab, and use adhesive tape to include them in your lab notebook.
6. Table of Calculated Results (and Sample Calculations)
   Any calculations necessary to yield final results should be neatly summarized in a Calculations Table or section of the record dedicated to this purpose. Clearly label all results and be sure to include units. For experiments where a calculation is repeated numerous times, it isn't necessary to show your work for every single calculation performed, but you must include a) all formulae (i.e., F = ma) used to calculate results, and b) samples of each different type of calculation performed (i.e., 120 N = 40.0 kg x 3.00 m/s² ). Showing these details will help the reader to understand your procedure, provide the instructor with evidence that you know what you're doing, and save you time if you need to perform this same type of calculation later on.
7. Graph(s) of Results
   Where appropriate (nearly always), plot the data from your Calculations Tables on a graph.
   
   A brief review of graphs: All graphs must have a descriptive title, and X and Y axes labeled with quantity and units. Adjust the scale of your graph so that the information being presented fills an entire page of the paper. Data points, if plotted by hand, should be plotted with very small "pinprick" of ink, surrounded by a small, more visible, circle. When appropriate (nearly always) you should attempt to draw a smooth "best fit" line connecting the points. If you are graphing several sets of data on the same graph, clearly identify the different sets of data by using different shapes around the data points (circles, squares, triangles, etc), and include a key to these symbols.
   
   Instead of plotting by hand, you may wish to use some form of graphing software or spreadsheet program to create your graph. These printed graphs should conform to all the requirements listed above, and be neatly trimmed and taped into your lab notebook. Specific data points used in your analysis in the lab must be clearly identified on these graphs.
8. Discussion
   The Discussion section of each entry is used to summarize your findings in paragraph form. Typically, the Discussion section will include:
   • Comparison with Known Values and Error Analysis
     Compare your results with known values -- how close are your findings to accepted values? Any time that your results have been obtained by more than a single method, or a standard value is available for your result, you must calculate the "percent difference" (in the case of results obtained by two different methods) or "percent error" (in the case of a result compared to a known quantity).
     
     To calculate the percent difference between two measured results, use the formula:
     
     
     To calculate the percent error between a measured result and a known value, use the formula:
     
   • Sources of Experimental Error
     [Thanks to E. Santochi and K. Hatch-Harrison for much of this section]
     
     "Experimental error" refers to variability in results due to limitations in the experimental design; it's the reason scientists perform multiple trials of any given experiment, and report their results as a statistical average with a plus or minus deviation included. (In this course, although we will often average measurements, we'll rarely do more advanced statistical analysis). In the "Sources of Experimental Error" section of the report, do list any observed reasons that you feel may have contributed to errors in your experiment, including specific problems with the equipment, difficulties in reading the equipment, or limitations in the design of the experiment. Don't just guess about why your experiment might have gone wrong—only mention specific sources of error that you have a legitimate reason to believe affected your results, and explain how those sources of error affected your results, e.g. "The average final velocity of the pendulum was 15.6% smaller than it was at the top, which we presume is at least partly caused by loss of energy due to air friction as the pendulum swung down."
     
     Please don't refer to "human error." Examples of so-called human error include misreading a ruler, adding the wrong reagent to a reaction mixture, mistiming the reaction, miscalculations, or any kind of mistake. Scientists would never report the results of an experiment affected by human error—instead, they repeat the experiment more carefully. It is presumed that if you realize that you've made a mistake, you'll go back to repeat the experiment, after class if necessary.
     
     Points will be deducted from your lab report if you discuss "human error" instead of "experimental error."
   • Questions and Answers
     Restate any questions posed in the lab handout that you haven't already answered in the course of your entry, and provide answers in complete sentences.
9. Summary of Results
   This is a brief, one-paragraph, summary of the results of this experiment, with a discussion of whether or not you think the experiment supports your hypothesis, or the subject material covered in class. Please note that science is unable to "prove" anything, so an experiment that measures forces, masses, and accelerations will never "prove" Newton's Second Law. We often try to "confirm" our physical descriptions of the world, however: your measurements might (and should!) confirm the validity of Newton's Second Law.
   
   By way of neatly summarizing the success of the lab, you should include a restatement of the most significant values calculated, which most often will simply be the percent error or difference you calculated.
   Please note that although the instructor hopes you enjoyed the lab experience, or at least appreciated it, it's not appropriate to include any remarks on how you felt about the assignment, nor whether or not you liked it.

An Example of a Lab Entry

You might find it instructive to see an actual example of a lab entry completed by a student. This particular entry was a 5-page report on a Ballistic Pendulum experiment. While the report differs somewhat from the format that you'll be using for your reports, it's an excellent example of the kind of "look-feel" that is expected in a lab record.

Lab Writeup: Ballistic Pendulum

Use of the Lab Time

Before the Lab Period
Complete the lab entry up to the Table of Observed Data. Make sure that you've looked over the lab and have a good idea of what you're going to be doing during the lab period.

During the Lab Period
On a typical lab day you'll come in, listen to the teacher's instructions on some of the finer points of using the lab equipment, and settle in to conducting the lab: getting equipment, setting it up, calibrating it, running the experiment, performing a few calculations to check that your results are satisfactory, and then breaking down the equipment and putting it away.

Most labs are designed for most people to complete within the allotted time period. Students who haven't prepared adequately, who perform the experiment poorly the first time, or even have a faulty piece of equipment, may find that they'll need a small amount of additional time to complete the experiment. If you manage to finish the lab and clean-up before the end of the lab period, you are expected to stay in the classroom to begin working on the remaining sections of their lab record.

After the Lab Period
Students will need to complete the remaining sections of the lab entry before turning in their completed lab on the Wednesday following the lab.

It is expected that you will collaborate with lab partners and other students in the class—you are encouraged to discuss the lab, and even to consult with others as you complete the lab entry. However, each students is expected to complete his or her own lab entry, with all calculations, analysis, and written entries completed in his or her own words.

Grading Rubric for Lab Writeups

Lab writeups are typically worth 20 points, but that value may vary with the difficulty of the experiment. The amount of points earned by a student on each entry or report will be determined according to the following rubric:

• 100%
  The student demonstrates an excellent understanding of the laboratory exercise. All parts of the lab notebook entry or the lab report are clearly labeled and in the correct order. The equipment and experimental procedures used are clearly identified and explained. The data and results tables clearly show correct data, calculations processes, and results. Graphs are appropriately and correctly drawn. The discussion is clear, displays logical reasoning, and correctly identifies and explains potential sources of error. Minor errors in the entry or report, if present, do not detract from overall understanding. Communication is clear and effective. The notebook is turned in on time.
• 80%
  The student demonstrates a solid understanding of the laboratory exercise. All parts of the lab notebook entry or the lab report are clearly labeled and in the correct order. The equipment and experimental procedures used are identified and explained. The data and results tables show data, calculations processes, and results. Graphs are appropriately and correctly drawn. The discussion is clear, displays logical reasoning, and identifies and explains potential sources of error. Minor errors in the entry or report may be present. Communication is mostly clear and effective.
• 60%
  The student demonstrates a basic understanding of the laboratory exercise. The student has included most of the parts of the lab notebook entry or the lab report. The data and results tables show data, calculations processes, and results. Graphs are appropriately and correctly drawn. Major errors may be present. Writing may hinder effective communication.
• 40%
  The student demonstrates a limited understanding of the laboratory exercise. The student has included some of the parts of the lab notebook entry or the lab report. Major errors and misconceptions are present. Writing may hinder effective communication.
• 20%
  The student demonstrates minimal understanding of the laboratory exercise. The student has included few of the parts of the lab notebook entry or the lab report. The data and results tables show data, calculations processes, and results. Major errors are present. Writing may hinder effective communication.

List of Labs for the 2010-2011 School Year

This is a tentative and incomplete list of labs and activities that will be performed by AP Physics C students this school year. For more specific information on labs, see the Current Unit Course Materials or the Course Calendar.

First Semester

• 1-D Kinematics: Acceleration of a Cart (Computer-based data collection)
• 2-D Motion: Catch the Ball
• Newton's Second Law of Motion (Inclined plane)
• Newton's Second Law of Motion (Friction)
• Circular Motion: Penny on a Turntable
• Conservation of Energy: Pendula
• Collision and Impulse (Air Track w/Photogates)
• Ballistic Pendulum
• Air Friction: Finklebottom & Priest
• Physics of Amusement Parks

Second Semester

• Electric Fields
• Millikan's Oil Drop Experiment
• Electric Field of a Flat Conducting Disk
• Ohm's Law
• RC Circuits
• Building an Electric Motor