Northwestern University

Undergraduate Physics Labs

Department of Physics and Astronomy

Lab Report Guidelines

The purpose of the lab report is to convey information about what you have accomplished during your experiment, just as it would if you were in a laboratory after graduation. The lab report is your opportunity to show the lab instructor what you have learned, but it is also practice for composing professional technical reports later on. Because of this, there are some things to keep in mind that will help you out when your report is graded.

Make your lab report easy to follow. Show how you get from one statement to the next. If a value is particularly important, put a box around it. Don't go overboard! We're not looking for reams of paper here; in fact, the shortest reports are often the best reports. Just make sure that someone who hasn't done the lab can understand what you did and how you did it. Including the correct material is required and will gain points, but we do not count off for including extraneous stuff.

Believe in your data. Contrary to popular opinion, grades are not assigned exclusively, or even predominantly, on how close the value obtained agrees with the one in "the back of the book". If you are supposed to verify a law of nature, and you end up disproving it, that's fine, provided that you say that you disprove it. If however, you cannot verify it, but say that you can, we can only assume that you didn't understand the experiment. If your results are completely different than established values, then you have probably measured or calculated something incorrectly.

Many times, questions about the experiment are scattered around the manual. Answering them is appropriate. Please use complete sentences! "The magnetic field of the Earth causes the electrons to bend to the left, i.e. counterclockwise", is a much better answer than just "left" or "no". Remember, the purpose of the lab report is to convey information and your reader does NOT have access to these questions nor would he know which question you are answering. These questions are included to allow you to check your comprehension and are not intended to be a weekly test. The answers often provide ideas to write about in your report.

Follow a well thought-out format for your lab write-up. We don't expect something publishable in the Physics Review Letters, but on the other hand, we don't expect four pages of stream-of-consciousness writing either. A sample format follows:

Title

Tell us what your experiment is.

Author Credits

Tell us your name and your partner(s) name(s).

Purpose

Tell us what your are trying to prove. If the purpose isn't immediately obvious, you might want to wait until you've finished the experiment.

Apparatus and Procedure

Tell us what you are measuring and how you are measuring it. This is not supposed to be a word-for-word copy of the lab manual; instead it should be much shorter (maybe half a page). The procedure is not meant to indicate the mathematical techniques use to obtain the final answer. It should not include phrases like "divide by the square of the mass and the sine of the angle" but a concise reference to analysis software when used is recommended. Generally, you should say such things as "We used a Fluke 87-5 multimeter to measure the voltages and currents in Table 2 and in Figure 3." Such forward references to your Data section are efficient and help tie your reports together. If the reader needs more details, he can consult Fluke for the specifications on their model 87-5. Sometimes a few words and/or an illustration of how the items are connected and interact are essential. Each item in your apparatus should be a noun somewhere in your Procedure.

Data

Data are what you measure! It's vital that your reader be told the difference between directly measured quantities and derived quantities. If your table contains both kinds of numbers, make sure that the caption makes this distinction. Also, most physical observables have units: 75 cm, 45 Joules, 23 degrees, etc. Without units, the number alone is meaningless because the units provide the scale. Put the units in the legend on top of the column, e.g. "length (cm)"; do not follow every number with a symbol. Contrarily, measurement uncertainties might vary from entry to entry and should be placed after the measurements. If a column has a common uncertainty, specify it only once (near the top).

Sample Calculations

One example calculation from each group of calculations is sufficient. These are physics labs and not algebra quizzes. We recommend allowing the computer to do the calculations, but you should always check one example with your calculator to be sure the formula(s) were entered correctly. In 1-2 labs your instructor will ask that you determine the uncertainties in these calculated quantities as instructed in Error Analysis, but most of the time we will skip this step.

Results

Results are the values calculated from the measured data. It may be convenient to combine them in the same data table as the data, but make sure that the distinction between measurements and calculations is apparent. Like data, most of the results have units, so your column legend should have these units clearly specified. Graphs and such should be included in this section. Make sure graphs are large enough to discern all of their important features. A good strategy is to reduce the graph's size in the analysis software before copying it to Word. When you enlarge the graph in Word, the text is enlarged also. Other software allows you to specify the font face and size. Experiment if necessary until you get it right.

Analysis

Analysis and Conclusions are by far the most important parts of the lab, and are graded accordingly. In Analysis we want to figure out what our data have demonstrated. First, do our data fit the model curve(s) on the graph(s)? This would strongly indicate that the equation(s) used to draw the model curve(s) are supported by the data. Second, do the parameters used to draw the model curve conform to the values used to collect your data? To answer this question we will use statistics as described in Section 2.9 of the instruction manual.

Third, what assumptions are required for the model to be valid? Are you sure that all of these are met? How might the apparatus' environment affect your data? Are there forces (or other effects) present that we have neglected? Investigate these to see whether one or more of them might explain your discrepancy. Is it likely that you have underestimated your measurement uncertainties? How might these "other error sources" affect your data relative to the model(s) under test? (Some instructors prefer that this appear in Conclusions.)

Conclusions

First, write down important measured results. For example, "We measured the acceleration due to the Earth's gravity and found it to be 9.9+/-0.3 m/s2." With this you should mention whether or not your value is consistent with theoretical predictions or other measurements. In this example our agreement with the standard g=9.8 m/s2 is better than one sigma.

In the above case, since the accepted value lies within the uncertainty of the measure value, we agree with "the back of the book". Do not express agreement as a relative error! This is fine for high school physics, but is very misleading in professional research. We will keep up with how well we know our data (using uncertainties) and we will use this knowledge to establish how well our data should agree. Doing otherwise assumes that there is an accepted value that we are 100% certain of (seldom the case) and second, it can make results that are perfectly fine appear faulty. In the above example, we have a relative error of 1% even though we confirmed the accepted value to the best of our ability.

Of course, there might be some reasons for some deviation from the accepted value. If so, list them and briefly explain why they throw off the results, and if you can, estimate the magnitude and direction that these effects have on your result. (Does this tip your results up or down? Enough to be measured?) Be careful! If your measurements of energy are higher than the accepted value, losing energy due to friction is not a good source of error. Although it is likely present, it makes your agreement worse and thus is NOT your primary source of error. (Many instructors prefer that this appear in Analysis.)

Review

As you are writing your Conclusions, it might pay to glance at the Purpose of the lab. Did you succeed in what you set out to do? Why or why not? Conversely, you might look in your Conclusions for ideas to write in your Purpose. Also review your Data and Results for measured physical constants that are worth emphasizing in Conclusions. Pick the best one that YOU measured to mention prominently.

Mention anything else that you have learned from the lab, even if it doesn't seem to be directly relevant. For example, noticing that Teledeltos paper might be useful in sending pictures by teletype wire. If someone is paying so much attention to the lab that they notice this sort of thing, this will reflect in their grade as bonus points. On the other hand, this is not at all mandatory. Most labs are very straightforward with no hidden meaning at all, but, if you see a potential application for an item, a strategy, or a result, this indicates that you have been paying attention.

Finally, what might you suggest as improvements to the lab to gather better data or to eliminate some sources or error?

One of the instruction manual appendices also details instructions for reports that mirror those above. There are a few differences in preferred placement of various material, but both formats require the same material. This is to convey that each journal has its specific format, but all formats contain the same information. Your instructor might prefer yet another subtle variation. Pay attention to his requests and warnings because it might eventually cost you points.