Northwestern University

Undergraduate Physics Labs

Department of Physics and Astronomy

Lab Report Guidelines

Note: If you are enrolled in the Physics 136-1 Laboratory, please check your CANVAS account for updated instructions on how to keep notes and how to write a report.

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. Because of this, there are some things to keep in mind that will help you out when your report is graded.

First, don't try to erase or paint over mistakes with white-out. Destroying data is unprofessional, and worse, you might have been right the first time, before you obliterated the page with a bath of ink. If you make a blunder, cross it out with an X. This way the instructor knows that this is not part of the lab, knows that you realized your error and took the time to correct it, and protects you in the event that you were right the first time. For the same reasons, keep a bound lab book. This includes not stapling pages in (with the exception of computer print-outs from lab equipment) or ripping them out.

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.

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.

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.

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.

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 (about five sentences). 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." If you must include calculations, mention them in the following manner: "from the angle and the mass, we can calculate the charge."

Materials

A list of what equipment was used and a drawing. A picture of the physical appearance of the equipment is not always necessary; for instance, a block diagram or electrical schematic drawing might be more appropriate. Whatever you draw, make sure it conveys the most information possible.

Data

Data are what you measure! It's vital that you know the difference between directly measured quantities and derived quantities. If your table contains both kinds of numbers, make sure that the instructor knows that you know the difference. Also, most physical observables have units: 75 cm, 45 joules, 23 degrees, etc. Without units, the number alone is meaningless. It saves writing to put the units in the legend on top of the column, e.g. length (cm), rather than to follow every number with a symbol.

Sample Calculations

One example calculation from each group of calculations is sufficient. These are physics labs and not algebra quizzes.

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 you should have these units clearly labeled. Graphs and such should be included in this section. Make sure graphs are large enough to discern all of their important features.

Conclusions

This is by far the most important part of the lab, and is graded accordingly. This section is your chance to show what you've learned. First, write down important calculated results. For example, "we measured the acceleration due to the Earth's gravity and found it to be 9.9+/-0.3 m/s/s." With this you should mention if your value is consistent with the accepted value or not.

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. First, it assumes that there is an accepted value that we are 100% certain of (often not the case) and second, it makes results that are perfectly fine appear faulty. In the above example, we have a relative error of 2% 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 (does this tip your results up or down? Enough to be measured?) that these effects have on your result. 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.

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?

Many times, questions about the experiment are scattered around the manual. Answering them in the conclusion section 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 that just "left" or "no". Remember, the purpose of the lab report is to convey information.

Finally, 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 wire. If someone is paying so much attention to the lab that they notice this sort of thing, this will reflect in their grade. On the other hand, this is not at all mandatory. Some labs are very straightforward, with no hidden meaning at all.