Goals of the Laboratory

The goal of the physics laboratories is to deepen your understanding of the physical concepts discussed in the classroom and to expose you to some approaches encountered in experimental research.

The laboratory course does not follow the development of physics concepts as encountered in the classroom, but rather focuses on a few well-defined physics problems that you may or may not have discussed in your physics lecture by the time you carry out your laboratory work.

The physics laboratory puts you in a situation very similar to what you will encounter after graduation. You will be expected to solve specific problems, regardless of whether you have solved similar problems in your college career or have never seen such problems before.

Lab Reports

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 when your report is graded.

First, write in pen, and 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. That 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 disproved it. If however, you cannot verify it, but say that you did, 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 Physics Review Letters, but on the other hand, we don't expect four pages of stream-of-consciousness writing either. A sample format could be:

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

Make a list of what equipment was used and use a full page drawing. If it is worth drawing, it is worth drawing on a full page. 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.

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, and should have these units clearly labeled. Graphs and such should be included in this section. As with the apparatus, if it is worth drawing, it is worth drawing on a full page. If the graph is too small, many of its important features are invisible.

Sample Calculations

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

Results and 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/s2." 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 than 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.

Changing Lab Sessions

You must attend the lab sessions you have registered for. If, having scheduled your classes, you encounter a conflict with the time of the lab for which you are registered, you may be able to change your lab session . Consult Dr. Schmidt in Tech F214, or by telephone at (847) 491-7477 during the first week of class.

If you are unable to attend a particular lab session for good reason, see Dr. Schmidt before your regular session to obtain an admit slip to reschedule you into a different lab group (within the one-week period that the experiment is being offered). You will not be allowed in a lab you are not registered for without an admit slip. Admit slips cannot be issued to allow study time for midterm exams.

Failure to attend the lab session for which you are registered will result in a reduction of your lab grade by 7 points. If this should happen repeatedly, you will not be allowed to continue taking the lab for this quarter nor will you be allowed to transfer your lab grades!

It is sometimes possible to make up missed labs at specially scheduled make-up lab sessions which may be offered at the end of the quarter. A limit of two labs maximum can be made up in this way, and then only if permission is obtained from the Dr. Schmidt prior to attending the make-up lab session.

You must complete at least 6 of the 8 labs to obtain a passing grade for the lab. If you fail the lab, you will be given a failing grade for the course.

Lab "Write-up"

The laboratory "write-up" has been written keeping the described situation in mind. It includes a brief description of the basic physics concepts and formulas which underlie the experiments you will be carrying out.

It describes what you must do, how to make measurements and complete the experiment. The write-up must be carefully studied before your laboratory session. It also includes a list of questions which should help you in testing your preparation for the laboratory.