THE LAB REPORT
WRITEUP
This introductory section is intended for you, the student, to use as a guide and reference for general physics laboratory. It is important that you read the practices and procedures as outlined in this introduction.
The labs are intended to provide you with
- hands-on experience with the concepts we have been going over in class;
- practice in taking measurements (a very important part of being in science or engineering); and
- practice working in groups.
Laboratory Report Format
Each group must submit a lab report for each experiment performed. The following format should be adhered to closely unless your instructor tells you otherwise. Use 8 1/2" x 11" paper and do not write on the back of the sheets. Write legibly or (even better) type, and use proper grammar. Points will be taken off for misspelled words and incorrect grammar. A small portion of your grade may be based on your in-lab performance.
1. Cover Sheet:
Title of experiment, your name, date
that experiment was performed, partner's names. (Provide first and last
names. Get the spelling right!)
2. Abstract:
1-3 sentences. Write a concise
statement of the principle result that is described in this report.
This should include what you were trying to measure (or do), a brief
description of the method (“marked locations at equal time intervals of a
falling object, then extracted acceleration from plots of average velocity vs time and displacement vs
time”) and then whether your measurement
(or tinkering) managed to agree with the expectations.
3. Theory:
1-2 SHORT
paragraphs. Summarize the basic physics of your experiment. Include
equations and other principle things the reader would need to know in order to
understand the experiment. Indicate how your measurements will be
converted to “the result,” i.e. if the
results of the experiment requires a derivation from these theories (or
principles), then you should complete such a derivation here. Do not discuss the procedure here. Keep it
short! You may need to recreate
a wiring diagram or draw the apparatus and define physical quantities (H, d, L,
etc.) in order to refer to it later during discussion. (This may be better in the Procedure section.)
4. Procedure:
Describe briefly how you carried out
the experiment. Do not include relatively trivial things like turning on a
switch. On the other hand, you should include descriptions of how you determine
things that are necessary to the anticipated results. This should be very
short as well. Mention the particular pitfalls in data taking that you
discovered and managed to maneuver around.
5. Raw data
Complete Step 1 of the
Data Analysis write-up here.
Present the raw data you took here, without
modification. Record all trials. Indicate the instruments used to make the
measured trials (and the instrumental uncertainty). Be careful to include ALL measurements needed
to determine a quantity. For example, if you have a meter stick and measure df
= 1.223 m, you had to make two measurements: d1 = 1.000 m and d2
= 0.223 m. Each measurement has an IU associated with it, so that the TU for df
is a combination of the IU’s for d1
and d2. Data should be easy to read, in tabular
form. This is important in order to find mistakes (yes, they happen!)
later on. Poor data recording skills
lead to poor write-ups. If your raw data and subsequent analysis are
illegible, the grade will suffer.
6. Propagation of errors
If you are asked to follow measurement
uncertainties in the lab, include that analysis in this section. Complete Steps 2 and 3 of Data Analysis as given
in the manual. Note, on occasion Steps 2
and 3 may be replaced by plotting linear fits (see Appendix B in the write-up
and 6B below). Show all work for
determining IU, RU and TU. Explicitly
show how you propagate the uncertainty into the final result.
A table is recommended to organize this step. An
example for a projectile lab is below. The quantity d is measured several times
so has an IU and an RU, while h is measured only once.
Method A:
|
Quantity |
IU (cm) |
RU = s/ÖN
(cm) |
TU (cm) |
|
davg = 36.2 cm |
0.05 |
0.20 |
0.21 |
|
h = 102.3 cm |
0.30 |
-- |
0.30 |
6B. Graphs (when required):
a. Hand-drawn graphs: Include title, labeled axes, smooth lines
through experimental data points, and slope calculations. Each graph should
convey a complete message and be fully understandable without referring to any
other section in the report. When
calculating a slope of a line on a graph, make sure to choose grid points (not
data points!) that are at the front and end of the line respectively in order
to have a large difference in x and y values.
Draw a triangle or otherwise connect the two points. Label Dx and Dy, and calculate the slope right there on
the graph. Scales should be chosen so
that the plot should takes up the whole page, so that plotting accuracy is
increased. An example of how to show the
slope calculation:
Example slope calculation
b. Computer-generated graphs: If
you use a plotting or spreadsheet program (e.g. Graphical Analysis or Excel) to
plot your data and fit a line, be sure to set scales so that the data takes up
the whole graph (as above) and that the equation for the fitting line (with fit
parameter errors!) is displayed on the graph as well as the line itself.
7. Assessment and Conclusion:
Complete Step 4 of the Data Analysis write-up. This is a very important section
of the lab! It is here that it becomes clear whether your data agree with
the accepted value(s) and/or are self-consistent. Calculate
final experimental results, standard or accepted values, if they exist, and
percent errors and/or percent differences. A table of results
(see below) will clearly lay out the supporting numbers for your argument.
This section should not just be a rehash of your results. Give possible reasons for errors, personal observations, suggestions, and any other comments you feel are pertinent. (Hint: In discussing errors, think carefully about the limits of the measuring apparatus.) Also think about how different issues contribute to problems in accuracy (systematic errors) vs. precision (measurement errors). How could you improve your results if you were to do the lab again?
Percent
error is used when
comparing a result to an accepted value.
% error = ( (X
- Xs) / Xs ) x 100 %
where Xs = a standard or
accepted value
X = an experimental value
Percent difference is used when comparing two results from
different experimental methods. The average of the two measurements is probably
closer to the actual value than either measurement, so the average is used in
the denominator.
% difference = ( (X1-
X2) / Xavg )x 100 %
where X1 = an experimental
value,
X2 = an experimental value
obtained by another method,
Xavg = (X1 + X2 )/2
= the average value of X1 and X2
Percent uncertainty or relative
error = DX / X x 100 % is
used to determine how uncertain you are in your measurement. It is calculated using propagation of errors
techniques, or through estimates. This
gets compared to the percent difference or error, and conclusions about the
success of your experiment may be made.
This section should include a concise, tabulated summary of your results. You may need a table like one or both of these, and each table may have more than one row.
Two examples follow:
1.
Comparing to an accepted value: Percent error
|
-- |
Accepted value |
Experimental
Result |
Percent Error |
|
Acceleration Due
to gravity |
980 cm/sec2 |
960 ± 45
cm/sec2 |
-2% |
2. Comparing two measurements
of the same quantity: Percent difference
|
-- |
Method #1 |
Method #2 |
Percent
Difference |
|
Initial Velocity
of Projectile |
5.02 ±0.15
m/sec |
4.78 ± 0.29
ft/sec |
+ 4.9% |
8. Answers to Post-Lab Questions: Discuss any questions at the end of the lab here. These questions are usually designed to see if you really understood the physics behind the experiment and/or to take you to the next step of understanding.
Lab write-ups grading:
|
|
Section or topic |
Points |
|
A. |
Sections 1-4; overall neatness, organization |
20 |
|
B. |
Raw Data – presentation and completeness, including IU’s (section 5) |
20 |
|
C. |
Error Analysis, graphs (section 6/6B) |
20 |
|
D. |
Conclusion (section 7) |
20 |
|
E. |
Post-lab questions (section 8) |
20 |
Last updated 4/29/2011
Scott
Nutter