Physics 110
INTRODUCTORY LABORATORY
WHY MAKE EXPERIMENTAL MEASUREMENTS?
When you can measure what you are speaking about and express it in numbers,
you know something about it; but when you cannot measure it, when you cannot
express it in numbers, your knowledge is of meager and unsatisfactory kind.
Lord Kelvin
As Lord Kelvin so aptly expressed, we measure things to know something about them--so
that we can describe objects and understand phenomena. Experimental measurement is
the cornerstone of the scientific method, which holds that no theory or model of nature is
tenable unless the results it predicts are in accord with experiment.
The main purpose of an introductory physics laboratory is to provide "hands-on"
experiences of various physical principles. In so doing, one becomes familiar with
laboratory equipment and procedures and with the scientific method.
In general, the theory of a physical principle will be presented in an experiment, and the
predicted results will be tested by experimental measurements. Of course, these well
known principles have been tested many times before, and there are accepted values for
certain physical quantities. Basically, in many cases, you will be comparing your
experimentally measured values to accepted theoretical or measured values. However,
most experiments do not have just one simple goal, the emphasis is not to obtain a low
percent error but to gain insight into a physical principle or to visualize a physical
phenomena by careful observation. In addition, the skills required to carefully design,
execute, and analyze an experiment will be of great value regardless of your chosen
career path.
GUIDELINES FOR LAB REPORTS:
The following guidelines should be used to complete your lab reports and all reports
should include the following information.
I.
Title
This section will include the names of your team members, goal(s) of the experiment, and
complete answers to any prediction questions that you are asked to complete.
II.
Description
A general description of the experiment includes all apparatus being used and an
explanation of the method of data collection. This should be a relatively short paragraph
giving the essence of the experiment while not being as specific as a step-by-step
procedure.
Physics 110
III.
Data
All experimental data should be organized in table format with the appropriate units
clearly indicated. Any notes taken during the experiment can be reproduced here or cited
from the original data sheet. Your original data sheet can be attached at this point or
appended to the report. (Eventually, your original data sheet will be well organized and
readable so that there should be no need to reformat any of the data.)
IV.
Analysis
The explanation of any method you use to scrutinize or discuss the meaning of the data.
This would include listing equations, identifying or defining variables, and explaining the
usage of each equation.
V.
Graphs
Any graphs asked for or deemed pertinent and efficacious.
VI.
Conclusions
This section is a summary of the important facts of the experiment and a comparison of
your results to the applicable theory. Discuss the results you obtained. Note, to aid in
this endeavor, the following definitions may be of use:
Precision: An index of the maximum amount a measurement varies from
the true value that it represents. This also indicates how reproducible the
experimental results are. For example, a measurement to 0.01 units is
more precise than a measurement to 0.1 units. The maximum error may
be expressed as a plus or minus value.
Accuracy: accuracy of measurement refers to how well the experimental
value agrees with the true value. However, the true value can never be
obtained, and the experimental value is compared with an accepted value
known as a standard.
% Of Error is defined as the absolute value of the difference between
experimental and standard value, divided by the standard value, and then
converted to a percent.
Discussing sources of error for the experiment is not a trivial thing. If you describe your
error as ‘human error’, what do you mean? If you state that there was some experimental
error, does this convey any real information about what might have gone awry with the
experiment? The following excerpt should help you distinguish between the different
types of experimental error.
Physics 110
Excerpt taken from Physics Laboratory Experiments, third edition by Jerry Wilson:
Types of experimental errors
Experimental errors can be generally classified as being of three types: personal,
systematic, and random.
PERSONAL ERROR----Personal errors arise from personal bias or carelessness in
reading an instrument, in recording observations, or in mathematical calculations. For
example:
1) In performing a series of measurements an observer may become biased in
favor of the first observation. Assuming this observation to be correct, the observer then
proceeds to attempt to make other observations agree with it.
2) Error in reading a scale, usually due to incorrect alignment of the line of sight.
3) Not observing the significant figures in a calculation.
SYSTEMATIC ERROR----Systematic errors are associated with particular
measurement instruments or techniques, such as an improperly calibrated instrument or
bias on the part of the observer. Conditions, which can result in systematic errors, are:
1) An improperly "zeroed" instrument or an instrument that is not properly
calibrated.
2) Human reaction time (0.1-0.2s) when timing is done be an observer who is
starting or stopping a clock.
4) Personal bias of an observer, who, for example, always takes a low reading of
a scale division.
Avoiding systematic errors depends on the skill of the observer to detect and to prevent or
correct them.
RANDOM ERROR----Random errors result from unknown and unpredictable variations
in experimental situations. The effect of random errors can be reduced and minimized by
improving and refining experimental techniques and repeating the measurement a
sufficient number of times so that the erroneous readings become statistically
insignificant.
Physics, an exact science? No measure quantity is ever exact. There are errors due to the
limits of ability to read the measuring instrument, the calibration of the measuring
instrument, and the inherent variations in many physical quantities. The error in a
calculated result may be found using the following rules:
1. When quantities are added or subtracted, the absolute errors add.
2. When quantities are multiplied or divided, the relative or percent errors add.