Introduction to 110 Labs

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.

III. Data Physics 110

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.

Excerpt taken from Physics Laboratory Experiments, third edition by Jerry Wilson: Physics 110 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.