General Physics Laboratory Information

GENERAL PHYSICS LABORATORY INFORMATION

I. LAB RULES AND REGULATIONS

1. General Responsibility

Your lab instructor is in charge of your lab section and may change any of the rules, regulations or procedures. Follow his/her directions.

Whenever you have a problem in the lab, ask the lab instructor to help you.. Do not try to guess how to adjust the apparatus, or how to read the scales, etc. When in doubt, ask!

2. Safety

Do not handle any unfamiliar apparatus unless you have proper instructions in order to avoid damaging the equipment or hurting yourself or others.

3. Lab Attendance

You are required to attend all meetings of your lab section. If you have a valid reason, you may miss a meeting, but you must obtain the approval of your lab instructor. You must make-up the missed experiment (see 4 below).

4. Make-Up Labs

If you miss your class meeting you can make-up the experiment within 2 weeks by attending any other lab. You must ask the permission of the instructor of that lab (as well as your own instructor). The apparatus for your experiment will be available on a side table. The instructor in charge will initial your data, but you should perform the experiment according to the requirements of your own lab instructor

5. Instruction Sheets

Read the instructions for experiments before coming to the lab. If your lecturer has not yet covered the material, it is your responsibility to read the theory from your textbook or other sources.

6. Apparatus and Samples

You and your partner (if any) will be assigned a lab station and apparatus. The place and apparatus assigned to you is marked by code labels. Do not exchange any of the marked apparatus with other stations. Record the code label of your apparatus. You and your partner are jointly responsibly for your equipment and the cleanliness of your table.

Some equipment or samples may be handed out to your personally by the instructor. If any equipment or samples are handed out to you by the lab instructor, you must return it to him/her in person.; Do not leave it on his or her desk. Other equipment may be shared by all students in the lab.

Familiarize yourself with the equipment before starting the experiment. Check whether it is in proper working order. If you think it is not, do not try to fix it. Call your instructor.

7. Data Sheets

All work you do in the lab must be properly recorded on data sheets. The data sheets must be initialed by your instructor in order to be accepted. Before submitting your data sheet to the instructor, you and your partner must account for all equipment and clean your table.

8. Lab Reports

A lab report is required for each experiment. No report will be accepted unless it contains properly initialed data sheets.

Important: whether or not your data are identical or different from your partner’s, your report must be your own. You must process and analyze your data independently of your partner and draw your own conclusions.

See the Report Writing Section below for a description of the laboratory report format.

9. Grading

For each experiment, you will be given a numerical grade from 0 to 20. A properly done and initialed data sheet without a report carries an automatic grade of 10. A higher grade depends on the quality of your report and your lab performance. A missed experiment which is not made up within two weeks carries an automatic grade of 0.

II. LAB REPORTS

A. Data Recording

1. Data Sheets

Each student must have their own set of data records. If you are working with a partner, each person must have a separate data sheet.

All work you do in the lab must be properly recorded on data sheets. As a rule, these will be sheets of special data paper which are available in the SJU bookstore. Occasionally your data will be graphical and will be recorded on unruled white paper. Other recording devices, such as tapes, may occasionally be used.

Data sheets must bear your name, your station, and the date. If you are working with a partner, list his or her name. All numerical data must recorded in ink. All graphical work must be done with a sharp pencil. No erasures are permitted. If you make an error, neatly cross-out the wrong data and write the correct data next to it.

Your data sheets must be initialed by your instructor in order to be accepted.

2. Measurement

Before recording measurements, familiarize yourself with the range of values that your apparatus can record. Notice the units involved.

Next, prepare a table for each set of measurements. Identify the measured quantity and the units you are using.

Observe the following rules:

(a) Estimate all scale readings to the best precision available (e.g. do not record “62” if you can estimate “61.5”. This precision will vary with the instrument used to make the measurement.

(b) All readings must be recorded to the same precision (e.g. do not record “3” and “3.25” in the same set of measurements. Record either “3.0” and “3.3” (with two significant digits) or “3.00” and “3.25” (with three significant digits). As discussed above, the precision you use depends on your measuring instrument.

Use the following customary procedures:

(d) Use scientific notation. Write 2.73 cm3 (or 2.73  10-6 m3) instead of 0.000027 m3.

B. Data Presentation and Analysis

1. Displaying your Calculations

A table is routinely used when there are several similar calculations which use the same formula. Display your calculations with columns for data and columns for calculated quantities. Fill the table with rounded off quantities as discussed below. Be sure to include the units. Show a sample calculation which quotes the units for all quantities.

If a calculation is unique, it should be displayed fully. Quote the formula, show the substituted values with their units, and give the rounded-off result along with its units.

2. Significant Digits and Rounding Off

Simply speaking, the number of significant digits is the number of digits in your value. A zero counts as a significant digit, even at the end of a number (but not at the beginning of a number). When you calculate new quantities from your original data, keep the same number of significant figures in your answer as in your original data. Round off spurious digits generated by your calculator or your arithmetic. For example 1.15 x 2.38 = 2.737. Round off to 3 significant digits (as in the original numbers) and record your answer as 2.74. Similarly 1.15 divided by 2.38 = 0.483193... (on the calculator). Round this off also to three significant digits and write your answer as 0.483. Note that the beginning zero is not counted as a significant digit.

3. Scientific Notation

Scientific notation is the conventional manner of expressing numbers in science. Each quantity is expressed as a number between 1 and 10 multiplied by the appropriate power of ten.

Examples:

(a) 0.026374 is written as 2.6374  10 -2 which may be rounded off to 2.64  10 -2.

(b) 26374 is written as 2.637  10 4 which may be rounded off to 2.64  10 4.

4. Systematic Errors and Mistakes

Systematic errors in measurement may be caused by some persistent factor such as improper procedure, incorrectly adjusted apparatus, errors in reading equipment, etc. Since such errors are often revealed when calculations are made, you are encouraged to make sample calculations while still in the lab in order to discover such errors.

Gross human error (mistakes), either in measurements or calculations may be discovered by repeated measurements and/or calculations.

You are responsible for discovering and eliminating systematic errors and mistakes before you present your conclusions in your report.

5. Graphs

In some of your lab reports you will be asked to make graphs (plots) of your data and measure the slope of your graph (if it is a straight line). To draw a good graph, it is suggested

that you observe the following:

(a) Use special graph paper which will be available either from the SJU bookstore or from a stationery store.

(b) Use a sharp pencil and a ruler. Do not use ink or free-hand drawings.

(d) Occasionally you will be asked to do other graphical constructions on sheets of white unruled paper. This type of graphical work should also be done using sharp pencils, rules, protractors, etc.

Attached is a sample of a properly drawn graph. Observe its features:

(a) The scales are well chosen so that data points are spread over the whole sheet.

(b) Axes are labeled. The physical quantities are well identified by names and/or symbols. The physical units are quoted. Both scales are conveniently chosen and marked.

(c) Data points are marked precisely by dots made with sharp pencil. The circles are for convenience in order to easily locate the points.

(d) The line of best fit is properly drawn. This line represents an average of the data and goes as close as possible to as many points as possible. DO NOT “CONNECT THE DOTS”. DO NOT JUST CONNECT THE FIRST AND LAST POINTS. Your instructor can help you draw this line correctly.

(e) The measurement of the slope from the line of best fit is clearly shown. The slope is the change in y (in this graph the change in y is the change in w = w)divided by the change in x (in this graph the change in x is the change in f = f). A large triangle is chosen for the measurement of the slope. The lengths of its sides are quoted in proper symbols with units and measured according to the scales shown. The value of the slope is displayed on the same sheet .

6. Averages

Random deviations in measurement are different from systematic errors. Every instrument has an inherent uncertainty. Therefore, measurements of the same physical quantity differ because of random deviations. To improve accuracy, several measurements are often made of each quantity and the average value is calculated. Each measurement should have the same number of significant digits. The greater the number of measurements, the more accurate your average. Consult your instructor if in doubt.

The average is calculated by adding all the measurements and dividing by the number of measurements. The average value must have the same number of significant digits as the individual measurements. The calculation of the average value should be displayed in the table of measurements.

7. Accuracy of your Results

Quite often, your final results are to be compared to the known value of the same physical quantity. If your result is “Z” and the known value is “Zo”, then you should display the % discrepancy or % error. This is calculated as follows:

The percent error must be rounded off. In some experiments your results will be expected to be within 2-3% of the known value, but in other experiments where the apparatus is crude, 10-15% discrepancy will be reasonable.

If there is no known value to which to compare your results, your instructor will ask you to quote either the average percent deviation or the maximum percent deviation. These are calculated as follows. First find the average of all your values. This average value is shown as “Z“. The deviation for each value is “Z - Z“. The maximum percent deviation is taken from the maximum deviation for the data set:

The average percent deviation is taken from the average of the individual deviations for this data set:

Other methods for estimating accuracy may be used as indicated in the instructions for individual experiments.

C. Report Writing

1. You must submit a report for each experiment you have performed in the lab. The report may be written in pencil except for the data sheet which must be in ink. It is not necessary to type it, but the handwriting must be legible and all presentations much be done neatly. The report should be on standard 8 1/2 x 11 inch paper stapled together.

Express yourself clearly using proper scientific terms. Your grade for the report will be based upon the quality of your results and not by the amount of work you put into it. Be sure you understand the theory thoroughly before writing the report. Otherwise you may waste a lot of time and effort.

2. The report should be your own except for identical data from your partner, if you have one. The entire contents of your report, including the data analysis, must be done by yourself, independently of your partner. Never copy even a part of your report from someone else.

This does not mean that you cannot compare your results to the results of your partner or other students. If you have difficulty, you are free to ask for help. But clearly distinguish between asking for help and copying the work of someone else.

3. The format of the report is as follows:

(a) TITLE PAGE: The title of the experiment, your name and the lab section.

(b) PURPOSE STATEMENT: State clearly and concisely in your own words what the experiment was about. Do not use more than a few sentences, and avoid copying the wording from your instruction sheet. (You will gain no credit for this.)

(c) DATA SHEETS: The next page(s) should be the data sheet(s), containing the date, station, and the name of your partners.

(d) CALCULATIONS: Present your calculations following the steps in the instruction sheet. Whenever asked to make tables, display your results directly by filling out the tables.

(i) State clearly how the individual entries in a table are calculated by displaying a sample calculation in detail.

(ii) State the physical units at the top of each column in the table.

(iii) Round off the computed entries (see “Data Processing”).

(iv) Understand roughly what your results should be. If any of your results are completely different from your expectations, it is a clear indication that you made an error. It is your responsibility to correct this error before submitting your report.

(v) Display the calculation of the percent discrepancy or percent error whenever a comparison of results is requested. Round off to two significant digits.

(e) CONCLUSIONS AND COMMENTS:

(i) Reflect the connection between theory and results.

(ii) Discuss the major causes of the discrepancy between theory and results.

(iii) Avoid statements like “calculation errors” “human error”. These errors are

likely to be mistakes and should be corrected!

(f) QUESTIONS: Answer all “Questions” asked in your Instructions.