120-lab-manual(1)

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CUNY LaGuardia Community College *

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240

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Physics

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Dec 6, 2023

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docx

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Contents Introduction page 3 Tutorial on Errors and Significant Figures 5 Tutorial on Graphs 7 Laboratory Exercises Electricity & Magnetism 21 Ohm's Law 9 22 DC Circuits 13 23 Force on a Current-Carrying Wire... 19 24 The Specific Charge of the Electron 23 25 The Oscilloscope 29 26 PN Junction Diode 35 Optics 27 The Speed of Light 41 28 Part I: Refraction 45 Part II: Focal Length 49 29 Mirrors and Lenses 53 30 Interference and Diffraction 57 31 The Grating Spectrometer 61 Radioactivity 32 Absorption of Radiation 65 1

This collection of Laboratory Exercises is the introductory physics laboratory manual used by Hunter College. The original exercises were developed by the Physics Faculty over thirty years ago. A number of revisions have since been made. In particular, the revision of 1994 led by Professor Robert A. Marino, introduced several new exercises involving modern optical and electronic equipment. We are indebted to the faculty and students who participated in the creation and revision of the manual over the years. Physics Faculty Hunter College July, 1999 © 1998, 1999 by Department of Physics and Astronomy, Hunter College of the City University of New York All rights reserved. 2

INTRODUCTION How to Succeed in Physics Lab Read the lab manual before coming to class to become familiar with the experiment. Lecture and Lab are NOT in perfect synch, so you may have to give the textbook a look also. You should take responsibility to learn safe operating procedures from the lab instructor. The lab manual is also occasionally a good source of safety tips. With electrical circuits, no power is to be supplied unless OK'd by instructor or lab tech. Report any accidents immediately! You will work with a lab partner to take data, but you are individually responsible for your own data. All subsequent calculations, graphs, etc. are also your own individual responsibility. Original data MUST be in ink. If you change your mind, cross out with a single stroke, and enter new datum nearby. Do not leave the lab room without obtaining the instructor's signature on your original data sheet. Without it, your lab report will not be accepted. No exceptions. The lab has been designed to be a "low pressure" experience. We hope it is an enjoyable one as you take the time to become familiar with new equipment and experiences. Still, you should aim to complete all data-taking, all necessary calculations, reach all conclusions, and at least sketch all graphs before you leave. It's well known (to those who know it well) that once you walk out that door, all work on lab reports will take longer. Besides, most of the grade for the course will come from the lecture part, so spend your time accordingly. Before taking good data, run through the experiment once or twice to see how it goes. It is often good technique to sketch data as you go along, whenever appropriate. The Report: Your Lab Report should be self-contained: It should still make sense to you when you pass it on to your grandchildren. It should include: a) Front page: your original data sheet with your name, partners and date. The original data MUST be in ink. Report not acceptable if original data is in pencil or if data sheet was not signed by your instructor. (So... don't leave lab room without it.) b) Additional pages with data and calculations in neat tabular form. If the original came out messy, you should rewrite your data before continuing with calculations. c) Any graphs. Neatness counts! It's one of the aims of this lab to produce students that know how to produce a decent graph. d) Answers to any Questions e) An Appendix made up of the pages from the lab manual that describes the experiment. Including them relieves you from having to rewrite the essential points of the procedure, description of equipment, etc. Lab reports are due the next time the lab meets. At the beginning of the period! It is department policy to penalize you for lateness in handing in lab reports. This is to discourage you from working on stale data with the lab experience no longer fresh in your mind. A schedule will be announced. 3

Laboratory Grade: The lab instructor will make up a grade 90% based on the average of your lab reports, and 10% on his/her personal evaluation of your performance in the laboratory. This grade is then reported to your lecturer for inclusion in the final course grade (15% weight factor). The list below will give you an idea of the criteria used by your lab instructor in grading your lab report: 1. Quality of measurements. Logical presentation of report contents. 2. Accuracy and correctness of calculations resulting from proper use of data and completion of calculations. 3. Orderly and logical presentation of data in tabular form, where appropriate. 4. Good-looking graphs, easiness to read, good choice of scales and labels. 5. Comparison with theory. 6. Answers to Questions; Conclusions. 7. Clarity, Neatness, Promptness. 4

TUTORIAL # 1 On Errors and Significant Figures Errors We could distinguish among three different kinds of "errors" in your lab measurements: 1. Mistakes or blunders. We all make these. But with any kind of luck, and some care, we catch them and then repeat the measurement. 2. Systematic Errors . These are due either to a faulty instrument ( a meter stick that shrank) or by an observer with a consistent bias in reading an instrument. 3. Random Errors . Small accidental errors present in every measurement we make at the limit of the instrument's precision. After blunders are eliminated, the precision of a measurement can be improved by reducing random errors ( by statistical means or by substituting a more precise instrument, i.e., one that yields more significant figures for the same measurement.) Accuracy can be increased by reducing any systematic errors as well as by increasing the precision. Significant Figures No measurement of a physical quantity can ever be made with infinite accuracy. As an honest experimentalist, you should relay to the reader just how good you think your measurement is. One simple way to relay this information is by the number of significant figures you quote. For example, 3.4 cm says one thing, 3.40 cm tells a different story. The last digit you write down can be your best estimate made between the markings of a scale, but it still represents a willfully reported number, it still is a significant figure. The placement of the decimal point does not change the number of significant figures. For example, 20.8 grams and 0.00208 grams each have three significant figures; each is assumed to be uncertain by at least ±1 in the last figure, i.e., ±1 part in 208, which is about ½ %. Normally, figuring out how many significant figures are in a stated number gives no problems, except when zeros are involved. For example, is it obvious how many significant figures are expressed in 5500 feet, 250 years, or $1,300,000 ? A good way to tell the reader which is, in fact, the last significant figure is by using scientific notation . For example, 5.50 x 10 3 feet, 2.5 x 10 2 years, and 1.300 Megabucks, telegraph that the number of digits in which any confidence can be placed was three, two, and four, respectively. Computations using raw data How do you combine your carefully gathered data with other numbers in an expression? With a little common sense, and a hand calculator, you can verify that the following rules should be followed:

Multiplication and Division: Report only as many significant figures in your final answer as there were in the least precise value. For example, 3.481 x 1.75 gets reported as 6.09, not 6.092. Of course, you should only round off the final answer. If a number is used again in another computation, you should not round it off in between, or you may make a small but significant error. Addition and Subtraction: Again, common sense rules: 1.11 x 10 3 + 3.33 x 10 4 is, unfortunately, just 3.44 x 10 4 . Note: To see this you have to write it out in ordinary notation (even better: line-up one under the other): 1,110 + 33,300 = 34,410 mathematically but the tens position is not significant in one of the terms, so it cannot be significant in the final sum. The answer is 34,400, or 3.44 x 10 4 . A more sophisticated way to "propagate errors" is to derive the proper expression using the methods of differential calculus. For those labs where this becomes necessary, you will be given the answer.

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