PHYSICS LABORATORY PRACTICE PHYSICS FOUNDATIONS ON COMPUTER SCIENCE E.T.S.I.N.F. Instructor: Jorge Más jmas@fis.upv.es
ASSESSMENT FINAL MARK THEORY AND PROBLEMS, LABORATORY PRACTICES TESTS AND ATTENDANCE 20% 80 % LABORATORY LABORATORY EXAM TASK 50% 50%
Lab practice program 1 Practice/week not always. Look the data at the web site. Groups of two tables (5 or 6 people) • 0: Errors on measurement. • 1ª: NI Multisim. • 2ª: Ohm’s law: Systematic errors. • 3ª: The Oscilloscope. • 4ª: The Junction diode. • 5ª: The capacitor. Discharging process. • 6ª: Receptors: the D.C. engine. • 7ª: Network analysis. • 8ª: Resonance and Filters. • 9ª: Laboratory exam (1/2 weeks).
Laboratory Rules • Practice sessions and Lab exam will all of them online via Teams, and they will start at the time fixed. You must be connected ON TIME. • One lab group is made up by two working tables (5 or 6 people divided into two tables). • Your group will be asked to write a report about the task completed each session.
Material for Laboratory sessions You’ll find all the material for Lab sessions on web site http://jmas.webs.upv.es/ffi/ / Practices You’ll find there the date of each session; be careful because there are several weeks without lab session. The most important document to carry out each practice is the PRACTICE GUIDE, explaining both the theoretical basis of experiment as every details you have to take in account to complet it. Besides the guide you’ll find videos, slides or software to explain some device or clarify the task to be done. Very implortant is the Experimental sheet, useful to write every measurement meanwhile you stay at the laboratory. You must read and/or see these documents before coming in laboratory because none explanation will be done in the beginning of lab session after the practice 1. You will have to start your task immediately when arriving in laboratory.
ABOUT THE REPORTS • After each lab session, each group (two tables) has to do a report about the task done. On web site (Practice 1) you’ll find the document “How a report should be done” . You must observe the rules given there: organization, length, cover page, … . • One person of group must send the report in PDF format to the teacher through Poliformat/Tasks. • Very important to observe the deadline given on Poliformat (usually 5 days after the date of practice). Only in justified cases, the report will be accepted after this time. I’ll return you the corrected report.
REPORTS ASSESSMENT a) General appearance. All required elements of report 10 % are present. Contents are well organized and neatly presented. b) Data taken during the lab session looks correct and is 25 % well organized and in table form If necessary. Units must always be written. c) Calculations and results are correct and consistent. 25 % The equations used must be given. Graphs correctly drawn and labeled. d) Errors must be properly computed and written as 20 % required. e) Final conclusions: clear well-founded conclusions and 20 % the answer to the questions (when required) must be written at the end of the report. These percentages are only illustrative, and they can be different for each report
ERRORS ON MEASUREMENT Every measurement must be given with its absolute error or absolute uncertainty (usually shorted as error), stating an interval where the “true” value we are measuring will be with some probability: x ± x or x( x) Example: 48,7 ± 0,3 m means that the “real” measurement we are looking for is “likely” inside the interval 48,4 , 49,0 . The relative error is the rate of absolute error to the “true” value. It is useful to compare different measurements: x ( ) x r x
ERRORS ON MEASUREMENT • There are two rules to express correctly a measurement : 1. The error mustn’t be written with more than two significant figures, but it can be rounded to only one. 2. The last significant figure of measurement shouldn’t have less decimal order than that of the error. Examples: Incorrect measurements Correct measurements 48,721 ± 0,32 V 48,72 ± 0,32 V 4,6 ± 0,0182 V 4,6 ± 0,018 V 563,1 ± 30 cm 563 ± 30 cm 872·10 -6 ± 0,8656·10 -4 N (8,72 ± 0,87)·10 -4 N 4,6782·10 -8 ± 4,61·10 -10 A (4,678 ± 0,046) ·10 -8 A 0,23±3 ºC 0±3 ºC Right to left: On first must be rounded the error and then, the measurement
ERRORS ON MEASUREMENT • Normal behaviour. Gauss’s bell When a variable depends on a great number of independent factors, Statistics says that this variable follows a “normal” behaviour. That’s the usual case at Physics laboratory, where any measurement depends on a lot of factors: the ability of the observer, the measuring device, the temperature, the humidity, the level of lighting, …… . These variables are governed by the Gauss’s bell: 2 ( x x ) 1 b 2 f x ( ) e 2 p a ( x b ) f x dx ( ) 2 2 a x x x b a x The probability that the “true” value : Mean value. The most likely value : Standard deviation was in the interval is 68 % x , x
ERRORS ON DIRECT MEASUREMENTS • Accuracy and Precision. Random and Systematic errors. Random: Low Random: Low Systematic ? Systematic ? Random: High Random: High Systematic ? Systematic ? Random errors can be minimized by taking several measurements (a sample of the population). Usually three measurements is enough. Systematic erros are not corrected with several measurements. Either the measuring method or the calibration of measuring devices should be checked.
ERRORS ON DIRECT MEASUREMENTS If several measurements are repeated: • (x 1 , x 2 , …… , x N ) are the N measurements taken x i • The mean value is calculated (PROMEDIO with Excel) i 1 N ... x N • The type A error (standard deviation of the mean) is calculated: 2 ( x x ) i DESVEST M . ( with Excel) i 1 N ... ( ) x A N N ( 1 ) N
ERRORS ON DIRECT MEASUREMENTS In any case: • We calculate the type B error, due to the unaccuracy of measuring device: – This error must be estimated according the instructions given by the manufacturer of measuring device in the technical sheets, or according the experience of the observer. – If no technical sheets exist, the type B error can be calculated from the resolution of measuring device (the minimum magnitude can read the measuring device): Digital device mA Resolution a=0,001 mA a ( ) x B 2 3 Analog device Resolution a=0,5 V
ERRORS ON DIRECT MEASUREMENTS If several measurements are carried out: • It is accepted that the “true” value is the mean value of the N measurements. • The error of the measurement is the highest of type A and B errors: ( ) max( x ( ), x ( )) x A B If only one measurement is carried out: • It is accepted that the “true” value is that of the taken measurement. • The error of the measurement is the type B error: ( ) x ( ) x B
ERRORS ON DIRECT MEASUREMENTS How many measurements are required?: • We start taking three measurements. • We calculate the dispersion of the taken measurements: x x min 100 max D x • If D < 2%, then the three taken measurements are enough. • If D > 2%, then we have to carry out three more measurements (6 as a whole) and verify that the resulting dispersion is < 8%. • If D > 8%, then we have to carry out nine more measurements (15 as a whole), and verify that the dispersion is < 12 %. • If D > 12 %, then we have to increase up to a minimum of 50 measurements and a statistical data processing.
NEXT PRACTICE: NI Multisim BEFORE COMING TO NEXT LAB SESSION YOU HAVE TO: • Write the report of first lab session according the rules given on “ How a report should be done ” . • At the Laboratory will be explained the operation of NI Multisim
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