Dr. Chaouki Boulahouache. Physics Laboratory. Rules to be followed. Slide 1

Transcription

1 Dr. Chaouki Boulahouache Physics Laboratory Rules to be followed Slide 1

2 Physics Laboratory Structure ² Lab is structured into three main parts: 1. Pre-Lab 2. In-Lab 3. Post-Lab Slide 2

3 Physics Laboratory: Pre-Lab Come prepared to perform the experiment. ² Pre-Lab, Individual activity: 1. Read the lab manual. 2. Solve problems and answer questions related to the lab. Slide 3

4 Physics Laboratory: In-Lab Don t just follow the directions and work through a procedure without understanding ² In-Lab, group activity: 1. Students need to come to the lab with a plan on how to perform the experiment. 2. They need to know what kind of data is needed. Slide 4

5 Physics Laboratory: Post-Lab Experiment s results need to be criticized ² Post-lab, data analysis and conclusions: 1. Use the experimental data to answer the questions. 2. Look for the physics, and evaluate theories quantitatively and critically (with uncertainties -next slide) è No experimental result will have a meaning without the uncertainty (random/systematics). Slide 5

6 "We sail within a vast sphere, ever drifting in uncertainty, driven from end to end B. Pascal ( ) Experimental Uncertainties ² Uncertainties: help us weigh the evidence. ² Students need to know how to: 1. Calculate uncertainties 2. Perform uncertainty propagation 3. Determine the number of significant figures Slide 6

7 Experimental Uncertainties: Assignment and Propagation ² Assignment of uncertainty depends on the measurement device: set to the lowest division that one can measure. For example: A meter stick has a 1mm uncertainty. ² Propagation of uncertainties: important subject è There are different ways to propagate the uncertainties: 1. Monte-Carlo method: relies on the use of a computer programming. 2. 2N+1 method: It is not an exact method to propagate the uncertainties è very simplified approximation of the Monte Carlo method. 3. Algebraic method. Slide 7

8 Propagation of Uncertainties: 2N+1 Method ² If a variable is a function of N parameters then we need 2N+1 calculations. For example for an area A=B*C where B=B 0 ±ΔB and C=C 0 ±ΔC è We need to make (2 x 2) + 1 = 5 calculations. ² Next slide describes the 2N+1 method in more details. Slide 8

9 Propagation of Uncertainties: 2N+1 Method (details) 1. Do the calculation using the best estimated values for all the input parameter, which usually are the average values. 2. Re-do the calculation with one of the parameters set to its upper limit, and leave the other N-1 parameters at their best estimated values. 3. Re-do the calculation with the same parameters as in the previous step set to its lower limit, and again leave the other N-1 parameters at their best - estimated values. 4. Repeat step b and c for the all other parameters. 5. The difference between the value calculated with the best estimated values and the minimum calculated value would give you the lower end of the uncertainty. 6. The difference between the maximum calculated value and the value calculated with the best estimated values would give you the higher end of the uncertainty. Slide 9

10 Propagation of Uncertainties: Algebraic Method v If we have multiple parameters: X, Y and Z. Each parameter known within its respective random error as follows: X = X 0 +!X Y = Y 0 +!Y Z = Z 0 +!Z v Mathematically, we can evaluate the uncertainty of a function that depend on these three parameters as follows: A = A 0 +!A A = X +Y! Z ² Case 1: è Where : A 0 = X 0 +Y 0 " Z 0 and :!A =!X 2 +!Y 2 +!Z 2 A = A 0 +!A A = X!Y Z ² Case 2: è Where : A 0 = X 0 "Y 0 Z 0 and :!A A 0 = # % $!X X 0 & ( ' 2 # + %!Y $ Y 0 & ( ' 2 # + %!Z $ Z 0 & ( ' 2 Slide 10

11 Summary: Experimental Measurement is an Art and Science ² To present a measurement, we need to adhere to the following rules: 1. Quote the unit (if it exists). 2. Report only the significant figures. 3. Report an estimate of the uncertainties.

12 Next: Lab Report

13 Lab Report q q Learning how to present the experimental results is an essential part of the university laboratory education è It is a skill, if acquired, will help the students during their professional career. Main information to be included in the lab report: 1. Title Page: Slide 13

14 Lab Report, Cont. q Main information to be included in the lab report [Continue]: 2. Pre-Lab: 1. Answers to questions. 2. Description of the experiment Goals/Aims 3. The tools needed to achieve these goals 4. The experimental method used in this lab: how do we perform the experiment? Slide 14

15 Lab Report, Cont. q Main information to be included in the lab report [Continue]: 3. In-Lab: 1. Experimental observations 2. DATA tables and Graphs (if necessary) 3. Calculation of the uncertainties. 4. Post-Lab: 1. Extra calculations and/or graphs (if needed). 2. Summary and Conclusion Slide 15

16 Laboratory: Grading The lab grade will be based on three lab portions: 1. Pre-Lab [20%] based on the student s preparation for the lab. 2. In-Lab [40%] based on the students performance, namely: note taking, and the quality and sufficiency of the data. 3. Post-Lab [40%] based on the student s analysis of the data, student s ability to criticize the data. Slide 16