Physics 2DL

Spring 2011!!

Jim Branson!

LECTURE 1

Physics 2DL

I  assume  you  know  Error  Analysis  from  2BL  or  2CL.    The  book  by  Taylor  (which  you  probably  own)  should  be  good  reference.    Your  2D  or  4E  textbook  should  provide  reference  for  some  of  the  physics.    I  will  try  to  supplement  this  with  some  addiGonal  reference  material.  

Six Experiments with Electrons and Photons

1.  Optical Spectra with a Diffraction Grating (~1854)!2.  e/m for electrons (1897)!3.  The Photoelectric Effect (1887-1902)!4.  Millikan Oil Drop: electron charge (1911)!

Or Michelson Interferometer and Coherence of Light???!5.  Frank Hertz Experiment (1914)!6.  Electron Diffraction (1927)!!There are 2 setups of each experiment and all six experiments are available each week, thus 12 setups. 16 students will only use 8 setups each week, so there is wiggle room.!(some New Equipment)!

How 2DLab Works

•  This week you will divide up among the six (maybe 5) experiments and pick a partner.!

•  You will take 2 weeks for the first experiment (weeks 2 and 3).!

•  There will be a signup sheet for following weeks (4-8).!

•  Lab Reports will be due in your lab section the next week.!

•  (Thus you will need two notebooks. Buy 2 cheap, quadrille-ruled, bound (not spiral) notebooks.)!

•  And the last report will be due in week 9.!•  If there are some problems, you may do an

experiment in week 9 by permission of the TAs, but only for a good reasons.!

The Final

•  The Final will be in lecture of week 9!!

•  (Monday of week 10 is Memorial Day.)!

Grades

•  6 Lab “Reports” " " " " " " "90%!•  Final (Monday week 9)" " " " "10%!

•  We will be done early, leaving you time to study for finals. " " " " "!

Lab Report Grading (20 points)

•  Apparatus Diagram " " " " " " " " "2!•  Electronics Diagram " " " " " " " " "2!•  Data Taking and Recording " " " " " "4!•  Graphs of Data " " " " " " " " " " "2!•  Graphs of computed results " " " " " "2!•  Error Analysis " " " " " " " " " " "4!•  Comparison to accepted measurements "2!•  Conclusions " " " " " " " " " " " "2!•  (experiments are not all the same)!•  Probably only integer grades, so try not to

lose any points on each part.!

Written and Oral Reports!

•  Propose 3 written reports and 3 video reports with question and answer.!

•  Grade on content, and presentation in the same way for both types.!

•  Can probe your understanding better in question and answer session.!

•  Propose alternate experiments have written and oral/video reports.!

•  (This is also new so the rules may change if there are problems.)!

A Quick Tour of the Experiments But First…

•  Electrons!– Matter particles with mass and charge!– Conserved number!

•  Photons!– Energy particles with no mass or charge!– Are created (light bulbs) and destroyed (black

surface)!

•  Both are always detected as single particles but their “probability amplitude ψ” follows a wave equation.!– P=|ψ|2 !

Optical Spectra with a Diffraction Grating

Emission  spectra  for  atoms:    Sun,  H,  He,  Hg  ,U.    

Interference  causes  bright  maximum  for  large  number  of  scratches.  

e/m of Electron

•  1897 !•  J.J. Thomson!•  Accelerate

“cathode rays” with E field.!

•  Bend them with B field.!

•  Other measurements too, imply “discovery of electron”.!

The Photoelectric Effect

•  Due to problems with calculation of Black Body Radiation, Planck had proposed E=hν for EM waves.!

•  Emission of electrons from clean metal surface in the presence of radiation was also hard to understand.!

•  Also explained by E=hν. •  Einstein Nobel prize.

Millikan Oil Drop Experiment

•  Levitate oil drops falling in gravity using E field to hold them up.!

•  Measure electron charge.!

•  Measure positive charge.!

•  Check quantization of charge.!

Franck-Hertz Experiment Measure electron energy loss in Hg vapor.!

Result shows quantized levels (not just photons)!

Electron Diffraction Experiment A perfect crystal of Ni presents an array of scattering centers.!There will be interference in 2 angles.!

Diffraction Patter for perfect crystal. !Amorphous crystal will display rings. Why?!

LECTURE 2

Six Experiments with Electrons and Photons

1.  Optical Spectra with a Diffraction Grating (~1854)!2.  e/m for electrons (1897)!3.  The Photoelectric Effect (1887-1902)!4.  Millikan Oil Drop: electron charge (1911)!5.  Frank Hertz Experiment (1914)!6.  Electron Diffraction (1927)!!(some New Equipment)!Use PASCO manual for the new experiments.!Do all the measurements described.!

Lab Report Grading (20 points)

•  Apparatus Diagram " " " " " " " " "1!•  Electronics Diagram " " " " " " " " "1!•  Data Taking and Recording " " " " " "6!•  Graphs of Data " " " " " " " " " " "2!•  Graphs of computed results " " " " " "2!•  Error Analysis " " " " " " " " " " "4!•  Comparison to accepted measurements "2!•  Conclusions " " " " " " " " " " " "2!•  (experiments are not all the same)!•  Probably only integer grades, so try not to

lose any points on each part.!

How to Ace Your Lab Reports •  Clear, complete, correct diagrams!

–  Make a diagram of the physics apparatus!–  Make a diagram of the electronics!

•  Collect sufficient data!–  Repeat measurements to check for unexpected problems!–  Vary conditions and repeat measurements to study systematic errors!

•  Record your data carefully!–  Do not erase, cross out mistakes and say why!–  Give errors and units for all measurements!

•  Include tables derived from the data you recorded, if useful.!•  Make all the relevant graphs!

–  Say why each graph is there. For example: “We expect the voltage to decay exponentially with time so we plot log(V) vs. time and expect a straight line.”!

–  Label both axes with variable name and units!–  Give the graph a title!–  Put error bars on points where possible!–  Use DataStudio software to make graphs!–  If there is a fit shown, give the fit parameters with errors.!

•  Propagate errors to get errors on derived quantities.!•  Draw clear (and correct) conclusions!

–  If you got the wrong answer, try to explain why!–  Never fudge the result to get the right answer!

Errors

•  Repeat measurements to search for errors you didn’t anticipate.!– And perhaps eliminate these problems.!

•  Vary conditions and repeat measurements to understand how results depend on conditions.!

•  Estimate errors reading scales…!•  Assign an error to each measurement.!•  Think about systematic errors.!•  Propagate errors from measurements to

derived quantities.!

The Photoelectric Effect

Black  body  radiaGon  Notes  Photoelectric  Effect  Notes  

The Physics

•  Conduction electrons are not bound to individual atoms. They are free to move inside metal but are bound in metal.!

•  May be some other energy lost.!

•  W is “work function”= minimum energy needed to remove electron.!

W  

Photon  E=hν

Electron  Max  KE=Tmax  

Tmax=hν-­‐W  

Simple Diagram

•  “Reverse bias” to push electrons back to cathode.!

•  Zero current when !–  Ve=Tmax=hν-W!

•  Vary ν to measure h/e!•  Check intensity

dependence…!

LECTURE 3

Atomic Spectra

•  Apparatus interfaces directly into DataStudio!

Efinal  +  hν=Eini1al  λν=c  mλ=d  sinθ  

Atomic  Spectra  Notes  DiffracGon  Hydrogen  Helium  

Optical Spectra with a Diffraction Grating

Emission  spectra  for  atoms:    Sun,  H,  He,  Hg  ,U.    

Interference  causes  bright  maximum  for  large  number  of  scratches.   Do  several  tubes  and  the  

absorpGon  experiment  

Electron Diffraction Experiment A perfect crystal of Ni presents an array of scattering centers.!There will be interference in 2 angles.!

Diffraction Pattern for perfect crystal. (Bragg Theory)!Amorphous crystal will display rings. Why?!

Davisson  Germer  Experiment  

Electron Diffraction Tube

Experimental Setup

Two Symmetries in Carbon

Really  there  are  (upto)  3  symmetry  vectors  (ax,ay,az)  and  …    

The (Small) Diffraction Angle

nλ=2d  sinθ