Physics 2DL Lectures

1
Physics 2DL Lectures

• Vivek Sharma
• Lecture 2
• Discussion of Experiments

2
House Keeping Details

• Every one reported to the lab in assigned
  sections ?
• Know which experiment you are doing this week ?
• Could download pdf or WORD file of experiment ?
• We switched the web server hardware on you on
  last friday
• But you could not tell (I hope) !
• Helpful reading material on E-reserve
• Modern Physics Serway, Moses, Moyer
• Modern Physics Beiser
• Resnick-Halliday-Walker Chapters on
  Interference Diffraction
• All should be accessible from UCSD computers of
  (UCSD web proxy) as PDF files
• Lectures Plan
• Start with a brief overview of Physics behind the
  experiments
• Statistical analysis of data advanced concepts
  in Probability

3
Lecture Schedule In Light of My Travel

• 2D last quarter kept me tied to UCSD, Scientific
  responsibilities require me to travel to Europe
  in April May

I would like to schedule makeup lectures early in
this quarter Wednesday same time OK with you ?
4
Additional Lecture this Week Early Final Exam
for 2DL ?

• Additional lecture to review experiments this WED
  same time ?
• Other dates Wed 16 Apr, Fri 18 Apr, May 14th,
  May 16th
• feedback please
• There are 6 labs ? (1)6 Weeks lt 10 Weeks
• ? plenty of time
• Schedule Final exam (50 minutes, 3 problems)
  prior to the finals week to get this course out
  of your way?
• Tentative Date 4th June ?
• Pl. keep checking the Class Announcements page at
  the 2DL web site http//hepweb.ucsd.edu/modphys/
  2dl03/an.html
• for late breaking information (rescheduled
  lectures, finals etc)

5
Experiments You Will Do In No Particular Order

• Optical Spectra Diffraction Grating
• Coherence of Light Interferometry
• Photoelectric Effect
• e/m of Electron
• Franck-Hertz Experiment
• Electron Diffraction
• Thermal Band Gap in Solids (optional because
  neither 2D or 4E will teach you this)

6
Determination of e/m for Electron

• e/m is characteristic of a particle electron Vs
  Cl- ion
• When Uniform magnetic field of strength B is
  established perpendicular to direction of motion
  of a charged particle, particle moves in a
  circular path of radius R

7
e/m, e, m of Electron Why Important
Realization that electron is much less massive
than the Hydrogen atom made physicists think
about the structure Inside atom
The electron was discovered just a bit over 100
years ago, triggered A scientific revolution
Thomsons idea Still used to measure Masses of
fundamental Particles or nuclei
8
The Photo Electric Effect Nobel Prize for
Einstein

• Maxwells Equations ? EM Wave Properties
• Hertz Electromagnetic Waves ( Experiment)
• Description of Photoelectric Effect
• Failure of classical physics (why?)
• Einsteins Quantum Interpretation inspired by
  Max Planck
• Bottomline
• How EM waves propagate
• Interaction of EM waves with Matter
• How to prove that Einstein was right Measure
  h
• Find it is same as in Blackbody radiation

9
Maxwells Equations EM Waves

permeability
permittivity
10
EM Waves

11
Hertz The EM Wave Guy

12
Hertz The EM Wave Guy

13
PhotoElectric Effect
14
PhotoElectric Effect
15
PhotoElectric Effect E h f Quantized
16
Franck-Hertz Experiment A prelude

• Bohr Atom Discrete orbit ? Emission
  Absorption line

17
Franck Hertz Experiment Playing Football !
Inelastic scattering of electrons Confirms Bohrs
Energy quantization Electrons ejected from
heated cathode At zero potential are drawn
towards the positive grid G. Those passing
thru Hole in grid can reach plate P and
cause Current in circuit if they have
sufficient Kinetic energy to overcome the
retarding Potential between G and P Tube
contains low pressure gas of stuff! If incoming
electron does not have enough energy to transfer
??E2-E1 then Elastic scattering, if electron has
atleast KE ?? then inelastic scattering and the
electron does not make it to the plate P ? Loss
of current

18
(J) Franck (G) Hertz Experiment

Current decreases because many Electrons lose
energy due to inelastic Scattering with the Hg
atom in tube And therefore can not overcome
the Small retarding potential between G ? P The
regular spacing of the peaks Indicates that ONLY
a certain quantity Of energy can be lost to the
Hg atoms ??4.9 eV. This interpretation can be
confirmed by Observation of radiation of photon
energy Ehf4.9 eV emitted by Hg atom when V0
gt 4.9V