Transcript Physics 2DL Lectures - University of California, San Diego

Physics 2DL Lectures
Vivek Sharma
Lecture # 2
Discussion of Experiments
House Keeping Details
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Every one reported to the lab in assigned sections ?
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Know which experiment you are doing this week ?
Could download pdf or WORD file of experiment ?
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We switched the web server hardware on you on last friday
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Helpful reading material on E-reserve
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Modern Physics: Serway, Moses, Moyer
Modern Physics : Beiser
Resnick-Halliday-Walker : Chapters on Interference &
Diffraction
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But you could not tell (I hope) !
All should be accessible from UCSD computers of (UCSD web
proxy) as PDF files
Lectures: Plan
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Start with a brief overview of Physics behind the experiments
Statistical analysis of data + advanced concepts in Probability
Lecture Schedule In Light of My Travel
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2D last quarter kept me tied to UCSD, Scientific responsibilities
require me to travel to Europe in April & May
Isola de Elba (Italy)
Ringberg Castle, Bavaria
I would like to schedule makeup lectures early in this quarter : Wednesday
same time OK with you ?
Additional Lecture this Week + Early Final Exam
for 2DL ?
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Additional lecture to review experiments this WED
same time ?
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Other dates : Wed 16 Apr, Fri 18 Apr, May 14th, May 16th
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There are 6 labs  (1)+6 Weeks < 10 Weeks
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 plenty of time
Schedule Final exam (50 minutes, 3 problems) prior
to the finals week to get this course out of your way?
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feedback please
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)
Experiments You Will Do In No Particular Order
1.
2.
3.
4.
5.
6.
7.
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)
Determination of e/m for Electron
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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
mu 2
mu
quB 
R
R
qB
If electrons have KE = qV
e
2V
then
 2 2
m B R
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
Thomson’s idea
Still used to measure
Masses of fundamental
Particles or nuclei
The Photo Electric Effect: Nobel Prize for Einstein
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Maxwell’s Equations  EM Wave Properties
Hertz & Electromagnetic Waves ( Experiment)
Description of Photoelectric Effect
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Einstein’s “Quantum” Interpretation inspired by Max
Planck
Bottomline :
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Failure of classical physics (why?)
How EM waves propagate
Interaction of EM waves with Matter
How to prove that Einstein was right : Measure “h”
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Find it is same as in Blackbody radiation
Maxwell’s Equations & EM Waves
permeability permittivity
EM Waves
Energy Flow in EM Waves :
1
Poynting Vector S =
( E  B)
0
Power incident on
1
 S . A   AE0 B0 Sin 2 (kx  t ) 
an area A
0
Intensity of Radiation I =
1
2 0 c
E02
Larger the amplitude of Oscillation
More intense is the radiation
Hertz : The EM Wave Guy
Hertz : The EM Wave Guy
PhotoElectric Effect
Light of intensity I, wavelength  and frequency  incident on a photo-cathode
Can tune I, f, 
Measure characteristics of current in the circuit as a fn of I, f, 
PhotoElectric Effect
PhotoElectric Effect : E = h f = Quantized
Franck-Hertz Experiment : A prelude
Bohr Atom : Discrete orbit  Emission & Absorption line
classical
Bohr’s
quantization
n2 2
rn 
, n  1 , 2,....
2
mke
n  1  Bohr Radius a0
 ke2  Z 2
En   
 2
 2a0  n
Franck Hertz Experiment: Playing Football !
Inelastic scattering of electrons
Confirms Bohr’s 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
(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
E=hf=4.9 eV emitted by Hg atom when
V0 > 4.9V