1. Class #8: Quantum Mechanics Born
Modern Physics (PC300)
Class #8: Quantum Mechanics Born

2. Test Assessment

5. Photoelectric Effect - + Photoelectrons. V0 - stopping potential
Minimum extra KE that allows electrons to escape is the work function

6. Photoelectric Effect: Intensity
-v0 – stopping potential (stops all photoelectrons no matter what the light intensity).
For a given light intensity, there is a maximum photocurrent which is reached as voltage increases from –ve to +ve
Makes Sense: more photons - more electrons

7. Photoelectric Effect: Frequency
1902: Lenard -
Maximum KE does not depend on intensity
Maximum KE of photoelectrons (material dependent) depends only on light frequency.
i.e. light of different frequency requires a different retarding potential (-v0) to stop the most energetic photoelectrons
Vo depends on frequency but not intensity.
Threshold Frequency (Work Function) below which no electrons are released
Lenard - nobel prize on cathode rays
1905 Lenard - Nobel Prize

8. 1905: Photoelectric Effect: Solution
Classical electromagnetic theory has problems explaining the independence of Kmax and light intensity, the linear dependence of Kmax on light frequency and the instantaneous (10-9s) response of the photocurrent.
Einstein - Light composed of irreducible "grains" - quanta (energy packets).
1905 What a year for Einstein
1921 Einstein - Nobel Prize

9. Photoelectric Effect: Work Functions
(Kmax)
The smaller the work function, the smaller the threshold frequency that light can eject photoelectrons.
No photoelectrons are released for frequencies below this threshold frequency.
Draw Graph – No photoelectrons is proportional to intensity (I.e. maximum photocurrent is proportional to the light intensity)

10. Various Work Functions
1923 Millikan - Nobel Prize
Why do different metals have different work functions?

12. Problem Set for Wednesday
To answer the first two questions you will first need to do a Simulation.
Run the Photoelectric Effect program by following the steps below.
P:\Academic\White\PC301 Modern Physics 2013\Programs\Photoelectric\Photoelectric_en
Take data to answer the following question.
1. Using five different wavelengths striking Sodium, calculate the ratio h/e. (Hint, make a graph in Excel of the Stopping Potential vs. frequency. Include this graph, with a trendline, in your homework solution. And remember, that all graphs need labels, a title, and a short caption…)
2. Find the Workfunction for each of the six elements (most of which are metals…including the unknown) used in the simulation. Make a simple chart of your findings. Write a few sentences that clearly describe what procedure you followed and how you arrived at these conclusions.

13. Sim #4: Photoelectric Effect
P-Drive\White\Modern Physics\Programs\Petutor\CTWINX
Click on Photo15.ctb then hit OK

14. Lab 6 – Photoelectric Effect

15. X Ray Production 1901 Roentgen - Nobel Prize
X-rays are produced when the e-'s are suddenly decelerated upon collision with the metal target; these x-rays are commonly called brehmsstrahlung or "braking radiation
Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high
enough, that radiation is in the x-ray region of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The curves above are from the 1918 data of Ulrey, who bombarded tungsten targets with electrons of four different energies.
1901 Roentgen - Nobel Prize

16. EM Spectrum: X-Rays (Duane-Hunt Rule)
Same min. wav. (max freq) -> E=hf => when an electron gives up all its KE and creates one photon (inverse photoelectric effect).
(W-tungsten, Mo-Molybdenum, Cr-Chromium
(Duane-Hunt Rule)

17. Quantum Homework Questions
Some Simulation Questions and an Electron Gun question
Also…
Thornton and Rex!
"Yeah, well I got mine first
Due Wed normal time

18. 1897 - J.J.Thompson 1906 - Nobel Prize
"identified cathode rays as electrons and measured e/m
Nobel Prize

19. X Ray Production
X-rays are produced when the electrons are suddenly decelerated upon collision with the metal target; these x-rays are commonly called brehmsstrahlung or "braking radiation
Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high
enough, that radiation is in the x-ray region of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The curves above are from the 1918 data of Ulrey, who bombarded tungsten targets with electrons of four different energies.

20. Measuring charge/mass (e/m)
Electron beam
Cathode Ray
CRT predecessor to TV etc
B field perpendicular to direction of motion of particles make them move in a circular motion.
Cathode Ray Tube
Derive in lab

21. Lab ? - e/m setup

22. 1909 - Robert Millikan Precise Measurement of e
Oil Drop Experiment