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Do Now (2/21/14):

Do Now (2/21/14):. What does the word “quantized” mean? Where have we seen quantization in Physics? What is the structure of an atom?. Objectives. Define photoelectric effect and evidence of particle properties of light. Define work function. Calculate energy of a photon and an electron.

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Do Now (2/21/14):

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  1. Do Now (2/21/14): What does the word “quantized” mean? Where have we seen quantization in Physics? What is the structure of an atom?

  2. Objectives • Define photoelectric effect and evidence of particle properties of light. • Define work function. • Calculate energy of a photon and an electron. • Determine Planck’s constant.

  3. Particles and Waves

  4. Quantum Theory • Max Planck (1900) recognized electromagnetic radiation is quantized as E=hf. • 1905, Einstein proposed photon theory of light. Supported by work in photoelectric effect.

  5. Photoelectric Effect • E = KE + W • Energy of impinging light equals KE of electron plus the work function. • Intensity increases will increase current. • Frequency changes affect KE.

  6. The Photoelectric Effect 2/28/12

  7. Newton • Thought of light as particles

  8. Maxwell’s Theory • Light is composed of crossed electric and magnetic fields which make up a wave.

  9. Experiments show that when light shines on a metal surface, the surface emits electrons.

  10. Planck’s Work What are some other examples of “quantization”? In 1900, Max Planck came up with a formula to explain radiation from objects, but the formula only made sense if the energy of a vibrating molecule was quantized.

  11. Planck’s Constant

  12. Einstein’s Theory • Based on Planck's work, Einstein proposed that light also delivers its energy in chunks • light consists of particles (quanta) called photons, each with an energy of Planck's constant times its frequency

  13. Photon a light quantum that is massless, has energy and momentum, and travels at the speed of light

  14. The Photoelectric Effect the emission of electrons produced when electromagnetic radiation falls on certain materials

  15. Threshold Frequency f0 the minimum frequency of incident light which can cause photo electric emission

  16. Energy of a photon E=hf h=Planck’s constant f=frequency

  17. Electron Volts 1 eV= 1.6x10-19 J λ=wavelength

  18. Example: Calculate the wavelength and the energy of a photon of light with frequency equal to 1.984 x 1014 Hz. • Calculating the wavelength, from : c=fλ  3x108=λ (1.984 x 1014 )=  1.51 x 10-6 m • Calculating the energy of the photon: E = hf           E = 6.628 x 10-34 x 1.984 x 1014              = 1.31 x 10-19 J

  19. KE of photon hf0=min. energy to release electron

  20. Stopping Potential (Vo) The negative potential at which the photo electric current becomes zero

  21. Example: The stopping potential of a certain photocell is 4 V. What is the KE given to the electrons by the incident light? KE=-W KE=-qV0 KE=-(1.6x10-19)(4)=+6.4x10-19J

  22. Work Function ϕ0 Minimum amount of energy which is necessary to start photo electric emission. It is a property of material. Different materials have different values of work function.

  23. Einstein’s Theory hf =  + ½ mv2 • hf : energy of each photon Source: http://www.westga.edu/~chem/courses/chem410/410_08/sld017.htm

  24. Kinetic energy of emitted electron vs. Light frequency • Higher-frequency photons have more energy, so they make electrons come out faster; same intensity but a higher frequency increases the max KE of the emitted electrons. • If frequency is the same but intensity higher , more electrons come out (because there are more photons to hit them), but they won't come out faster, because each photon still has the same energy. • if the frequency is low enough, then none of the photons will have enough energy to knock an electron out. If you use really low-frequency light, you shouldn't get any electrons, no matter how high the intensity is. if you use a high frequency, you should still knock out some electrons even if the intensity is very low. Source: http://online.cctt.org/physicslab/ content/PhyAPB/lessonnotes/dualnature/ photoelectric.asp

  25. Simple Photoelectric Experiment Source: http://sol.sci.uop.edu/~jfalward/particlesandwaves/phototube.jpg

  26. Photoelectric Effect Applications

  27. Applications • The Photoelectric effect has numerous applications, for example night vision devices take advantage of the effect. Photons entering the device strike a plate which causes electrons to be emitted, these pass through a disk consisting of millions of channels, the current through these are amplified and directed towards a fluorescent screen which glows when electrons hit it. Image converters, image intensifiers, television camera tubes, and image storage tubes also take advantage of the point-by-point emission of the photocathode. In these devices an optical image incident on a semitransparent photocathode is used to transform the light image into an “electron image.” The electrons released by each element of the photoemitter are focused by an electron-optical device onto a fluorescent screen, reconverting it in the process again into an optical image

  28. Applications: Night Vision Device http://www.lancs.ac.uk/ug/jacksom2/

  29. Photoelectric Effect Applications • Photoelectric Detectors In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm. Source: http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm

  30. Photoelectric Smoke Detector Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg

  31. Applications • Solar panels are nothing more than a series of metallic plates that face the Sun and exploit the photoelectric effect. The light from the Sun will liberate electrons, which can be used to heat your home, run your lights, or, in sufficient enough quantities, power everything in your home. Source: www.futureenergy.org/ picsolarpannelsmatt.jpg

  32. Work Cited Amar, Francois G. The Photoelectric Effect. 25 Sep 2003. Section of Chemistry 121 for fall 03. 11 May 2006 <http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html> Blawn, Jeramy R. and Colwell, Catharine H. Physics Lab: Photoelectric Effect. 10 Jun 2003. Mainland High School: Online Physics Labs. 11 May 20006 <http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp> Helmenstine, Anne Marie. Photoelectric & Ionization Smoke Detector. 25 Feb 2006. About.com. 11 May 2006 <http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm> Einstein, Albert. “Concerning an Heuristic Point of View Toward the Emission and Transformation of Light.” American Journal Of Physics 5 May 1965: 137. Nave, Rod. HyperPhysics. 19 Aug. 2000. Georgia State University. 06 May 2006 <http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html> . Thornton T., Stephen, and Rex, Andrew. Modern Physics for Scientists and Engineers. Canada : Thomson Brooks/Core, 2006 Photoelectric Effect. 24 Apr. 2006. Wikipedia Free Encyclopedia. 05 May 2006. <http://en.wikipedia.org/wiki/Photoelectric_effect>.

  33. Do Now (2/25/14): In your own words, describe the photoelectric effect. Use the words “work function,” “threshold frequency,” “electron,” and “photon,” at least once in your paragraph.

  34. White Board Competition! • Work in groups • For each correct question, make a tally in the upper right hand corner of your board. BE HONEST!!! • The teams with the most points at the end will receive extra credit!

  35. #1 According to Einstein, the energy of a photon depends on the _________ of the electromagnetic radiation. momentum B. speed C. frequency D. intensity

  36. #2 The work function of iron is 4.7 eV. What is the threshold wavelength of iron? 2.60 nm B. 260 nm C. 470 nm D. 2600 nm

  37. #3 The stopping potential, V0, that prevents electrons from flowing across a certain photocell is 6.0 V. What is the kinetic energy in J given to the electrons by the incident light? 9.6 x 10-19 J 1.60 x 10-19 J 6.9 x 10-19 J D. 6.4 x 10-19 J

  38. #4 When light is directed on a metal surface, the kinetic energies of the electrons vary with the intensity of light vary with the speed of light vary with the frequency of the light are random

  39. #5 The threshold frequency for photoelectric emission in copper is 1.1 x 1015 Hz. What is the maximum kinetic energy in eV of the photoelectrons when light of frequency 1.5 x 1015 Hz is directed on a copper surface? 2.65 eV B. 2.12 eV C. 1.66 eV D. 1.03 eV

  40. #6 What will likely happen if a light whose frequency is below the threshold frequency hits a clean metal surface? • no electron will be ejected from the metal • fewer electrons will be ejected from the metal • more electrons will be ejected from the metal • ejected electrons will have higher kinetic energy

  41. #7 What is the work function of a metal whose threshold frequency is 3.5 x 1015 Hz? 2.32 x 10-18 J B. 3.11 x 10-18 J C. 3.65 x 10-18 J D. 4.01 x 10-18 J

  42. #8 What is the maximum wavelength of light that will cause photoelectrons to be emitted from sodium if the work function of sodium is 2.3 eV? 1.75 x 10-7 m B. 3.44 x 10-7 m C. 5.40 x 10-7 m D. 5.88 x 10-7 m

  43. #9 What will the maximum kinetic energy of the photoelectrons be if 200-nm light falls on a sodium surface (work function is 2.3 eV)? 2.96 x 10-19 J B. 4.73 x 10-19 J C. 5. 21 x 10-19 J D. 6.26 x 10-19 J

  44. #10 When 230-nm light falls on a metal, the current through the photoelectric circuit is brought to zero at a reverse voltage of 1.64 V. What is the work function of the metal? 4. 39 x 10-19 J B. 5.38 x 10-19 J C. 6.01 x 10-19 J D. 7.11 x 10-19 J

  45. #11 The current in a photoelectric effect experiment decreases to zero when the retarding voltage is raised to 1.25 V. What is the maximum speed of the electrons? 6.63 x 105 m/s B. 5.53 x 105 m/s C. 4.78 x 105 m/s D. 4.19 x 105 m/s

  46. #12 What is the maximum speed of an electron ejected from a sodium surface whose work function is 2.28 eV when illuminated by light of wavelength 450 nm? 3.25 x 105 m/s B. 4.10 x 105 m/s C. 4.85 x 105 m/s D. 5.25 x 105 m/s

  47. #13 Light is incident on the surface of metallic sodium, whose work function is 2.3 eV. The maximum speed of the photoelectrons emitted by the surface is 1.2 x 106 m/s. What is the wavelength of the light? 1.95 x 10-7 m B. 2.42 x 10-7 m C. 2.86 x 10-7 m D. 3.01 x 10-7 m

  48. #14 Ultraviolet radiation (wavelength 250 nm) falls on a metal target and electrons are liberated. If the maximum kinetic energy of these electrons is 1.00 x 10-19 J, what is the lowest frequency of electromagnetic radiation that will initiate a photocurrent on this target? 1.05 x 1015 Hz B. 1.35 x 1015 Hz C. 1.65 x 1015 Hz D. 1.78 x 1015 Hz

  49. #15 Photons of wavelength 220 nm on a metal target and liberate electrons with kinetic energies ranging from 0 to 61 x 10-20 J. Determine the threshold wavelength of the metal. 1.68 x 10-7 m B. 1.95 x 10-7 m C. 2.06 x 10-7 m D. 6.77 x 10-7 m

  50. #1 http://lrt.ednet.ns.ca/PD/ict_projects/photoelectric/index.htm

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