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History of Atomic Models (I) 1. Single indivisible particle, “atomos” Greek. Democritus(460-370 BC) 2. Plum Pudding Model J.J. Thomson(1856-1940), England, used his CRT to discover the electron.
Some Early Apparatus Thomson’s CRT (photo) Schematic Drawing Question: What does CRT stand for? Cathode Ray Tube
Robert Millikan (1868-1953) and his Oil-Drop Experiment in 1909. Proved that the elementary unit of charge, 1.6x10 -19 C only occurred in certain amounts (Quantized).
History of Atomic Models (II) 3. Planetary Model - imagine the solar system with the Sun as the nucleus and the planets as the electrons. Ernest Rutherford(1871-1937), England, Gold-foil Experiment. Results: (1) most of the atom is empty space (2) almost all the mass is in the nucleus.
Contributors to the Photoelectric Effect and Quantum Theory Heinrich Hertz 1857-1894 Max Planck 1858-1947 Albert Einstein 1879-1955 LNK2LRN
The Photoelectric Effect Light is sent to a metal surface. If the frequency of the light is high enough, the light kicks electrons off the surface of the metal. Emitter is connected to the negative terminal of a battery. Collector is attached to the positive terminal. Result: The Photocell. Discovered for the first time by Max Planck and Heinrich Hertz, Germany, in the late 1800’s. Carbon Arc Light
GIVES ALL ITS ENERGY TO ONE ELECTRON e ONE PHOTON
The emission of electrons from a material that is bombarded with radiation in the form of light. E photon = hf with h = 6.6 x 10 -34 J ·s LNK2LRN The Explanation of the Photoelectric Effect
Problem #1. Calculate the energy of a photon of light having a frequency of 2.5x10 15 Hz. (Show your work.) Given: f = 2.5x10 15 Hz We know Planck’s Constant. h = 6.6x10 -34 Js E = ___________ Equation: E = hf Solution: E = 6.6x10 -34 Js · 2.5x10 15 Hz E = 1.7x10 -18 J
All parts of the Electromagnetic Spectrum move at the same speed through a vacuum: 3.0 x 10 8 m/s. We call this the speed of light and assign it the letter, c. But all possess the 5 properties of waves, so they must obey the wave equation, c = f·λ. The energy is carried by mass-less photons, and computed with the equation, E = hf. ( h = 6.6 x 10 -34 Js )
Summary of the Equations for Photoelectric Effect and Quantum Theory E = h·f Energy of a photon of light h = 6.6 x 10 -34 J ·s c = f · Wave equation for light, c = 3.0 x 10 8 m/s W o = h · f o Work function, with f o = threshold frequency KE max = q·V o Maximum kinetic energy of photoelectrons based on stopping potential, V o KE max = E - W o Maximum kinetic energy of photoelectrons based on photon energy and work function E = mc 2 Einstein’s equation for conversion of matter to energy
Combine these three equations to get a single equation for the wavelength of a matter particle. E = mc 2 c = f · E = h·f The equation is: = h / (mv) “If waves (light) can act like particles, why can’t particles act like waves”. Prince Louis Victor deBroglie (1892-1987), France.
Matching Exercise. Democritus J. Thomson Ernest Rutherford Robert Millikan Max Planck Heinrich Hertz Albert Einstein Found charge on electron with Oil-drop experiment Unit of frequency named after him Made the first CRT Gave us the equation E = mc 2 Atom is single indivisible particle Planetary atomic model His constant is 6.6x10 -34 Js
Can you then come-up with the equation for the momentum of a photon? Remember, if particles can act like waves, why can’t waves act like particles? The equation is: p = h / Useful constants: 1 eV = 1.6 x 10 -19 J mass of electron = 9.11 x 10 -31 kg mass of proton = 1.67 x 10 -27 kg charge on the electron = -1.6 x 10 -19 C charge on the proton = 1.6 x 10 -19 C
Problem #2. Calculate the threshold frequency of a metal having a work function of 6.4 eV. (Show your work.) Given: W o = 6.4 eV We know Planck’s Constant. h = 6.6x10 -34 Js f o = ___________ Equation: W o = hf o Solution: f o = W o / h = ( 6.4 eV x 1.6 x 10 -19 J / 1 eV) / 6.6x10 -34 Js f o = 1.6x10 15 Hz
Problem #3. A metal has a threshold frequency of 3.3 x 10 14 Hz. Light with a frequency of 2.0 x 10 15 Hz shines on the metal. Find the maximum kinetic energy of the photoelectrons. (Show your work.) Given: f o = 3.3x10 14 Hz f = 2.0x10 15 Hz Planck’s Constant, h = 6.6x10 -34 Js KE max = ___________ Equation: KE max = hf – hf o = h(f – f o ) KE max = 6.6x10 -34 Js ( 2.0x10 15 Hz – 3.3x10 14 Hz ) KE max = 1.1x10 -18 J
Problem #4. Calculate the de Broglie wavelength of a cannon ball with a mass of 100.0 g and a velocity of 7.0x10 2 m/s. (Show your work.) Given: m = 100.0 g =.1000 kg v = 7.0x10 2 m/s λ = ___________ Equation: λ = h/mv h = 6.6x10 -34 Js Solution: λ = 6.6x10 -34 Js / (.1000 kg · 7.0x10 2 m/s) λ = 9.4x10 -36 m
History of Atomic Models (III) 4. Planetary-Quantum model was proposed to align atomic theory with the new physics of Quantum Theory. Niels Bohr(1885-1962), Denmark, determined the equations for r n = 5.3 x 10 -11 m x n 2 and E n = -13.6 eV x 1/n 2
Antoine Henri Becquerel 1/2 of the prize (France) The Nobel Prize in Physics 1903 "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity" "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel" Pierre Curie France 1/4 of the prize Marie Curie, née Sklodowska France 1/4 of the prize Contributors to the Study of Nuclear Physics This is …