lect22 handout

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Published on October 15, 2007

Author: Charlie

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Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality :  Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality Today’s Lecture will cover textbook sections 27.2-3, 28.1 Physics 102: Lecture 22 Everything comes unglued:  Everything comes unglued The predictions of “classical physics” (Newton’s laws and Maxwell’s equations) are sometimes completely, utterly WRONG. classical physics says that an atom’s electrons should fall into the nucleus and STAY THERE. No chemistry, no biology can happen. classical physics says that toaster coils radiate an infinite amount of energy: radio waves, visible light, X-rays, gamma rays,… The source of the problem:  The source of the problem It’s not possible, even “in theory” to know everything about a physical system. knowing the approximate position of a particle corrupts our ability to know its precise velocity (“Heisenberg uncertainty principle”) Particles exhibit wave-like properties. interference effects! The scale of the problem:  The scale of the problem Let’s say we know an object’s position to an accuracy Dx. How much does this mess up our ability to know its speed? Here’s the connection between Dx and Dv (Dp = mDv): That’s the “Heisenberg uncertainty principle.” h  6.610-34 J·s Atomic scale effects:  Atomic scale effects Small Dx means large Dv since Example: an electron (m = 9.110-31 kg) in an atom is confined to a region of size Dx ~ 510-11 m. How fast will the electron tend to be moving? Plug in, using h = 6.610-34 to find Dv > 1.1106 m/sec Quantum Mechanics!:  Quantum Mechanics! At very small sizes the world is VERY different! Energy is discrete, not continuous. Everything is probability; nothing is for certain. Particles often seem to be in two places at same time. Looking at something changes how it behaves. Blackbody Radiation:  Hot objects glow (toaster coils, light bulbs, the sun). As the temperature increases the color shifts from Red to Blue. The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature.) Blackbody Radiation Blackbody Radiation Spectrum:  Blackbody Radiation Spectrum Higher temperature: peak intensity at shorter l Blackbody Radiation: First evidence for Q.M.:  Blackbody Radiation: First evidence for Q.M. Max Planck found he could explain these curves if he assumed that electromagnetic energy was radiated in discrete chunks, rather than continuously. The “quanta” of electromagnetic energy is called the photon. Energy carried by a single photon is E = hf = hc/l Planck’s constant: h = 6.626 X 10-34 Joule sec Preflights 22.1, 22.3:  Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? The least energy? Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange ACT: Colored Light:  ACT: Colored Light Which bulb’s filament is hottest? 1) Red 2) Green 3) Blue 4) Same ACT: Photon:  ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red 2) Green 3) Same Wien’s Displacement Law:  Wien’s Displacement Law To calculate the peak wavelength produced at any particular temperature, use Wien’s Displacement Law (max ~ 1 / T ): max = 0.2898*10-2 m·K / T in Kelvin! Nobel Trivia:  Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special Relativity E=mc2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didn’t receive a Nobel prize. Photoelectric Effect:  Photoelectric Effect Light shining on a metal can “knock” electrons out of atoms. Light must provide energy to overcome Coulomb attraction of electron to nucleus Light Intensity gives power/area (i.e. Watts/m2) Recall: Power = Energy/time (i.e. Joules/sec.) Photoelectric Effect Summary:  Photoelectric Effect Summary Each metal has “Work Function” (W0) which is the minimum energy needed to free electron from atom. Light comes in packets called Photons E = h f h=6.626 X 10-34 Joule sec Maximum kinetic energy of released electrons K.E. = hf – W0 Photoelectric Effect: Light Intensity:  Photoelectric Effect: Light Intensity What happens to the rate electrons are emitted when increase the brightness? What happens to max kinetic energy when increase brightness? Photoelectric Effect: Light Frequency:  Photoelectric Effect: Light Frequency What happens to rate electrons are emitted when increase the frequency of the light? What happens to max kinetic energy when increase the frequency of the light? Photoelectric Table:  Photoelectric Table Wave Particle Result Increase intensity, but leave frequency unchanged Rate Increase Increase Increase KE Increase Unchanged Unchanged Increase frequency from infrared to ultraviolet Rate Unchanged Unchanged Unchanged KE Unchanged Increase Increase Light is composed of particles: photons Preflights 22.4, 22.6:  Preflights 22.4, 22.6 Which drawing of the atom is more correct? This is a drawing of an electron’s p-orbital probability distribution. At which location is the electron most likely to exist? 3 2 1 Is Light a Wave or a Particle?:  Is Light a Wave or a Particle? Wave Electric and Magnetic fields act like waves Superposition, Interference and Diffraction Particle Photons Collision with electrons in photo-electric effect Both Particle and Wave ! Are Electrons Particles or Waves?:  Are Electrons Particles or Waves? Particles, definitely particles. You can “see them”. You can “bounce” things off them. You can put them on an electroscope. How would know if electron was a wave? Look for interference! Young’s Double Slit w/ electron:  Young’s Double Slit w/ electron Screen a distance L from slits Source of monoenergetic electrons L Electrons are Waves?:  Electrons are Waves? Electrons produce interference pattern just like light waves. Need electrons to go through both slits. What if we send 1 electron at a time? Does a single electron go through both slits? ACT: Electrons are Particles:  ACT: Electrons are Particles If we shine a bright light, we can ‘see’ which hole the electron goes through. (1) Both Slits (2) Only 1 Slit Electrons are Particles and Waves!:  Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we don’t look, electron goes through both slits. If we do look it chooses 1. Schroedinger’s Cat:  Schroedinger’s Cat Place cat in box with some poison. If we don’t look at the cat it will be both dead and alive! Poison Here Kitty, Kitty! More Nobel Prizes!:  More Nobel Prizes! 1906 J.J. Thompson Showing cathode rays are particles (electrons). 1937 G.P. Thompson (JJ’s son) Showed electrons are really waves. Both were right! Quantum Summary:  Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment (coming soon!) See you next lecture!:  See you next lecture! Read Textbook Sections 27.5, 28.2, 4

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