Quantum 5/6/2013

Chapter 29 Introduction 1.What is the Wave Particle Duality? 2.Describe black body radiation and its result. 3.What is the photoelectric effect? What is the energy of a photon? 4.What is the momentum of a photon? How was this discovered? 5.What is the de Broglie wavelength? 6.What is the Heisenberg Uncertainty Principle? What does this mean about conservation of energy?

Quantum History 1887 Heinrich Rudolf Hertz – Photoelectric Effect first observed 1900 Max Planck – Quantum Hypothesis + Blackbody Radiation Explanation 1902 Philipp Lenard – Max energy of emitted electrons independent intensity of incident light 1905 Albert Einstein – Explains Photoelectric Effect (introduces quanta of light) 1913 Robert Millikan – Electron charge measured (Oil Drop Experiment) 1916 Robert Millikan – Photoelectric Effect tested accurately

The Birth of the Quantum

Implications of Planck’s Law Depending on your units h = 6.626x J. s h = 4.14x eV. s The energy levels of the molecules must be discreet Only transitions by an amount E=hf are allowed The implication is that light is discreet or quantised These quantum levels are now known as number states energy 4hf 3hf 2hf 1hf 0 energyn

Photoelectric Effect

Can you describe these phenomena? PhenomenaWavesParticle ReflectionYesNo RefractionYes InterferenceYesNo DiffractionYesNo PolarizationYesNo Photoelectric EffectNO!YES!

Photoelectric Effect

Waves vs. Quanta WavesQuanta Increased intensity means higher maximum kinetic energy Increased intensity will increase the number of photons but not change the maximum kinetic energy Frequency should not effect maximum kinetic energy As frequency increases so does the maximum kinetic energy (linearly) The longer the exposure the more likely electrons will be emitted No time dependence There is a minimum energy required to release electrons Assume monochromatic light for the photoelectric effect

What can be shown? 1. There is a linear relationship maximum kinetic energy of emitted electrons and frequency of light that freed the them. 2. That the slope of the maximum kinetic energy vs. frequency line is numerically equal to h (or Planck’s constant) 3. There is a critical frequency that is the lowest frequency at which the metal can be photoelectrically active.

Maximum Kinetic Energy vs. Frequency Frequency (Hz) K max (J) The slope represents Planck’s constant The frequency axis intercept is the minimum frequency for electron emission and is related to the work function for the material

Further Applications Photomultipliers Solar Cell Burglar Alarms (UV or IR for invisibility) Automatic Door Openers (Drop in current trips solenoid switch) Some Smoke Detectors Some Film sound track Photocell -> Photodiode Spacecraft Moon dust

Practice CQ’s p.935 #2,5 Problems p.936 #1, 5, 7

Bellwork 5/6 Can an object with no mass have momentum?

Photon Momentum

Compton Scattering Compton scattering is an inelastic scattering of a photon by a free charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which maybe an X-ray or gamma ray photon), called the Compton effect. Part of the energy of the photon is transferred to the scattering electron.

Compton Scattering Calculating

Quantum mechanics Wave-particle duality Waves and particles have interchangeable properties This is an example of a system with complementary properties The mechanics for dealing with systems when these properties become important is called “Quantum Mechanics” particle_duality.ogv particle_duality.ogv

Wave Properties of Matter

Quantum Theory Particles act like waves?! The wave associated with a particle is a wave of probability (this gives us the electron cloud). The best we can do is predict the probability that something will happen. Heisenberg Dirac Schrodinger

Heisenberg Uncertainty Principle The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa. Heisenberg, uncertainty paper, 1927 X position of particle P is linear momentum of particle

The Uncertainty Principle Measurement disturbes the system

Uncertainty Principle Consequences

Practice CQ’s p.936 #11 Problems p.936 #13, 14, 22, 24