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Physics 1202: Lecture 31 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like problems –Team in class HW 9 this FridayHW 9 this Friday.

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Presentation on theme: "Physics 1202: Lecture 31 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like problems –Team in class HW 9 this FridayHW 9 this Friday."— Presentation transcript:

1 Physics 1202: Lecture 31 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like problems –Team in class HW 9 this FridayHW 9 this Friday Modern physics

2 Modern Physics

3 Quantization Physical quantities come in small but finite quantities –Quantum (or quanta for many of them) –Not continuous Atomic Spectra: a)Emission line spectra for hydrogen, mercury, and neon; b)Absorption spectrum for hydrogen.

4 Blackbody and temperature Peak gives main color

5 Black Body Radiation Intensity of blackbody radiation Planck’s expression h = 6.626  10 -34 J · s : Planck’s constant Assumptions: 1. Molecules can have only discrete values of energy E n; 2. The molecules emit or absorb energy by discrete packets - photons Max Planck (1899):

6 Quantum energy levels Energy E 0 1 3 4 5 2 n hf 2hf 3hf 4hf 0 5hf

7 Photoelectric effect In 1887, Heinrich Hertz –shining ultra-violet light on metal in vacuum –If V not large enough, no current

8 Photoelectric effect Kinetic energy of liberated electrons is where  is the work function of the metal

9 Photoelectric effect Explained by Einstein in 1905 –Based on quantum of light (Planck) –Nobel Prize in 1914

10 Photon properties Recall (for electromagnetic wave) E = pc Quantization (Planck): E = hf = hc / So  = h / p Recall from relativity Conclusion: m 0 = 0 (photons have no mass ! )

11 Compton effect In 1920’s, Arthur Compton experiments with X-rays –Wavelength longer after scattering –Using quantization he derived C : Compton wavelength

12 The waves properties of particles In 1924, Louis de Broglie postulate: because photons have both wave and particle characteristics, perhaps all forms of matter have both properties Momentum of the photon De Broglie wavelength of a particle

13 Example: An accelerated charged particle An electron accelerates through the potential difference 50 V. Calculate its de Broglie wavelength. Solution: Energy conservation Momentum of electron Wavelength

14 Birth of quantum mechanics Erwin Schrödinger –Wave function & Hamiltonian Werner Heisenberg –Uncertainty principle

15 5 steps methods Draw and list quantitites Concepts and equations needed Solve in term of symbols Solve with numbers Checks values and units


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