Major Concepts in Physics Lecture 21.

1. Major Concepts in Physics Lecture 21. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. Announcements • Exam 3 – Monday April 14 in class • Material: • everything since exam 2. eg temperature, heat, work, laws of thermodynamics. • Photon. Photoelectric effect. Wave-particle duality. Electron volts. Bohr model of H atom – basic ideas. • Electron as a wave. Diffraction, interference. Uncertainty principle. Pauli exclusion principle Lasers, chemical bonding PHY102

3. Reviews • In class Wednesday • In workshop Thurs/Friday • Practice exam – workshop • Hw5 due this week. Hw6 goes out • Solutions to quiz3, hw3, hw4 etc online PHY102

4. Thermodynamics • What to know: • Temperature T (Kelvins) thermal equilibrium. • Definition of heat Q: energy in transit. Flows from hot to cold (2nd law) • Definition of work W: for gas at constant pressure: -PDV. Care with sign. • 1st law. DU=W+Q • Entropy S: measure of molecular disorder. Tends to increase with time (2nd law again). DS=Q/T PHY102

5. Gases • Ideal gas. Molecules in rapid, random motion • Temperature measures mean kinetic energy • Pressure measures average force exerted on walls container due to molecular collisions K=3/2kT=1/2mv2rms PV=NkT where k – Boltzmann’s constant. N = total number of molecules • Neglect interactions – U=NK. Real gases U (internal energy) includes energy from intermolecular forces PHY102

6. Photons etc • Light sometimes behaves like particle with energy E=hf – h Planck’s constant • Photon can collide with electron in metal and kick it out – photoelectric effect hf=f+K or K=hf-f • K>0  threshold frequency. Maximum wavelength for effect. K independent of photon beam intensity …. PHY102

7. Wave-particle duality • Electrons, photons, … possess both particle and wave-like properties • De Broglie wavelength l=h/p • See interference/diffraction. • Uncertainty principle: quantum particles have an intrinsic uncertainty in their positions and momenta. DxDp>h PHY102

8. Discrete energy levels • Wave picture explains why discrete energy levels seen in atoms  only certain wavelength can fit inside … • Bohr – electron orbits must allow integer number of de Broglie wavelengths … • E=-13.6/n2 eV • Excited atoms: photons emitted with DE=hf PHY102

9. Energy level diagram for hydrogen PHY102

10. Fig. 28.23 PHY102

11. Quantum effects • Pauli principle: only one electron allowed in quantum state (energy level) • Multielectron atoms. Fill up levels respecting Pauli. Outer electrons are only loosely bound. • Chemical bonding: sharing electrons between atoms lowers energy due to uncertainty principle PHY102

12. Lasers etc • Know about absorption, spontaneous emission and stimulated emission. • Know roughly how a laser works: • Pumping stage : put atoms in metastable excited state • Stimulated emission. Electrons drop back to ground catalyzed by presence of same photon as emitted in decay – chain reaction • Many identical, coherent photons produced PHY102

13. Problems …. • Distance between atoms in NaCl is 0.28 nm. Crystal studied through neutron diffraction. How fast must the neutrons be moving (approx) ? PHY102

14. X-ray diffraction experiment uses 16keV X-rays. What should the electron kinetic energy be to produce the same diffraction pattern ? PHY102

15. Electrons are accelerated through a potential difference of 38 V. Beam passes through a slit. Width of central fringe formed on screen 1.0 m away is 1.13 mm. What is the slit width ? PHY102

16. Calculate the wavelength of light emitted when an electron makes a transition from the n=5 state to the n=2 state of H PHY102