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Major Concepts in Physics Lecture 21. PowerPoint Presentation

Major Concepts in Physics Lecture 21.

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### Major Concepts in Physics Lecture 21.

Prof Simon Catterall

Office 309 Physics, x 5978

http://physics/courses/PHY102.08Spring

PHY102

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

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

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

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

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

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

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

Fig. 28.23

PHY102

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

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

Problems ….

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

PHY102

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

PHY102

- 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

- 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

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