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. Announcements. Exam 3 – Monday April 14 in class Material: everything since exam 2. eg temperature, heat, work, laws of thermodynamics.

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Major Concepts in Physics Lecture 21.

Prof Simon Catterall

Office 309 Physics, x 5978




  • 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



  • In class Wednesday

  • In workshop Thurs/Friday

  • Practice exam – workshop

  • Hw5 due this week. Hw6 goes out

  • Solutions to quiz3, hw3, hw4 etc online



  • 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



  • Ideal gas. Molecules in rapid, random motion

  • Temperature measures mean kinetic energy

  • Pressure measures average force exerted on walls container due to molecular collisions


    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


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 ….


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.



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


Energy level diagram for hydrogen


Fig. 28.23


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


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


Problems ….

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


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


  • 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 ?


  • Calculate the wavelength of light emitted when an electron makes a transition from

    the n=5 state to the n=2 state of H


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