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Modern Physics IV Lecture 5

Fromm Institute for Lifelong Learning University of San Francisco. Modern Physics for Frommies IV The Universe - Small to Large Lecture 5. Modern Physics IV Lecture 5. 1. Agenda. Administrative matters Normal Conductivity The Discovery of Super Conductivity. Modern Physics IV Lecture 5.

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Modern Physics IV Lecture 5

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  1. Fromm Institute for Lifelong Learning University of San Francisco Modern Physics for Frommies IV The Universe - Small to Large Lecture 5 Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 1

  2. Agenda • Administrative matters • Normal Conductivity • The Discovery of Super Conductivity Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 2

  3. Administrative Matters Reduced mass: Checked, correct as is Very Short Introductions: Oxford University Press https://www.oup.co.uk/vsi Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 3

  4. Normal Conductivity Current is flow of charge. Units of Coulombs (C)/sec = Amperes (A) Analogy with the flow of water through a pipe There is frictional resistance, R, to the flow. h P = rWgh F A l One might expect that the resistance decreases with the cross sectional area of the pipe and increases with the length of the pipe. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 4

  5. Current flowing through conductor with resistance R R Ohm’s Law V l is length of conductor A is conductor’s cross sectional area ris called the resistivity (Wm) and is a characteristic of the material For small DT ais called the temperature coefficient of resistance of the material. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 5

  6. r r0 Near room temperature T T0 (conventional) curremt density Ohm’s Law Electrons drift against electric field bouncing off atoms Reach a more or less steady average speed Drift velocity vd << average random speed vdis not the velocity of a signal, there is a “pipeline” effect. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 6

  7. As temperature increases, the number of collisions increases, increasing the resistance to electrical flow. Collisions generate heat. Energy id dissipated into the environment. Joule heating. As T is reduced to low levels reventually plateaus r Plateau dominated by scattering from impurities. Dependent on purity and thermal history T Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 7

  8. The Road to Extremely Low Temperatures Absolute Zero: Is there a lowest temperature? (1740 – 1845) Kinetic Theory of Gasses Measure the pressure of a constant volume of various gasses as a function of temperature. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 8

  9. Liquifaction of Gasses: Elements and compounds that are gasses at STP 1833 Michael Faraday liquefies chlorine with water ice. High pressure raises boiling point. At STP Cl2 boils at -34 ○C Cl2H2O crystals LCl2 HOT COLD Technique also succesful for liquefying NH3, H2S, NO2, SO2, and solidifying CO2. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 9

  10. 1870 Louis Cailletet attempts to liquefy C2H2 . His apparatus springs a leak. Rapid release of pressure causes cooling. In 1877 he succeeds in liquefying oxygen. At the same time Pierre Picet also succeds with oxygen by cascading Faraday’s technique Cailletet goes on to do nitrogen and carbon monoxide The rapid expansion effect is today known as Joule - Thomson cooling. Hydrogen remains stubborn as does the newly discovered (1868 in solar spectrum) helium !898 James Dewar succeeds in liquefying H2. Boiling point – 253 ○C or 20 K Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 10

  11. 1908 H. Kammerlingh Onnes (Leiden) liquefies He at 4.2 K. 1853 - 1926 1911 Discovers superconductivity in mercury Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 11

  12. Low Temperature Summary Substance and Event T (○C) T (K) Water boils 100 373 Water freezes 0 273 SO2 boils 263 Cl2 boils 195 H2S boils 212 CO2 sublimates195 CH4 boils 112 O2 boils 90 N2 boils 77 H2 boils 20 He boils 4.2 Absolute zero = - 273 ○C Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 12

  13. Now that Ommes had LHe he could measure resistivities at temperatures in the 1 K region. Tried various materials including very pure mercury (Hg) Most materials Hg Tc≈ 4K Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 13

  14. Critical temperature, marks phase change: normal superconducting Superconductivity is destroyed by a sufficiently high field or equivalently by a high enough current Superconductivity appears in metals whose normal conductivity is not particularly high, e.g. Hg, Sn, Pb, Ga, Nb. 1931: Superconductivity found in alloys. Component elements may be non superconducting. Some alloys will support much higher fields before going normal, e.g. NbSn. Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 14

  15. Meissner effect: • Magnetic fields are expelled from super conductors Surface currents are generated which screen the interior of the superconductor. Currents give rise to an exterior magnetic field which will act to repel the original magnet.  Levitation Modern Physics IV Lecture 5 Modern Physics IV Lecture 5 15

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