Modern Physics IV Lecture 5115 February 2012Modern Physics IV Lecture 51 Modern Physics for Frommies IV The Universe - Small to Large Lecture 5 Fromm Institute for Lifelong Learning University of San Francisco

Modern Physics IV Lecture 5215 February 2012Modern Physics IV Lecture 52 Agenda Administrative matters Normal Conductivity The Discovery of Super Conductivity

Modern Physics IV Lecture 5315 February 2012Modern Physics IV Lecture 53 Administrative Matters Reduced mass: Checked, correct as is Very Short Introductions: Oxford University Press

Modern Physics IV Lecture 5415 February 2012Modern Physics IV Lecture 54 Normal Conductivity Current is flow of charge. Units of Coulombs (C)/sec = Amperes (A) Analogy with the flow of water through a pipe h P =  W gh F There is frictional resistance, R, to the flow. One might expect that the resistance decreases with the cross sectional area of the pipe and increases with the length of the pipe. l A

Modern Physics IV Lecture 5515 February 2012Modern Physics IV Lecture 55 Current flowing through conductor with resistance R R V Ohm’s Law  is called the resistivity (  m) and is a characteristic of the material  is called the temperature coefficient of resistance of the material. l is length of conductor A is conductor’s cross sectional area For small  T

Modern Physics IV Lecture 5615 February 2012Modern Physics IV Lecture 56  T T0T0  (conventional) curremt density Electrons drift against electric field bouncing off atoms Reach a more or less steady average speed Drift velocity v d << average random speed v d is not the velocity of a signal, there is a “pipeline” effect. Ohm’s Law Near room temperature

Modern Physics IV Lecture 5715 February 2012Modern Physics IV Lecture 57 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  eventually plateaus T  Plateau dominated by scattering from impurities. Dependent on purity and thermal history

Modern Physics IV Lecture 5815 February 2012Modern Physics IV Lecture 58 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 5915 February 2012Modern Physics IV Lecture 59 Liquifaction of Gasses: Elements and compounds that are gasses at STP 1833Michael Faraday liquefies chlorine with water ice. Cl 2 H 2 O crystals HOTCOLD LCl 2 High pressure raises boiling point. At STP Cl 2 boils at -34 ○ C Technique also succesful for liquefying NH 3, H 2 S, NO 2, SO 2, and solidifying CO 2.

Modern Physics IV Lecture February 2012Modern Physics IV Lecture Louis Cailletet attempts to liquefy C 2 H 2. 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 H 2. Boiling point – 253 ○ C or 20 K

Modern Physics IV Lecture February 2012Modern Physics IV Lecture H. Kammerlingh Onnes (Leiden) liquefies He at 4.2 K Discovers superconductivity in mercury

Modern Physics IV Lecture February 2012Modern Physics IV Lecture 512 Low Temperature Summary Substance and EventT ( ○ C)T (K) Water boils Water freezes 0273 SO2 boils 263 Cl2 boils195 H2S boils212 CO2 sublimates 195 CH4 boils112 O2 boils 90 N2 boils 77 H2 boils 20 He boils 4.2 Absolute zero = ○ C

Modern Physics IV Lecture February 2012Modern Physics IV Lecture 513 Now that Ommes had LHe he could measure resistivities at temperatures in the 1 K region. Tried various materials including very pure mercury (Hg) Hg T c ≈ 4 K Most materials

Modern Physics IV Lecture February 2012Modern Physics IV Lecture 514 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 February 2012Modern Physics IV Lecture 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 February 2012Modern Physics IV Lecture 516

Modern Physics IV Lecture February 2012Modern Physics IV Lecture 517

Modern Physics IV Lecture February 2012Modern Physics IV Lecture 518