What Puts the Super in Superconductors? Saturday Morning Physics November 22, 2003 Dr. Sa-Lin Cheng Bernstein Conductors John Wiley & Sons, Inc. Outer electrons of the atoms in conductors are loosely bound and free to move through the material
FREE electrons Metals are conductors Energy is carried by charge fro power plant to appliances Resistance Life is tough for free electrons Resistance:
Repulsion from other electrons Vibration of atoms Impurities Life is tough for free electrons, especially on hot days Energy is wasted http://regentsprep.org/Regents/physics/phys03/bresist/default.htm Extreme Low
Temperature Kelvin (1824-1907): electrons freeze and resistance increases Onnes (18531926): Resistance drops to zero Temperature Conversion Kelvin (K): K = C + 273.15 K = 5/9 F + 255.37
Fahrenh eit 212 32 -300.42 -452.11 -459.67 Celsiu Kelvin comments s 100 373.15 water boils 0 273.15 water freezes -195.79 77.36 liquid nitrogen boils liquid helium boils
-268.95 4.2 absolute zero -273.15 0 Discovery of Superconductivity Heike Kamerlingh Onnes 1908 - liquefied helium (~4 K = - 452F )
1911- investigated low temperature resistance of mercury 1913 - Nobel Prize in physics Conductors vs. Superconductors Normal conductors: =0 at T=0 Superconductors: = at T
(superconducting state) Tc = critical temperature Magnetic Fields Magnet has two poles: North and South Like poles repel, unlike poles attract Detect magnetic
field iron filings John Wiley & Sons, Inc. p://www.school-for-champions.com/science/magnetism.htm Electromagnet Current flowing in a loop of wire creates a magnetic field Current loop can be imagined to be a phantom bar magnet
= John Wiley & Sons, Inc. http://www.windows.ucar.edu/spaceweather/info_mag_fields.htm Which side is north pole? Right hand rule N John Wiley & Sons, Inc. Lenzs Law A conductor opposes any change in
externally applied magnetic fields. S S Induced current N N = N S
Meissner Effect 1933 Walther Meissner and Robert Ochsenfeld T
Demo Superconductor: YBa2Cu3O7 Tc ~ 90 K (5.90 mV) Voltmeter: measure voltage across superconductor V=IR 0 0
Superconducting State Superconducting state: T
Tc T Hc = critical magnetic field Type I Superconductors Perfect diamagnetism Strength of diamagnetism increases linearly as the applied field increases When the applied field = Hc Field uniformly crosses
sample QUENCH!! Examples of Type I Mostly elements Highest Tc ~ 22 K (Phosphorus) Highest Hc ~ 800 G (lead) Element Mercury
Lead Aluminu m Tin Zinc Titanium Phosphor us Tc (K) 4.153 7.193 1.196 3.722 0.85 0.39 14~22
(pressure ) The But. . . Tc too low (highest: 22K) Hc too low (highest: 800 G) Little potential for applications Discouraged!!! RIVALRY SATURDAY! GO BLUE!
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