1. SuperconductivitySuperconductivity Dr. Viswanath Tanikella

2. Conductors 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 from power plant to appliances © John Wiley & Sons, Inc.

3. 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

4. Extreme Low Temperature Kelvin (1824-1907): electrons freeze and resistance increases Onnes (1853-1926): Resistance drops to zero

5. Temperature Conversion Kelvin (K): –K = C + 273.15 –K = 5/9 F + 255.37 Fahrenhei t CelsiusKelvincomments 212100373.15 water boils 320273.15 water freezes -300.42-195.7977.36 liquid nitrogen boils -452.11-268.954.2 liquid helium boils -459.67-273.150 absolute zero

6. Heike Kamerlingh Onnes 1908 - liquefied helium (~4 K = - 452°F ) 1911- investigated low temperature resistance of mercury 1913 - Nobel Prize in physics Discovery of Superconductivity

7. Conductors vs. Superconductors Normal conductors: = 0 at T=0 Superconductors: = at T<Tc (superconducting state) Tc = critical temperature

8. Magnetic Fields Magnet has two poles: North and South Like poles repel, unlike poles attract Detect magnetic field iron filings © John Wiley & Sons, Inc. http://www.school-for-champions.com/science/magnetism.htm

9. Electromagnet Current flowing in a loop of wire creates a magnetic field Current loop can be imagined to be a phantom bar magnet = http://www.windows.ucar.edu/spaceweather/info_mag_fields.html © John Wiley & Sons, Inc.

10. Which side is north pole? Right hand rule N © John Wiley & Sons, Inc.

11. Lenzs Law A conductor opposes any change in externally applied magnetic fields. N S N S N S = Induced current

12. Meissner Effect 1933 – Walther Meissner and Robert Ochsenfeld T<Tc: external magnetic field is perfectly expelled from the interior of a superconductor Strong external magnetic fields can destroy superconductivity http://www.jsf.or.jp/sln/aurora_e/step2.html

13. Superconductor: YBa2Cu3O7 Tc ~ 90 K (5.90 mV) Voltmeter: measure voltage across superconductor V = I R Demo 0 0

14. Superconducting State Superconducting state: T<Tc H<Hc Hc = critical magnetic field Normal State Superconducting State T Hc Tc H

15. Perfect diamagnetism Strength of diamagnetism increases linearly as the applied field increases When the applied field = Hc Field uniformly crosses sample QUENCH!! Type I Superconductors

16. Examples of Type I Mostly elements Highest Tc ~ 22 K (Phosphorus) Highest Hc ~ 800 G (lead) ElementTc (K) Mercury4.153 Lead7.193 Aluminum1.196 Tin3.722 Zinc0.85 Titanium0.39 Phosphoru s 14~22 (pressure)

17. The But... Tc too low (highest: 22K) Hc too low (highest: 800 G) Little potential for applications Discouraged!!!