SuperconductivitySuperconductivitySuperconductivitySuperconductivity

Dr. Viswanath TanikellaDr. Viswanath Tanikella

ConductorsOuter electrons of the atoms in conductors are loosely bound and free to move through the material FREE electronsMetals are conductors

Energy is carried by charge from power plant to appliances

© John Wiley & Sons, Inc.

Resistance• Life is tough for free

electrons• Resistance:

Repulsion from other electrons

Vibration of atomsImpurities

• 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 (1853-1926): Resistance drops to zero

Temperature Conversion• Kelvin (K):

– K = C + 273.15– K = 5/9 F + 255.37

Fahrenheit

Celsius Kelvin comments

212 100 373.15 water boils

32 0 273.15 water freezes

-300.42 -195.79 77.36 liquid nitrogen boils

-452.11 -268.95 4.2 liquid helium boils

-459.67 -273.15 0 absolute zero

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

Conductors vs. Superconductors

• Normal conductors:=0 at T=0

• Superconductors:

= at T<Tc(superconducting

state)

Tc = critical temperatureTc = 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.

http://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

=

http://www.windows.ucar.edu/spaceweather/info_mag_fields.html© John Wiley & Sons, Inc.

Which side is north pole?

• Right hand rule

N

© John Wiley & Sons, Inc.

Lenz’s Law• A conductor opposes any change

in externally applied magnetic fields.

N

S

N

S

N

S

=

Induced current

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

• Superconductor: YBa2Cu3O7 Tc ~ 90 K (5.90 mV)

• Voltmeter: measure voltage across superconductorV = I R

Demo

0 0

Superconducting State• Superconducting

state:T<TcH<Hc

Hc = critical magnetic fieldHc = critical magnetic field

Normal State

SuperconductingState

T

Hc

Tc

H

• 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

Examples of Type I• Mostly

elements• Highest Tc ~

22 K (Phosphorus)

• Highest Hc ~ 800 G (lead)

Element Tc (K)

Mercury 4.153

Lead 7.193

Aluminum 1.196

Tin 3.722

Zinc 0.85

Titanium 0.39

Phosphorus

14~22 (pressure)

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