# Superconductivity - Dropbox

Urban and Civil

Nov 15, 2013 (4 years and 5 months ago)

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Superconductivity

Dr. Viswanath Tanikella

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.

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 (1853
-
1926):

Resistance drops to zero

Temperature Conversion

Kelvin (K):

K = C + 273.15

K = 5/9 F + 255.37

Fahrenhei
t

Celsius

Kelvin

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:

r
=
r
0

at T=0

Superconductors:

r
=
0

at
T<Tc

(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.

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.

=

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
superconductor

V = I R

Demo

0

0

Superconducting State

Superconducting
state:

T<Tc

H<Hc

Hc = critical magnetic field

Normal
State

Superconducting

State

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 ~

Element

Tc (K)

Mercury

4.153

7.193

Aluminum

1.196

Tin

3.722

Zinc

0.85

Titanium

0.39

Phosphoru
s

14~22
(pressure)

The “But”. . .

Tc too low (highest: 22K)

Hc too low (highest: 800 G)

Little potential for applications

Discouraged!!!