Superconductivity - Dropbox

winkwellmadeUrban and Civil

Nov 15, 2013 (3 years and 8 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

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:

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 ~
800 G (lead)





Element

Tc (K)

Mercury

4.153

Lead

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!!!