speech.ppt 547KB Jun 23 2011 10:31:42 AM
Constructing and Studying a
Levitating Frictionless
Bearing
Ruth Toner
Senior Project Speech
6-10-03
-First discovered 1911 by
Heike Kamerlingh Onnes.
Superconductors:
The Basics
-Above critical temperature,
superconductor behaves like
normal material, with high
resistivity
- Below Tc, has zero
resistance
- If current is established in
loop of superconducting
material, will continue
indefinitely.
- Other conditions:
superconductor only works
when current density and
magnetic field are below
critical values Jc and Hc.
Background: www.superconductors.org
Type I Superconductors -- The Meissner Effect
-Zero resistivity of superconductor
means that material can act as “perfect
dimagnet”
-When superconductor is exposed to
magnetic flux, field induces current
on surface
-Induced current creates opposing
magnetic field which leads to force of
repulsion between magnet and
superconductor
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
-In case of Type I superconductor,
magnetic field is completely expelled
from superconductor
- force strong enough to cause
levitation
http://www.imagesco.com/articles/supercond/06.html
Type II Superconductors – Flux Pinning
-Type II Superconductor: contains small impurities which allows some
magnetic flux to pass through filaments in the material
-flux lines become “pinned” in place: any attempt to move the superconductor
up or down will create a restoring force
-combination of Meissner Effect repulsive force and flux pinning restorative
force causes levitation
-Advantages:
-Higher critical temperatures
- horizontal position of
superconductor also fixed
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
Materials
YBCO Superconductor:
NdFeB magnet:
Critical Temperature 90°K (-183°C)
Surface strength = 1.6 Tesla
(32000x the earth’s magnetic field
Creating the Mount
AutoCAD Drawing:
[CAD drawing]
Materials – base: aluminum
handle: G10
A Levitating Frictionless Bearing: Photos
Before:
The magnet rests on supports on top of
the superconductor, not levitating.
During cooling:
The mount is lowered into liquid
nitrogen and allowed to cool to
77°K, under YBCO’s critical
temperature. The YBCO becomes
superconductive.
A Levitating Frictionless Bearing: Photos
The mount is removed from the liquid nitrogen, and the supports are
knocked out. The magnet floats in midair, and can only be moved by
applying strong pressure.
Studying the Bearing – Part #1:
Finding the Spring Constant and Resonant Frequency
-The restoring force F applied by objects like the bearing can be described by Hooke’s law: F=-kx,
where k is some constant
k
-The frequency of vibration f is described by
f
m
2
-Increments of weight were placed on the magnet at three different initial heights, and the resulting
displacement was measured; these data points were graphed, and the regression line slope was used
to calculate constant k, and then frequency f:
At 4 mm:
k=1.7547
f=16.88 s-1
At 9 mm:
k=1.0761
f=13.22 s-1
At 16 mm:
k=.8057
f=11.44 s-1
Studying the Bearing – Part #2:
Finding the Spin Down Time Constant
-Because the bearing doesn’t make surface contact with anything, it is presumed nearly frictionless
-Some drag forces do exist, however (e.g., air drag), so that the rotational frequency f behaves
t
according to f f 0 e , where τ is the time constant for rotational decay, the time it takes for f to
decrease by 63%.
-The time constant was calculated by monitoring the number of rotations in a 10 second period every
minute; a regression time was plotted to achieve a value for τ. This was tested at four separate heights.
Example: rotational frequency decay at 12.70 mm
Initial elevation
(mm)
Time constant
(seconds)
3.00
246.81
6.54
814.11
9.67
1162.79
12.70
1602.05
Levitating Frictionless
Bearing
Ruth Toner
Senior Project Speech
6-10-03
-First discovered 1911 by
Heike Kamerlingh Onnes.
Superconductors:
The Basics
-Above critical temperature,
superconductor behaves like
normal material, with high
resistivity
- Below Tc, has zero
resistance
- If current is established in
loop of superconducting
material, will continue
indefinitely.
- Other conditions:
superconductor only works
when current density and
magnetic field are below
critical values Jc and Hc.
Background: www.superconductors.org
Type I Superconductors -- The Meissner Effect
-Zero resistivity of superconductor
means that material can act as “perfect
dimagnet”
-When superconductor is exposed to
magnetic flux, field induces current
on surface
-Induced current creates opposing
magnetic field which leads to force of
repulsion between magnet and
superconductor
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
-In case of Type I superconductor,
magnetic field is completely expelled
from superconductor
- force strong enough to cause
levitation
http://www.imagesco.com/articles/supercond/06.html
Type II Superconductors – Flux Pinning
-Type II Superconductor: contains small impurities which allows some
magnetic flux to pass through filaments in the material
-flux lines become “pinned” in place: any attempt to move the superconductor
up or down will create a restoring force
-combination of Meissner Effect repulsive force and flux pinning restorative
force causes levitation
-Advantages:
-Higher critical temperatures
- horizontal position of
superconductor also fixed
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html
Materials
YBCO Superconductor:
NdFeB magnet:
Critical Temperature 90°K (-183°C)
Surface strength = 1.6 Tesla
(32000x the earth’s magnetic field
Creating the Mount
AutoCAD Drawing:
[CAD drawing]
Materials – base: aluminum
handle: G10
A Levitating Frictionless Bearing: Photos
Before:
The magnet rests on supports on top of
the superconductor, not levitating.
During cooling:
The mount is lowered into liquid
nitrogen and allowed to cool to
77°K, under YBCO’s critical
temperature. The YBCO becomes
superconductive.
A Levitating Frictionless Bearing: Photos
The mount is removed from the liquid nitrogen, and the supports are
knocked out. The magnet floats in midair, and can only be moved by
applying strong pressure.
Studying the Bearing – Part #1:
Finding the Spring Constant and Resonant Frequency
-The restoring force F applied by objects like the bearing can be described by Hooke’s law: F=-kx,
where k is some constant
k
-The frequency of vibration f is described by
f
m
2
-Increments of weight were placed on the magnet at three different initial heights, and the resulting
displacement was measured; these data points were graphed, and the regression line slope was used
to calculate constant k, and then frequency f:
At 4 mm:
k=1.7547
f=16.88 s-1
At 9 mm:
k=1.0761
f=13.22 s-1
At 16 mm:
k=.8057
f=11.44 s-1
Studying the Bearing – Part #2:
Finding the Spin Down Time Constant
-Because the bearing doesn’t make surface contact with anything, it is presumed nearly frictionless
-Some drag forces do exist, however (e.g., air drag), so that the rotational frequency f behaves
t
according to f f 0 e , where τ is the time constant for rotational decay, the time it takes for f to
decrease by 63%.
-The time constant was calculated by monitoring the number of rotations in a 10 second period every
minute; a regression time was plotted to achieve a value for τ. This was tested at four separate heights.
Example: rotational frequency decay at 12.70 mm
Initial elevation
(mm)
Time constant
(seconds)
3.00
246.81
6.54
814.11
9.67
1162.79
12.70
1602.05