PHYSICS 231 Lecture 25: Viscosity ch'9 and Temperature ch' 10

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PHYSICS 231Lecture 25 Viscosity (ch.9) and
Temperature (ch. 10)

• Remco Zegers
• Walk-in hour Thursday 1130-1330 am Helproom

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Applications of Bernoullis law the golf ball
Neglecting the small change in height between
the top and bottom of the golf ball
P1½?v12 P2½?v22 P1-P2 ½?(v22- v12)
P1
P1-P2 ½?(v22- v12)0 v2v1 No pressure
difference, no lift
P2
P1
P1-P2 ½?(v2-v)2-(v1 v) 20 P2gtP1 so Upward
force the ball goes higher and thus travels
faster
P2
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Viscosity
Viscosity stickiness of a fluid One layer of
fluid feels a large resistive force when
sliding along another one or along a surface of
for example a tube.
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Viscosity
Contact surface A
moving
F?Av/d ?coefficient of viscosity unit
Ns/m2 or poise0.1 Ns/m2
fixed
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Poiseuilles Law
How fast does a fluid flow through a tube?
?R4(P1-P2)
(unit m3/s)
Rate of flow Q ?v/?t
8?L
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Example
Flow rate Q0.5 m3/s Tube length 3 m ?1500E-03
Ns/m2
PP106 Pa
P105 Pa
What should the radius of the tube be?
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Chapter 10 Temperature
Potential Energy
Kinetic energy T
0
-Emin
R
The curve depends on the material, e.g. Emin
is different for water and iron
R
2 atom/molecules
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Solid (low T)
Kinetic energy T
Potential Energy
Rmin
0
R
-Emin
The temperature (and thus kinetic energy) is so
small that the atoms/molecules can only oscillate
around a fixed position Rmin
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Liquid (medium T)
Potential Energy
Kinetic energy T
Rmin
0
R
-Emin
On average, the atoms/molecules like to stick
together but sometimes escape and can travel far.
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Gas (high T)
Kinetic energy T
Potential Energy
Rmin
0
-Emin
R
The kinetic energy is much larger than Emin and
the atoms/molecules move around randomly.
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What happens if the temperature of a substance is
increased?
RminRave(T0)
Kinetic energy T
0
R
T0 Average distance between
atoms/molecules Rmin
-Emin
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Temperature scales
Conversions TcelsiusTkelvin-273.5 Tfahrenheit9/5
Tcelcius32
We will use Tkelvin. If Tkelvin0, the
atoms/molecules have no kinetic energy and
every substance is a solid it is called
the Absolute zero-point.
Celsius
Fahrenheit
Kelvin
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Thermal expansion
?L
length
?L?Lo?T
surface
?A?Ao?T ?2?
?V?Vo?T ?3 ?
volume
L0
? coefficient of linear expansion different
for each material
Some examples ?24E-06 1/K Aluminum ?1.2E-04
1/K Alcohol
TT0?T
TT0
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Thermal equilibrium
Thermal contact
High temperature High kinetic energy Particles
move fast
Low temperature Low kinetic energy Particles move
slowly
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Zeroth law of thermodynamics
If objects A and B are both in thermal
equilibrium with an object C, than A and B are
also in thermal equilibrium.
There is no transfer of energy between A, B and C
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Thermal expansion an example
In the early morning (T30oF272.4K) a person is
asked to measure the length of a football field
with an aluminum measure and finds 109.600 m.
Another person does the same in the afternoon
(T60oF289.1K) using the same ruler and finds
109.566 m. What is the coefficient of linear
expansion of the ruler?
?L?Lo?T so ? ?L/(L0?T) ?T16.7K L0109.60
?L109.644-109.6000.044 So ?24E-06 1/K
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A heated ring

• A metal ring is heated. What is true
• The inside and outside radii become larger
• The inside radius becomes larger, the outside
  radius becomes smaller
• The inside radius becomes smaller, the outside
  radius becomes larger
• The inside and outside radii become smaller

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Demo bimetallic strips
?top
?bottom
Application contact in a refrigerator
?toplt?bottom if the temperature increases, The
strip curls upward, makes contact and switches on
the cooling.
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Demos
Bimetallic coil Expanding and contracting rod
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Water a special case
Coef. of expansion is negative If T drops the
volume becomes larger
Coef. Of expansion is positive if T drops the
volume becomes smaller
Ice is formed (it floats on water)
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Ice
? (g/cm3)
liquid
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Phase transformation
0.917
ice
Ice takes a larger volume than water!