phy 231 1 physics 231 lecture 25: viscosity (ch.9) and temperature (ch. 10) remco zegers walk-in...
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PHY 2311
PHYSICS 231Lecture 25: Viscosity (ch.9) and
Temperature (ch. 10)
Remco ZegersWalk-in hour: Thursday 11:30-13:30 am
Helproom
PHY 2312
Applications of Bernoulli’s law: the golf ball
P1
P2
Neglecting the small change in height between thetop and bottom of the golf ball:
P1+½v12= P2+½v2
2
P1-P2= ½(v22- v1
2)
P1
P2
P1-P2= ½(v22- v1
2)=0v2=v1
No pressure difference, no lift
P1-P2= ½(v2-v)2-(v1+ v) 2=0
P2>P1 so:Upward force: the ball goes higherand thus travels faster
PHY 2313
Viscosity
Viscosity: stickiness of a fluidOne layer of fluid feels a largeresistive force when slidingalong another one or along asurface of for example a tube.
PHY 2314
ViscosityContact surface A
fixed
movingF=Av/d
=coefficient of viscosityunit: Ns/m2
or poise=0.1 Ns/m2
PHY 2315
Poiseuille’s Law
How fast does a fluid flowthrough a tube?
Rate of flow Q= v/t=R4(P1-P2)
8L(unit: m3/s)
PHY 2316
Example
PP=106 Pa P=105 Pa
Flow rate Q=0.5 m3/sTube length: 3 m=1500E-03 Ns/m2
What should the radius of the tube be?
Rate of flow Q=R4(P1-P2)
8L
R=[8QL/((P1-P2))]1/4=0.05 m
PHY 2317
Chapter 10 Temperature
R
Potential Energy
0
R
2 atom/molecules
-Emin
The curve depends onthe material, e.g. Emin isdifferent for water andiron
Kinetic energy ~ T
PHY 2318
Solid (low T)
R
Potential Energy
0
Kinetic energy ~ T
-Emin
The temperature (and thus kinetic energy)is so small that the atoms/molecules can onlyoscillate around a fixed position Rmin
Rmin
PHY 2319
Liquid (medium T)
R
Potential Energy
0
Kinetic energy ~ T
-Emin
Rmin
On average, the atoms/molecules like tostick together but sometimes escape andcan travel far.
PHY 23110
Gas (high T)
R
Potential Energy
0
Kinetic energy ~ T
-Emin
Rmin
The kinetic energy is much larger thanEmin and the atoms/molecules move aroundrandomly.
PHY 23111
What happens if the temperature of a substance is increased?
R
0
Kinetic energy ~ T
-Emin
Rmin=Rave(T=0)
T=0: Average distance between atoms/molecules: Rmin
Rave(T>0) > Rmin
T>To: The average distance between atoms/molecules is larger than Rmin:
the substance expands
PHY 23112
Temperature scales
ConversionsTcelsius=Tkelvin-273.5Tfahrenheit=9/5*Tcelcius+32
We will use Tkelvin.
If Tkelvin=0, the atoms/moleculeshave no kinetic energy and everysubstance is a solid; it is called theAbsolute zero-point.
Kelvin
Celsius Fahrenheit
PHY 23113
Thermal expansion
L=LoT
L0
L
T=T0T=T0+T
A=AoT =2
V=VoT =3
length
surface
volume
Some examples:=24E-06 1/K Aluminum=1.2E-04 1/K Alcohol
: coefficient of linear expansion different for each material
PHY 23114
Thermal equilibrium
Low temperatureLow kinetic energyParticles move slowly
High temperatureHigh kinetic energyParticles move fast
Thermal contact
Transfer of kinetic energy
Thermal equilibrium: temperature is the same everywhere
PHY 23115
Zeroth law of thermodynamics
If objects A and B are both in thermal equilibriumwith an object C, than A and B are also in thermalequilibrium.
There is no transfer of energy between A, B and C
PHY 23116
Thermal expansion: an example
In the early morning (T=30oF=272.4K) a person is askedto measure the length of a football field with an aluminummeasure and finds 109.600 m. Another person does the same in the afternoon (T=60oF=289.1K) usingthe same ruler and finds 109.566 m. What is the coefficientof linear expansion of the ruler?
L=LoT so = L/(L0T) T=16.7K L0=109.60 L=109.644-109.600=0.044So: =24E-06 1/K
PHY 23117
A heated ring
A metal ring is heated. What is true:a) The inside and outside radii become largerb) The inside radius becomes larger, the outside
radius becomes smallerc) The inside radius becomes smaller, the outside
radius becomes largerd) The inside and outside radii become smaller
PHY 23118
Demo: bimetallic strips
Application: contact in a refrigerator
top
bottom
top<bottom if the temperature increases,
The strip curls upward, makes contact and switcheson the cooling.
PHY 23120
Water: a special case
Coef. of expansion isnegative: If T dropsthe volume becomeslarger
Coef. Of expansion ispositive: if T drops the volume becomes smaller
Ice is formed (it floats on water)