PHY 113 C Fall 2013 Lecture 20 1 PHY 113 C General Physics I 11 AM 1215 P M TR Olin 101 Plan for Lecture 20 Chapter 19 The notion of temperature Review of fluid physics Temperature ID: 134588
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Slide1
11/07/2013
PHY 113 C Fall 2013 -- Lecture 20
1
PHY 113 C General Physics I
11 AM - 12:15
P
M TR Olin 101
Plan for Lecture 20:
Chapter
19: The
notion of temperature
Review of fluid physics
Temperature
equilibrium
Temperature scales
Temperature in ideal gasesSlide2
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The physics of fluids.
Fluids include liquids (usually “incompressible) and gases (highly “compressible”).
Fluids obey Newton’s equations of motion
, but because they move within their containers, the application of Newton’s laws to fluids introduces some new forms.
Pressure: P=force/area
1 (N/m
2
) = 1
PascalDensity: r =mass/volume 1 kg/m3 = 0.001 gm/ml
Review: Slide4
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Review of equations describing static fluids in terms of pressure P and density
r:
Note that for compressible fluids (such as air), the relationship between pressure and density is more complicated.
Buoyant force for fluid acting on a
solid – net force due to
volume
V
displaced
being displaced in fluid:
F
B=rfluidV
displaced
gSlide5
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Bernoulli’s equation:Slide6
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Webassign
questions on fluids (Assignment #17)
A large man sits on a four-legged chair with his feet off the floor. The combined mass of the man and chair is 95.0 kg.
If the chair legs are circular and have a radius of 0.500 cm at the bottom, what pressure does each leg exert on the floor?
mg
mg/4
P=F/A=(mg/4)/ASlide7
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Webassign
questions on fluids (Assignment #17)
A swimming pool has dimensions 32.0 m
✕
7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water. (a) What is the force exerted by the water on the bottom?
(b) What is the force exerted by the water on each end? (The ends are 7.0 m.)
(c) What is the force exerted by the water on each side? (The sides are 32.0 m.)Slide8
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Webassign
questions on fluids (Assignment #17)
A swimming pool has dimensions 32.0 m
✕
7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water. (a) What is the force exerted by the water on the bottom?
h=2.5m
F
bottom
=
PA=rghASlide9
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Webassign
questions on fluids (Assignment #17)
A swimming pool has dimensions 32.0 m
✕
7.0 m and a flat bottom. The pool is filled to a depth of 2.50 m with fresh water. (
b) What is the force exerted by the water on each end? (The ends are 7.0 m
.)
h=2.5m
w=7.0mSlide10
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Webassign
questions on fluids (Assignment #17
)Slide11
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D
y
D
zSlide12
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Webassign
questions on fluids (Assignment #17
)
The
gravitational force exerted on a solid object is 5.30 N. When the object is suspended from a spring scale and submerged in water, the scale reads 3.50 N (figure). Find the density of the object.Slide14
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Webassign
questions on fluids (Assignment #17
)
A light balloon is filled with 373 m
3
of helium at atmospheric pressure. (a) At 0°C, the balloon can lift a payload of what mass
? Note:
r
air
= 2.9 kg/m3 : rHe = 0.179 kg/m3 Slide15
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Webassign
questions on fluids (Assignment #17
)
A hypodermic syringe contains a medicine with the density of water (see figure below). The barrel of the syringe has a cross-sectional area
A
= 2.40 10
-5
m
2
, and the needle has a cross-sectional area a = 1.00 10-8 m2. In the absence of a force on the plunger, the pressure everywhere is 1.00 atm. A force of magnitude 2.65 N acts on the plunger, making medicine squirt horizontally from the needle. Determine the speed of the medicine as it leaves the needle's tip. Slide16
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Dictionary definition:
temperature
– a measure of the the warmth or coldness of an object or substance with reference to some standard value. The temperature of two systems is the same when the systems are in thermal equilibrium.
“Zeroth” law of thermodynamics:
If objects A and B are separately in thermal equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other.
Temperature
T
1
T
2
T
3
Not equilibrium:
Equilibrium:Slide17
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Constant temperature “bath”
T
T
At equilibrium:Slide18
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Temperature scales
T
F
=9/5 T
C
+ 32
Kelvin scale:
T = T
C
+ 273.15
o
T
0Slide19
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iclicker
question:
Suppose you find yourself in a hotel in Europe or Canada. Which Celsius temperature would you set the thermostat for comfort?
-20
o
C
+20
o
C
+40oC+60oC+80oCSlide20
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There is a lowest temperature:
T
0
= -273.15
o
C = 0 K
Kelvin (“absolute temperature”) scale
T
C
= -273.15 + TK Example – Room temperature = 68o F = 20o C = 293.15 KSlide21
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Effects of temperature on matter
Solids and liquids
L
i
(equilibrium bond length at T
i
)
Model of a solid composed of atoms and bonds
D
L
Thermal exansion:
D
L =
a
L
i
D
TSlide23
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Typical expansion coefficients at T
C
= 20
o
C:
Linear expansion:
D
L =
a
Li DTSteel: a = 11 x 10-6/ oCConcrete: a = 12 x 10-6/ oCVolume expansion: V=L3 DV = 3a Vi DT = b Vi DT Alcohol: b = 1.12 x 10
-4/ oC Air: b = 3.41 x 10
-3
/
o
CSlide24
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iclicker
question
On the last slide – we suggest that
b
=3
a
. Is this result
One of those mysteries of physics that has no explanation?
A result that we can derive?
L
L+D
L
V=L
3
V+
D
V=(L+
D
L)
3
@
V(1+3(
D
L/L))Slide25
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Brass
SteelSlide27
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Switch in thermostat
Modern thermostats use electrical circuits to detect temperatureSlide28
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Effects of temperature on materials – continued
strange case of water:Slide29
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Effects of temperature on materials – continued -- ideal gas “law” (thanks to Robert Boyle (1627-1691), Jacques Charles (1746-1823), and Gay-Lussac (1778-1850)
pressure in
Pascals
volume in m
3
# of moles
temperature in K
8.314 J/(
mol
K)
1 mole corresponds to 6.022 x 10
23
moleculesSlide30
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P
0
=12.6
atm
T
0
=27.5
o
C
n
0
P=?
T=81.0
o
C
n=n
0
/3Slide31
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Assuming that air behaves like an ideal gas, what is the density of air at T=0
o
C and P=1
atm
?Slide32
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Typical composition of air:
url:
http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html