Announcements

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• Announcements
• Schedule
• Chapter 19 macroscopic view of heat (today)
• Chapter 20 microscopic view of heat (Tuesday
  11/18)
• Review Chapters 15-20 (Thursday 11/20)
• Exam 3 (Tuesday 11/25)
• Physics colloquium today Professor Gary Pielak
  from UNC will speak about Protein Biophysics in
  Cells.
• Todays lecture Chapter 19
• Temperature
• Heat
• The first law of thermodynamics

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Temperature, heat thermodynamics
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.
Equilibrium
Not equilibrium
T1
T2
T3
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Temperature scales TF9/5 TC 32
Kelvin scale T TC 273.15o T ? 0
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There is a lowest temperature T0 -273.15o
C 0 K Kelvin (absolute temperature) scale
TC -273.15 TK Example
Room temperature 68o F 20o C 293.15 K
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Effects of temperature on matter Solids and
liquids
Model of a solid composed of atoms and bonds
Li (equilibrium bond length at Ti)
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Typical expansion coefficients at TC 20o
C Linear expansion DL a Li DT Steel
a 11 x 10-6/ oC Concrete a 12 x
10-6/ oC Volume expansion VL3 ? DV 3a
Vi DT b Vi DT Alcohol b 1.12 x
10-4/ oC Air b 3.41 x 10-3/ oC
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BrassSteel
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Heat Q -- the energy that is transferred
between a system and its environment because
of a temperature difference that exists between
them.
Sign
convention
Qlt0 if T1 lt
T2

Qgt0 if T1 gt T2
T1
T2
Q
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Heat withdrawn from system
Thermal equilibrium no heat transfer
Heat added to system
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Units of heat Joule Other units
calorie 4.186 J
Kilocalorie 4186 J Calorie Heat capacity C
amount of heat which must be added to the
system to raise its temperature by 1K (or
1o C). Q
C DT Heat capacity per mass
Cmc Heat capacity per mole (for ideal
gas) CnCv

CnCp
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Some typical specific heats
Material J/(kgoC) cal/(goC)
Water (15oC) 4186 1.00
Ice (-10oC) 2220 0.53
Steam (100oC) 2010 0.48
Wood 1700 0.41
Aluminum 900 0.22
Iron 448 0.11
Gold 129 0.03
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Heat and changes in phase of materials
Example A plot of temperature versus Q added to
1g 0.001 kg of ice (initially at
T-30oC)
Q2260J
Q333J
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Some typical latent heats
Material J/kg
Ice ÞWater (0oC) 333000
Water Þ Steam (100oC) 2260000
Solid N Þ Liquid N (63 K) 25500
Liquid N Þ Gaseous N2 (77 K) 201000
Solid Al Þ Liquid Al (660oC) 397000
Liquid Al Þ Gaseous Al (2450oC) 11400000
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Peer instruction question Suppose you have a
well-insulated cup of hot coffee (m0.25kg,
T100oC). In order to get to class on time you
add 0.25 kg of ice (at 0oC). When your cup comes
to equilibrium, what will be the temperature of
the coffee. (A) 0oC (B) 10oC (C) 50oC
(D) 100oC
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Review question Suppose you have a well-insulated
cup of hot coffee (m0.25kg, T100oC). In order
to get to class on time you add 0.25 kg of ice
(at 0oC). When your cup comes to equilibrium,
what will be the temperature of the coffee.
Q m cw (Tf 100oC) m Lice m cw (Tf -
0oC) 0 2 cw Tf cw 100 - Lice
Tf cw 100/(2 cw ) - Lice /(2 cw )
Tf 50 333000/(2 4186) 10oC
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Energy in the form of work Work done by
the system due to volume change
Sign convention Wgt0 if Vf gt Vi (expansion)
Wlt0 if Vf lt Vi (contraction)
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Work done by a gas Isobaric (constant
pressure process)
Po
P (1.013 x 105) Pa
W Po(Vf - Vi)
Vi
Vf
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Work done by ideal gas Isovolumetric
(constant volume process)
Pf
W0
P (1.013 x 105) Pa
Pi
Vi
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Work done by a gas Isothermal (constant
temperature process)
For ideal gas PV nRT
Pi
P (1.013 x 105) Pa
Vi
Vf
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Work done by a gas Adiabatic (no heat flow in
the process process)
Qif 0
For ideal gas PVg PiVig
Pi
P (1.013 x 105) Pa
Vi
Vf
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Thermodynamic statement of conservation of energy
First Law of Thermodynamics
DEint Q - W
Work done by system
Heat added to system
Internal energy of system
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Internal energy EintEint(T,V,P) (for an
ideal gas, EintEint(T) can be
changed by interaction with the system
DEint Q - W
W
environment
Note Thermal equilibrium implies a uniform
temperature. In this example, the system and the
environment are presumed to be in thermal
equilibrium within themselves but not in thermal
equilibrium with eachother.
Tsys
Q
system
Tenv
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Examples process
WnetW(ABCDA)WBCWDA (Pf-Pi) (Vf-Vi)
Pf
C
B
DEint(ABCDA) DEint(AB) DEint(BC) DEint(CD)
DEint(DA)0 ?QnetQ(ABCDA)Wnet
P (1.013 x 105) Pa
D
A
Pi
Vf
Vi
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1 m3
6 m3
30 Pa
10 Pa
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• Mechanisms of heat transfer
• Thermal convection
• Heat transmitted by heated fluid flow. (Ex. --
  home heating units, stove top cooking)
• Radiation
• Heat transmitted by electromagnetic radiation.
  (Ex. sun, microwave cooking)
• Thermal conduction
• Heat transfer from one side of a material to
  another due to a temperature gradient.

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Quantitative statement of thermal conduction
Units 1 J/s 1 Watt (W)
cross-sectional area
thermal conductivity coefficient
Material k (W/(moC))
Copper 238
Glass 0.8
Water 0.6
Air 0.0234
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Examples of thermal conduction
T1
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Example T120oC, T2 0oC Single pane
glass window A1m2 , L2 0.002m, k20.8
W/moC Double pane glass window
L10.01m, k10.0234 W/moC (air)