Chapter 20 Heat and the first law of thermodynamics

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Chapter 20 Heat and the first law of
thermodynamics September 5,8 Specific heat 20.1
Heat and internal energy Internal energy All the
energy of a system that is associated with its
microscopic components. The system is viewed in
the center-of-mass frame. Internal energy
includes 1) kinetic energy of the translational,
vibrational and rotational motion of the
molecules, 2) potential energy within the
molecules, and 3) potential energy between the
molecules. Internal energy does not include the
bulk kinetic energy of the system. Heat
Transfer of energy across the boundary of the
system due to temperature difference. An analogy
(heat, internal energy) vs. (work, mechanical
energy). Change of internal energy does not
necessarily need heat transfer. Internal energy
can be changed by work. E.g., compressing gas
with a piston. Unit of heat, calorie The energy
transfer that raises 1 g of water from 14.5 ºC to
15.5 ºC.
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The mechanical equivalent of heat Joules
experiment 2mgh 4.186 (J/g ºC) DT M
1 cal 4.186 J
20.2 Specific heat and calorimetry Heat capacity
The energy needed to raise the temperature of a
sample by 1 ºC. C Q/DT Specific
heat The heat capacity per unit mass.
Measuring how thermally insensitive the substance
is to the addition of energy. The energy
required to raise the temperature of a substance
(with mass m) by DT is
The specific heats for gases measured at constant
pressure are quite different from those measured
at constant volume. Water has the highest
specific heat of common materials.
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Table 20.1
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Quiz 20.1
Calorimetry The technique of measuring specific
heat. The sample is first heated and then put
into water. The specific heat is calculated from
the equilibrium temperature. Conservation of
energy Qcold -Qhot
?Sign convention
Example 20.2
Suggestion Do not plug in numbers first. Try to
solve for the unknown first and you will have
more information.
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Read Ch20 1-2 Homework Ch20 (1-12)
2,7,11 Due September 12
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September 10 Latent heat 20.3 Latent heat Phase
change Change of the physical characteristics of
a substance. Such as from solid to liquid and
from liquid to gas, as well as sublimation. In
the course of phase changes the transfer of
energy results in a change in internal energy,
but does not cause a change in temperature. Latent
heat The amount of energy required to change
the phase of a sample with unit mass L Q/m. L
depends on the substance as well as the actual
phase change.
The energy transfer required to change the phase
Sign convention Positive sign is used when
energy is transferred into the system, negative
sign is used when energy is transferred out of
the system. Latent heat of fusion (melting)
(numerically) Latent heat of solidification
(freezing) Latent heat of vaporization (boiling)
( numerically) Latent heat of condensation
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Table 20.2
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Example Changing 1 gram ice into steam
Molecular view of phase changes 1) Phase changes
can be described in terms of the rearrangement of
molecules. 2) Liquid to gas work must be done
to separate the molecules. The latent heat of
vaporization is the energy per unit mass needed
to do this separation. 3) The latent heat of
vaporization is greater than the latent heat of
fusion.
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Example 20.4 For steam
For water glass
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Read Ch20 3 Homework Ch20 (13-20)
13,17,19 Due September 19
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Phase diagram of water
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September 12 Thermodynamic processes 20.4 Work
and heat in thermodynamic processes State
variable Describing the characteristic of a
system in thermal equilibrium. Pressure, volume,
temperature, internal energy. Transfer variable
Describing the characteristic of a process in
which energy is transferred between a system and
its environment. Heat, work. Work done on a gas
Quasi-static process A process that is slow
enough to allow the system to remain essentially
in thermal equilibrium at all times.
Total work done on a gas
Note the negative sign when dVlt0, work done on
the gas is positive.
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P-V diagrams The work done on a gas is the
negative of the area under the curve on a P-V
diagram calculated between the initial and final
states. The work done on a gas from an initial
state to a final state depends on the path
between these states.
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The energy transfer by heat also depends on the
initial, final, and intermediate states of the
system. Energy reservoir A source of energy that
is considered to be so great that a finite
transfer of energy does not change its
temperature. Path A Gas does work energy
transferred to gas by heat. Path B (adiabatic
free expansion) No energy transfer. No work done.
Summary of energy transfer 1) Energy transfers
by heat, like the work done, depend on the
initial, final, and intermediate states of the
system. 2) Both work and heat depend on the path
taken. 3) Neither work nor heat can be determined
solely by the end points of a thermodynamic
process.
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• 20.5 The first law of thermodynamics
• A special case of the law of conservation of
  energy. It takes into account changes in internal
  energy and energy transfers by heat and work.
• Both work and heat can change the internal energy
  of a system.
• From an initial state to a final state, Q and W
  both depends on the path. However, Q W (the
  change of internal energy) is independent of the
  path.

The first law of thermodynamics states that
For an infinitesimal change,

• The internal energy of an isolated system (Q W
  0) remains constant.
• In a cyclic process, DEint 0, Q -W.
• The net work done on the system in a cyclic
  process equals the area enclosed by the
• path on a P-V diagram.

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Read Ch20 4-5 Homework Ch20 (21-29)
21,24,26 Due September 19
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• September 15 The first law of thermodynamics -
  applications
• 20.6 Some applications of the first law of
  thermodynamics
• Adiabatic process A process during which no
  energy is transferred by heat. Q 0.
• Achieved by Thermal isolation, rapid processes.
• ?DEint W.
• Adiabatic free expansion Adiabatic expansion to
  vacuum. A unique process. No work is done. Q 0,
  W 0.
• ? DEint 0. The initial and final internal
  energies are equal.
• The internal energy of an idea gas depends only
  on its temperature (shown later).
• For an ideal gas the temperature does not change
  during an adiabatic free expansion.
• Isobaric process A process that occurs at a
  constant pressure. P constant.
• ?Work done on the gas W -P (Vf - Vi).

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Isovolumetric process A process that occurs at a
constant volume. V constant. ?Work done on the
gas W 0. DEint Q.
Isothermal process A process that occurs at a
constant temperature. T constant. ?For an ideal
gas A hyperbolic curve on P-V diagram
(isotherm), DEint 0, Q -W.
Work done in an isothermal process (Ideal gas
and quasi-static processes)
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Quiz 20.4 (Question Why curve B is below/above
curve C ?)
Example 20.5
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Read Ch20 6 Homework Ch20 (30-36)
30,34,35 Due September 26
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September 17 Energy transfer 20.7 Energy
transfer mechanisms Thermal conduction The
process of energy transfer by heat. Mechanism
The exchange of kinetic energy between colliding
molecules or electrons. Energy transfer through
a conducting slab The rate (power) of energy
transfer by heat
The law of thermal conduction k thermal
conductivity dT/dx temperature gradient
Thermal conductors metals Thermal insulators
nonmetals (exceptions), gases
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• Steady state thermal conduction
• The temperature at each point of the medium T(x)
  is constant in time.
• ? Energy transfers at the same rate at all
  points.
• ? For a rod, within the same medium
• ? The rate of energy transfer

Example 20.8 Energy transfer through two slabs
in series
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Analogy to a R-circuit
For a compound slab containing several materials
In engineering, L/k for a substance is called the
R value.
Two other energy transfer methods convection and
electromagnetic radiation. Convection Energy
transfer by the movement of a warm substance
(matter transfer). Radiation Energy transfer by
electromagnetic waves. Stefans law The rate
at which an object radiates energy P energy
rate of radiation s constant, 5.669610-8
W/m2K4 A surface area e emissivity, between 0
and 1. T surface temperature
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Read Ch20 7 Homework Ch20 (37-)
38,39,45,62 Due September 26