Transcript Thermodynamics

Thermodynamics
CHAPTER 15
15.1 Thermodynamic Systems and Their Surroundings
 Thermodynamics is the branch of physics that is built
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upon the fundamental laws that heat and work obey.
The work done on an object (or system) is equal to the
change in kinetic energy
A system is the collection of objects being studied or
analyzed.
Diathermal walls permit heat to flow through the
surroundings of a system.
Adiabatic walls are perfectly insulated and would
therefore not permit heat to flow between the systema nd
the surroundings.
15.2 The Zeroth Law of Thermodynamics
 Two systems individually in thermal equilibrium
with a third system are in thermal equilibrium with
one another.
 Thermal equilibrium is established when no net flow
of heat exists between systems.
 Temperature is the indicator of thermal equilibrium.
15.3 The First Law of Thermodynamics
 Another way of stating the Law of Conservation of
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Energy.
When a system gains heat, the internal energy of the
system increases.
Q is positive when a system gains heat and negative
when a system loses heat.
Internal energy of a system can decrease if the
system does work on its surroundings.
Work is positive when it is done by the system and
negative when it is done on the system.
1st Law Summarized
 The internal energy of a system changes from an
initial value Ui to a final value Uf due to heat Q and
work W.
 Q is positive when the system gains heat and
negative when it loses heat. W is positive when work
is done by the system and negative when work is don
on the system.
Read
Example 1
and 2 in
text!
U  U f  U i  Q  W
U, Q, W ALL Types of Energy (JOULES)
U
• Internal energy
• Depends only on the state of a system
Q
• Heat energy
• Dealt with in Chapters 12-14
W
• Work
• A change in kinetic energy due to
forces acting over a distance.
15.4 Thermal Processes
 Quasi-Static – the process whereby heat and work
interact with surroundings. The process occurs
slowly enough that a uniform pressure and
temperature exist throughout all regions of the
system.
 Isobaric process occurs at constant pressure.
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Positive value for work done by a system when it expands
isobarically.
Negative value for work when work is done one the system to
compress it.
W  PV  P(V f  Vi )
More Thermal Processes
 Isochoric process – occurs at constant volume
 Heat will only serve to change the internal energy.
 Isothermal process – occurs at constant temperature
 Adiabatic process – occurs without the transfer of
heat
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Internal energy will decrease by exactly the amount of work
done when work is done by a system.
Internal energy will increase when work is done on a system.
15.5 Thermal Process Using an Ideal Gas
 Isothermic Expansion or Compression occurs when a
contained gas expands or contracts due to a slowly
increasing or decreasing force.
W  nRT ln(

Vf
Vi
)

PiVi  Pf V f
15.6 Specific Heat Capacities
 Instead of using mass to solve for specific heat,
number of moles is often a helpful method.
 Letter C refers to the molar specific heat capacity.
 Use Kelvin as the unit for temperature.
 Cp and Cv must be used depending on constant
pressure or volume conditions.
Q  CnT
The difference in C values exists
because work is done when the gas
expands in response to the addition of
heat under constant pressure. No
work is done under constant volume.
5.7 The 2nd Law of Thermodynamics
 Heat flows spontaneously from a substance of higher
temperature to a substance at a lower temperature
and does not flow spontaneously in the reverse
direction.
15.8 Heat Engines
 Any device that uses heat to perform work.
 Three features
 Q is supplied to the engine at a high input temperature from a
hot reservoir.
 Part of the input Q us used to do work by the working
substance of the engine, which is the material within the
engine actually doing the work.
 The remaining input Q is rejects to a place called a cold
reservoir, which has a temperature lower than the input
temperature.
 No negative values will exist for Q for W
Efficiency
 The ratio of the magnitude of the work done by the
engine to the magnitude of the input heat.
 Often expressed as a %
 In reality, can never by 100%
e  1
Qc
QH
Carnot’s Principle and the Carnot Engine
 An alternative statement for the 2nd Law of
Thermodynamics
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No irreversible engine operating between two reservoirs at
constant temperatures can have a greater efficiency than a
reversible engine operating between the same temperatures.
Furthermore, all reversible engines operating between the
same temperatures have the same efficiency.
 Reversible process: both the system and its
environment can be returned to exactly the states
they were in before the process occurred.
Carnot Engine
 A reversible engine in which all input
heat originates from a hot reservoir at
a single temperature and all rejected
heat goes into a cold reservoir at a
single temperature.
TC
e  1
TH
15.10 Refrigerators, Air Conditioners, and Heat
Pumps
 Fridge – takes heat from inside and deposits outside
 Air Conditioner – room is cold reservoir and
outdoors is hot
 Heat Pump – makes heat from cold outdoors into a
warm house
These devices
MAKE energy flow
against the natural
tendency of hot to
cold!
15.11 Entropy
 A function of the state or condition of a system
 Can be defined as the partial loss of efficiency in a machine
due to the inability of some energy to do work.
 Reversible processes do not alter the total entropy of the
universe.
 Any irreversible process increases the entropy of the
universe.
 In terms of the 2nd Law : the total entropy of the universe
does not change when a reversible process occurs and
increases when an irreversible process occurs.
Order and Disorder
 Heat flow into a system increases the disorder of a
system according to the equation
Systems
naturally
move toward
a more
disordered
state
Q
S   
 T R
15.12 The 3rd Law of Thermodynamics
 It is NOT possible to lower the temperature of any
system to absolute zero (T = 0 Kelvin) in a finite
number of steps.