Download Thermodynamic Processes

Thermodynamic Processes
Chapter 11-2
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First Law of Thermodynamics
Imagine a roller coaster that operates
without friction.
The car is raised against the force of
gravity by work.
Once the car is freely moving, it will
have a certain kinetic energy (KE) and
a certain potential energy (PE).
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First Law of Thermodynamics
The coaster will move slower at the
top of the rise and faster at the low
points in the track. However the
mechanical energy, KE + PE, remains
constant throughout the rides
duration.
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First Law of Thermodynamics
If friction is taken into account,
mechanical energy is not conserved.
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First Law of Thermodynamics
A steady decrease in the car’s total
mechanical energy occurs because of
work being done against the friction
between the car’s axles and its
bearings and between the car’s wheels
and the coaster track.
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First Law of Thermodynamics
Mechanical energy is transferred to
the atoms and molecules throughout
(a system) the entire roller coaster.
Thus, the roller coaster’s internal
energy increases by an amount equal
to the decrease in the mechanical
energy.
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First Law of Thermodynamics
Most of this energy is then gradually
dissipated to the air surrounding the
roller coaster as heat and sound.
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First Law of Thermodynamics
If the internal energy for the roller
coaster and the energy dissipated total
energy will be constant.
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First Law of Thermodynamics
The principle of energy conservation
that takes into account a system’s
internal energy as well as work and
heat is called the First law of
thermodynamics.
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First Law of Thermodynamics
In all the thermodynamic processes
described so far, energy has been
conserved. To describe the overall
change in the system’s internal energy,
one must account for the transfer of
energy to or from the system as heat
and work.
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First Law of Thermodynamics
The total change in the internal energy
is the difference between the final
internal energy value (Uf) and the
initial internal energy value (Ui).
∆U = Uf - Ui
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First Law of Thermodynamics
Energy conservation requires that the
total change in internal energy from its
initial to its final equilibrium conditions
be equal to the transfer of energy as
both heat and work.
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First Law of Thermodynamics
∆U =Q – W
U - internal energy
Q – heat energy
W – work energy
SI Units : Joules
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First Law of Thermodynamics
According to the first law of
thermodynamics, a system’s internal
energy can be changed by transferring
energy as either work, heat, or a
combination of the two.
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Cyclic Processes
If a system’s properties at the end of
the process are identical to the
system’s properties before the process
took place, the final and initial values of
internal energy are the same and the
change in the internal energy is zero.
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Cyclic Processes
∆Unet = 0
Qnet = Wnet
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Cyclic Processes
This process resembles an isothermal
process in that all energy is transferred
as work and heat. But now the process
is repeated without no net change in
the system’s internal energy. This is a
cyclic process.
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Cyclic Processes
In a refrigerator, the net amount of
work done during each complete cycle
must equal the net amount of energy
transferred as heat to and from the
refrigerant.
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Cyclic Processes
Energy is transferred as heat from the
cold interior of the refrigerator to the
even colder evaporating refrigerant
(Qcold or Qc).
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Cyclic Processes
Energy is also transferred as heat from
the hot condensing refrigerant to the
relatively colder air outside the
refrigerator (Qhot or Qh)
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Cyclic Processes
Therefore, the difference between Qh
and Qc equals the net energy
transferred as heat-thus the net work
done-during one cycle of the
refrigeration process.
Wnet = Qh – Qc
Where Qh > Qc
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Cyclic Processes
The colder you want the inside of a
refrigerator to be the greater the net
energy transferred as heat (Qh-Qc)
must be. The net energy transferred as
heat can be increased only if the
refrigerator does more work.
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Cyclic Processes
A refrigerator uses mechanical work
to create a difference in temperature
and thus transfers energy as heat.
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Cyclic Processes
A heat engine is a device that does the
opposite.
It uses heat to do mechanical work.
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Cyclic Processes
Instead of using the difference in
potential energy to do work, heat
engines do work by transferring
energy from a high temperature
substance to a lower temperature
substance.
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Cyclic Processes
For each complete cycle of the heat
engine, the net work done will equal
the difference between the energy
transferred as heat from high
temperature substance to the engine
(Qh) and the energy transferred as
heat from the engine to a lower
temperature substance.
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Cyclic Processes
The larger the difference between the
amount of energy transferred as heat
into the engine and out of the engine,
the more work the engine can do.
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Cyclic Processes
The internal combustion engine found
in most vehicles is an example of a
heat engine.
Internal combustion engines burn fuel
within a closed chamber.
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Cyclic Processes
The potential energy of the chemical
bonds in the reactant gases is
converted to kinetic energy of the
particle products of the reaction.
These gaseous products push against a
piston and thus do work on the
environment.
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Cyclic Processes
Although the basic operation of any
internal combustion engine resembles
that of an ideal heat engine certain
steps do not fit in the model.
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Cyclic Processes
When gas is taken in or removed from
the cylinder, matter enters and leaves
the system so that the matter in the
system is not isolated.
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Cyclic Processes
No heat engine operates perfectly.
Only part of the available internal
energy leaves the engine as work done
on the environment; most of the
energy is removed as heat.
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