Transcript Thermodynamic Processes

Thermodynamic Processes
Illustrate how the 1st law of thermodynamics is a
statement of energy conservation
Calculate heat, work, and the change in internal energy
by applying the 1st law of thermo
Apply 1st law of thermo to cyclic processes
How does the potential energy vary as
the car rolls up and down the track?
The PE depends only on the car’s elevation.
How would a 4th bar representing ME
relate to the U bars?
The new ME bar should equal KE + PE in each case.
Thus, it will get shorter from (b) to (e) as the U bar
gets taller by the same amount.
The First Law of Thermodynamics
• Closed system: ΔU=Q-W
• U Internal energy: all the energy of the
molecules
• Q : energy transferred to/from system as heat
+ for heat added; - for heat lost
• W: energy transferred to/from system as work
+ work done by the system; - work on system
Signs for heat and work in a system are
summarized as follows…
1st law of thermodynamics
mathematically
ΔU = Uf- Ui
• 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.
• According to the 1st law of thermo, a systems internal energy
can be changed by transferring energy as either work, heat, or
a combination of the two.
KNOW
THIS
CHART
FOR
TEST
Thermodynamic Processes
• Isothermal: delta T=0; delta U=0, Q=W
• Adiabatic: Q=0; delta U=-W
ISOBARIC (constant P)
Delta P=0
W=Fd=P(ad)=P delta V
Q=delta U+W=delta U+P delta V
ISOVOLUMETRIC
delta V=0
W=0, Q=deltaU
CYCLIC PROCESSES
• A thermodynamic process in which a system returns
to the same conditions under which it started.
• Final internal energy = initial internal energy
• The change in internal energy of a system is ZERO in
a cyclic process
Unet = 0 + Qnet = Wnet
Qnet = Wnet
CYCLIC PROCESS
• A cyclic process represents an isothermal process in
that all energy is transferred as work and heat.
• Energy is transferred as heat from the cold interior of
the refrigerator to the even colder evaporating
refrigerant. (Qcold or Qc)
• Energy is also transferred as heat from the hot
condensing refrigerant to the relatively colder air
outside the refrigerator. (Qhot or Qh)
Cyclic Process
• Qnet = Qh – Qc = Wnet
• Wnet = Qh – Qc
Where Qh > Qc
The colder you want the inside of a
refrigerator to be, the greater the net
energy transferred as heat (Qh – Qc)
must be.
Cyclic Process
*A refrigerator performs work to create a temperature
difference between its closed interior and its
environment.
*Transfers energy from a body at low temperature to
one at a high temperature.
*Uses work performed by an
electric motor to compress
the refrigerant.
Cyclic Process
In each of the 4
steps of a
Refrigeration
cycle, energy is
transferred to or
from the
refrigerant either
by heat or by work.
OPEN YOUR TEXTBOOK
TO PAGE 414
Cyclic Process Frige
Refrigerator Cyclic Process
1: Electrically-run compressor does work on the Freon gas, increasing
the pressure of the gas.
High Pressure and High Temperature
2: High Pressure Freon Gas released into external heat enters exchange
coil on the outside of the refrigerator
3: Heat flows from High Temperature gas to the lower-temperature air
of the room surrounding the coil. This heat loss causes the Freon to
condense to a liquid releasing heat to outside of the refrigerator.
(space behind the frige)
4: As Freon passes through the expansion valve it expands and
evaporates
now at low pressure and low temperature
5: Evaporating Freon absorbs heat from inside the refrigerator. This
causes the refrigerator to cool as its heat is absorbed by the
evaporating Freon in the internal coils. Thus, temperature inside
refrigerator is reduced
6: When all Freon changes to gas, the CYLCE REPEATS
Cyclic Process
• The fact that the refrigerant’s final internal
energy is the same as its initial internal energy
is very important!
– (consistent with 1st law of thermodynamics)
• This is true for all machines that use heat to
do work or that do work to create
temperature differences
Heat Engine
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Efficiency (x100%)
e=W/QH
=(QH-QL)/QH
=1- QL/QH
e<1