Transcript Thermodynamics-d2

Warm-up
Complete the Free response you picked
up at the door.
Do Now (11/4/13):
What are the Laws of Thermodynamics?
Objectives
Describe thermodynamic processes on
P-V diagrams.
State and apply second law principles.
Apply the laws of thermodynamics to
the Otto cycle.
Today’s Plan
Finish efficiency discussion.
Discuss Thermodynamics.
Discuss the Otto cycle.
Homework problems due Thursday!
Quiz Friday.
Specific Heat
How much heat is required to
raise the temp of an empty 20-kg
vat made of iron (c=450J/kg°C)
from 10°C to 90°C
Calorimetry
If 200cm3 of tea (c=4186J/kg°C)
at 95°C is poured into a 150-g
glass cup (c=840J/kg°C) initially
at 25°C, what will be the final
temperature T of the mixture
when equilibrium is reached,
assuming no heat flows to the
surroundings?
First Law
U=Won + Qinto
Where U=internal energy, Q=net heat added to
system, W=net work done on the system.
Conventionally, heat added is +,
lost is negative
Work done on system is +, done by
system is negative
Statement of energy conservation
Work
Energy transfer between system and
surroundings due to organized motion
in the surroundings. (rubbing a block
of wood vigorously, stir a glass of
water, allow a gas to expand against an
external pressure.)
Heat
Energy transfer between system and
surroundings as a result of random
motion in the surroundings.
Flows spontaneously from high temp to
low temp. Work can be used to make
heat flow opposite natural flow
direction.
Efficiency
Ratio of work done by the gas to the heat
that flows into the system.
e=Wby / Qin
Develop a procedure to determine the
efficiency of a microwave and a hot plate.
Which is more efficient?
Pressure-Volume Work
P= F/A
F=P*A
Wby = F*d = PV
Graphical representation.
Area under curve = work done by gas
Example:
Gas in a cylinder is at a pressure of 8000
Pa and the piston has an area of 0.10
m2. As heat is slowly added to the gas,
the piston is pushed up a distance of 4
cm. Calculate the work done on the
surroundings by the expanding gas.
32 J
PV Diagrams
In general, the
work done in an
expansion from
some initial state
to final state is the
area under the
curve on a PV
diagram
Cyclical
Direction matters
Example:
Find the work done for each process
What is the total work done?
Example:
To find the work done by the gas, find the area under
each segment, remembering the sign convention.
Example:
For a constant volume process like B -> C, no work
is done by the gas.
The total work done for the entire cycle is the area
enclosed within the graph. In this example, the
sum of the work is
Which is the area of the enclosed triangle
Heat engines such
as automobile
engines operate
in a cyclic
manner, adding
energy in the
form of heat in
one part of the
cycle and using
that energy to do
useful work in
another part of
the cycle.
Thermodynamic Systems
Isothermal—work done by the gas
equals the heat added to the gas. U=0
Adiabatic—no heat is allowed to flow
into or out of the system. Q=0
therefore, U=Won.
Isobaric—pressure is constant
Isochoric (isovolumetric) —volume is
constant
Isothermal
Constant temperature
PV=constant
Graphical representation and isotherms
As heat is added slowly, gas expands at
constant temperature. Work is done by
the gas. U=0, so Wby = Qin.
Adiabatic
No heat is exchanged between the system and
surroundings. Q = 0.
U = Won
Internal energy decreases as gas expands.
(U=3/2 NkT, so temperature will decrease.)
Isobaric
Pressure of system remains constant.
W= PV
Isochoric
Volume remains constant
W = 0.
Second Law
Heat flows naturally from a hot object to a
cold object; heat will not flow spontaneously
from a cold object to a hot object.
http://www.entropylaw.com/
http://hyperphysics.phyastr.gsu.edu/Hbase/thermo/seclaw.html#c1
Heat Engines
Heat Engines
Carnot
Otto Cycle
Rankine
2 stroke engine
Refrigeration cycle
http://www.grc.nasa.gov/WWW/K12/airplane/thermo.html
http://www.entropylaw.com/
http://hyperphysics.phyastr.gsu.edu/Hbase/thermo/heaeng.html
Heat Engines
Mechanical energy is obtained from
thermal energy when heat is allowed to
flow from high to low temperature
regions.
QH=W+QL
e=W/QH = 1-(QL/QH)
Theoretical efficiency in Carnot (ideal)
cycle: eideal=1-TL/TH
Heat Engine
Carnot Engine
Theoretical heat engine where all
processes are considered reversible.
(very slow processes)
Actual cycles have turbulence and
friction.
Ideally, QH and QL are proportional to
TH and TL.
Theoretical efficiency : eideal=1-TL/TH
Practice:
Complete problem #5 in your textbook