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Transcript
Thermodynamics
Thermodynamics
AP Physics Notes
2010-2011
Thermodynamics
• Heat
• Internal Energy
• Work
• Heat Reservoir / Heat Sink
Thermodynamics
Thermodynamics
• Zeroth Law
• First Law
• Second Law
Thermodynamics
• Adiabatic
• Isobaric
• Isochoric (Isovolumetric)
• Isothermic
Thermodynamics
• Carnot Theorem
• Thermal Efficiency
• Carnot Efficiency
Thermodynamics
• Entropy
• Reversible
• Irreversible
Thermodynamics
Thermodynamics
• Thermodynamics - study of properties and
movement of thermal energy (Q).
Q is measured in Joules like all other forms
of energy
Thermodynamics
Thermodynamics
• Laws of Thermodynamics - each are
associated with a variable.
Zeroth law
First law
Second law
Temperature, T
Internal energy, U
Entropy, S
Thermodynamics
• Adiabatic
A thermodynamic process that occurs
without gain or loss of heat and without a
change in entropy
Look for gases that are insulated from
the environment
From Greek a “not“, dia “through”, and
batos “passable”
Thermodynamics
• Isobaric
A thermodynamic process that occurs
while the pressure remains constant
From Greek isos "equal“ and baros
"weight"
Thermodynamics
• Isochoric (Isolvolumetric)
A thermodynamic process that occurs
while the volume remains constant
Look for gases that are contained in a
closed or fixed container
From Greek isos "equal“ and choro
"place"
Thermodynamics
• Isothermal
A thermodynamic process that occurs
while the temperature remains constant
Usually a relatively slow process to
allow the gas to maintain its temperature
From Greek isos "equal“
and therme "heat"
Thermodynamics
• Combustion Engine
Thermodynamics
• What is work as defined by the Collegeboard?
Work is the work done by a gas
Therefore…
Positive work is the compression of a gas
Negative work is expansion of a gas
• What is work as defined by the textbook?
Work is the work done on a gas
• To avoid confusion, we use the
Collegeboard’s definition only
Thermodynamics
W = PDV
In order for work to be accomplished by the gas, it
must expand or contract (change volume)
A change in pressure, only, will not result in any
work being accomplished
In most examples a piston or object atop the gas must
be moved for work to be accomplished
Since we are considering the work done by the gas, as
the piston moves, the gas loses energy
Expansion is negative work; contraction is positive
work
Thermodynamics
PV Diagram basics
P(Pa)
V(m3)
Thermodynamics
PV Diagram basics
Given a point on the
diagram we can use
P(Pa)
PV = nRT
to find the gas’s
temperature (K)
V(m3)
Thermodynamics
PV Diagram basics
P(Pa)
If we see a line or curve
connecting points then
we know the gas has
changed its properties in
some way
V(m3)
Thermodynamics
PV Diagram basics
P(Pa)
When we see an arrow
on that line, then we
know the original and
final states of the gas
V(m3)
Thermodynamics
PV Diagram basics
P(Pa)
Movement to the right
shows expansion
(remember this is
negative work)
V(m3)
Thermodynamics
PV Diagram basics
Movement to the left
shows compression
(remember this is
positive work)
P(Pa)
V(m3)
Thermodynamics
PV Diagram basics
Movement directly
up or down shows
no change in
volume (remember
this is zero work)
P(Pa)
V(m3)
Thermodynamics
PV Diagram basics
P(Pa)
When the path
closes then we
have one complete
cycle
The gas has
returned to its
original pressure,
temperature, and
volume
V(m3)
Thermodynamics
Carnot Cycle and PV Diagram (Animation)
Ideal Diesel Engine (Animation)
Rubber Bands as Spokes Engine
( Explanation of Rubber Bands as Spokes Engine )
Thermodynamics
A. Be able to perform simple Carnot efficiency calculations.
B. Be able to describe an ideal gas.
C. Be able to interpret PV diagrams.
1. Which part of the cycle has DU greater than zero?
2. Which part of the cycle has Q greater than zero?
3. Which part of the cycle W greater than zero?
4. In which part of the cycle is no work done?
5. At which part of the cycle is the gas at is highest temperature?
6. What is the net work done?
7. Which part of the cycle does the most work?
8. Compare the temperatures at different points in the cycle.
9. Which part of the cycle represents an adiabatic, isothermal,
isochoric (isovolumetric), or isobaric process?
D. Be able to perform simple calculations using the ideal gas law or the
combined gas law. For example, pressure is doubled and volume is
quadrupled. What would be the relationship between the initial and final
temperatures?
E. Know the characteristics of the processes.
F. Predict what would happen to temperature if kinetic energy is changed.
G. Be able to perform simple calculations using the first law of
thermodynamics.
Thermodynamics
A. Be able to interpret PV diagrams.
1. Use ideal gas law (or combined gas law) to calculate the temperature at
different points.
2. Sketch the PV diagram from information given.
3. Predict whether positive, negative, or no work is done for the cycle.
4. Calculate the work done for the cycle (or part of a cycle).
5. Calculate the heat absorbed or given off for a cycle (or part of a cycle).
6. Predict which segments absorb the most heat.
7. Predict if the cycle represents a heat engine or refrigerator.
B. Remember the equation for power (P=W/t or P=Fv) and that P=F/A.
C. Sometimes they ask about the rate at which heat is discarded by an engine.
This can be found knowing that W=QH-QL
D. Be able to perform simple Carnot efficiency calculations.