Devil physics The baddest class on campus IB Physics...
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DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS IB PHYSICS
Transcript of Devil physics The baddest class on campus IB Physics...
DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS
IB PHYSICS
TSOKOS LESSON 3-4 THERMODYNAMICS
IB Assessment Statements
Topic 10.1., Thermodynamics:
10.1.1. State the equation of state for an ideal gas.
10.1.2. Describe the difference between an ideal gas and a real gas.
10.1.3. Describe the concept of the absolute zero of temperature and the Kelvin scale of temperature.
10.1.4. Solve problems using the equation of state of an ideal gas.
IB Assessment Statements
Topic 10.2., Thermal Physics: Processes
10.2.1. Deduce an expression for the work involved in a volume change of a gas at constant pressure.
10.2.2. State the first law of thermodynamics.
10.2.3. Identify the first law of thermodynamics as a statement of the principle of energy conservation.
10.2.4. Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas.
IB Assessment Statements
Topic 10.2., Thermal Physics: Processes
10.2.5. Draw and annotate thermodynamic processes and cycles on P-V diagrams.
10.2.6. Calculate from a P-V diagram the work done in a thermodynamic cycle.
10.2.7. Solve problems involving state changes of a gas.
IB Assessment Statements
Topic 10.3., Second Law of Thermodynamics and Entropy:
10.3.1. State that the second law of thermodynamics implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.
10.3.2. State that entropy is a system property that expresses the degree of disorder in the system.
IB Assessment Statements
Topic 10.3., Second Law of Thermodynamics and Entropy:
10.3.3. State the second law of thermodynamics in terms of entropy changes.
10.3.4. Discuss examples of natural processes in terms of entropy changes.
Objectives
Understand the meaning of internal energy
Calculate work when a gas expands or compresses using
VPW
Objectives
State the relationship between changes in the internal energy, the work done and the thermal energy supplied through the first law of thermodynamics
Define the terms adiabatic, isothermal, isobaric, and isochoric and show these on a pressure-volume diagram
WQU
Objectives
Understand what is meant by irreversibility and disorder
Understand that entropy is a measure of disorder
State the second law of thermodynamics
Understand the meaning of energy degradation
Internal Energy
The total kinetic energy of the molecules of the gas plus the potential energy associated with the intermolecular forces
For an ideal gas, the intermolecular forces are assumed to be zero
Internal Energy
Average kinetic energy of the molecules is given by
where
the Boltzmann constant
kTvmEk2
3
2
1 2
1231038.1 JKxN
RkA
Internal Energy
Internal energy, U, of an ideal gas with N molecules is given by
and since
then
NkTENU k2
3
nRTPV
NnN
NRk
A
A
PVnRTU2
3
2
3
Internal Energy
In this equation, n is the number of moles
The change in internal energy due to a change in temperature is thus
PVnRTU2
3
2
3
TnRU 2
3
Internal Energy
This formula shows that the internal energy of a fixed number of moles of an ideal gas depends only on temperature and not on the nature of the gas, its volume, or any other variable
TnRU 2
3
System
Means the complete set of objects under consideration
Open system – mass can enter or leave
Closed system – mass cannot enter or leave
Isolated system – no energy in any form can enter or leave
System State
Refers to the complete set of parameters that define the system
Not to be confused with states of matter
Any process that change the state of an ideal gas (temperature, pressure or volume) is called a thermodynamic process
Examples are adding/subtracting heat or doing work on the system
Work Done On or By A Gas
Compressing a gas with a piston, the (small) work done is given by:
VPW
VsA
sPAW
PAF
sFW
Work Done On or By A Gas
For large changes in volume, the pressure will increase with each incremental change so it must be integrated
VPW
Work Done On or By A Gas
The total work done when a gas expands by an arbitrary amount is the area under the graph of a pressure-volume diagram.
VPW
Work Done On or By A Gas
The net work done is the work done by the gas minus the work done on the gas.
It equals the area enclosed by the closed loop (i.e., the area between the upper and lower curves) VPW
Thermodynamic Processes
Isobaric – The gas expands or contracts under constant pressure
Thermodynamic Processes
Isochoric – The volume of the gas remains the same as pressure increases or decreases
Thermodynamic Processes
Isothermal – The temperature of the gas remains the same as pressure and volume change
Adiabatic – the gas does not absorb or release any thermal energy (Q = 0)
Thermodynamic Processes
Isothermal – The temperature of the gas remains the same as pressure and volume change
Adiabatic – the gas does not absorb or release any thermal energy (Q = 0)
First Law of Thermodynamics
When a small amount of thermal energy δQ is given to a gas, the gas will absorb that energy and use it to increase its internal energy and/or do work by expanding.
Conservation of energy demands that
WQU
First Law of Thermodynamics
+ δQ = thermal energy absorbed by the gas (Qin)
- δQ = thermal energy lost by the gas (Qout)
+ δW = work done by the gas (Wout)
- δW = work done on the gas (Win)
+ δU = increase in internal energy/temperature
- δU = decrease in internal energy/temperature
WQU
First Law of Thermodynamics
If the amounts of thermal energy and work are not small, then
WQU
WQU
What is going on here?
WQU
What is going on here?
WQU
TA = 400K
UA-B = 7.2 kJ
UC-A = 2.2kJ
Find:
TB
QA-B
QB-C
Net Work
We Stopped Here on 2/8
Second Law of Thermodynamics
Impossible processes consistent with the First Law
The spontaneous (i.e. without the action of another agent) transfer of thermal energy from a cold body to a hotter body
The air in a room suddenly occupying just one half of the room and leaving the other half empty
A glass of water at room temperature suddenly freezing, causing the temperature of the room to rise
Second Law of Thermodynamics
Order and Disorder – Consider a bowling ball rolled down the hall in between classes
Between collision with students and friction, the ball eventually comes to a stop.
Second Law of Thermodynamics
Once the ball comes to a stop, the ordered kinetic energy can’t be recovered because it has been degraded.
All that is left is the disordered heat energy caused by friction
Second Law of Thermodynamics
The Second Law of Thermodynamics deals with the limitations imposed on heat engines, devices whose aim is to convert thermal energy (disordered energy) into mechanical energy (ordered energy).
Second Law of Thermodynamics
Reversibility
All natural processes are irreversible
They lead to a state of increased disorder
Measure of Disorder – Entropy
State function – once the state of the function is specified, so is the entropy
Entropy
During an isothermal expansion of a gas, the gas must receive heat
During an isothermal compression of a gas, the gas expels heat
The net change in entropy is zero
T
QS
0
E
E
S
T
QS
0
C
C
S
T
QS
Entropy
The entropy of heat flowing from a hot object to a cold one
The hot object loses heat, the cold one gains heat
Entropy increases
T
QS
0
11
E
HC
CH
S
TTQS
T
Q
T
QS
Entropy
The entropy of heat flowing to a hot object from a cold one
The hot object gains heat, the cold one loses heat
Entropy decreases
Ain’t Gonna Happen!
T
QS
0
11
E
CH
CH
S
TTQS
T
Q
T
QS
Second Law of Thermodynamics (finally!)
General Statement:
The entropy of an isolated system never decreases.
Statement due to (Santos) Clausius is
It is impossible for thermal energy to (spontaneously) flow from a cold to a hot object.
Energy Degradation
The flow of energy from a hotter to a colder object tends to equalize the two temperatures and deprives us of the opportunity to do work.
WQU
Energy Degradation
It is a consequence of the second law that energy, while always being conserved, becomes less useful – this is energy degradation.
Energy Degradation of the Universe
Consider the universe to be a big bag of gas
As far as we know, the universe is a closed system – no heat is entering the universe
Since the universe is expanding, positive work is being done
0
0
U
WU
Energy Degradation of the Universe
Thus the internal energy of the universe, i.e. its temperature, is constantly decreasing
This aligns with the idea that the temperature during the BIG BANG was millions of degrees and today, wavelength measurements of electromagnetic radiation left over from the BIG BANG show the temperature
of the universe to be about
2.7K 0
0
U
WU
Energy Degradation of the Universe
The inevitable result of the constant cooling of the universe is,
0
0
U
WU
Energy Degradation of the Universe
HEAT DEATH OF THE UNIVERSE
COMING TO A THEATER NEAR YOU,
JUN 6, 2014
Summary
Do you understand the meaning of internal energy?
Can you calculate work when a gas expands or compresses using?
VPW
Summary
Can you state the relationship between changes in the internal energy, the work done and the thermal energy supplied through the first law of thermodynamics?
Can you define the terms adiabatic, isothermal, isobaric, and isochoric and show these on a pressure-volume diagram?
WQU
Summary
Do you understand what is meant by irreversibility and disorder?
Do you understand that entropy is a measure of disorder?
Can you state the second law of thermodynamics?
Do you understand the meaning of energy degradation?
IB Assessment Statements
Topic 10.1., Thermodynamics:
10.1.1. State the equation of state for an ideal gas.
10.1.2. Describe the difference between an ideal gas and a real gas.
10.1.3. Describe the concept of the absolute zero of temperature and the Kelvin scale of temperature.
10.1.4. Solve problems using the equation of state of an ideal gas.
IB Assessment Statements
Topic 10.2., Thermal Physics: Processes
10.2.1. Deduce an expression for the work involved in a volume change of a gas at constant pressure.
10.2.2. State the first law of thermodynamics.
10.2.3. Identify the first law of thermodynamics as a statement of the principle of energy conservation.
10.2.4. Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas.
IB Assessment Statements
Topic 10.2., Thermal Physics: Processes
10.2.5. Draw and annotate thermodynamic processes and cycles on P-V diagrams.
10.2.6. Calculate from a P-V diagram the work done in a thermodynamic cycle.
10.2.7. Solve problems involving state changes of a gas.
IB Assessment Statements
Topic 10.3., Second Law of Thermodynamics and Entropy:
10.3.1. State that the second law of thermodynamics implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.
10.3.2. State that entropy is a system property that expresses the degree of disorder in the system.
IB Assessment Statements
Topic 10.3., Second Law of Thermodynamics and Entropy:
10.3.3. State the second law of thermodynamics in terms of entropy changes.
10.3.4. Discuss examples of natural processes in terms of entropy changes.
QUESTIONS?
#1-15
Homework
