Thermometry Concept of temperature Boyles Law Charles Law Thermometers Thermocouple Calorimetry Platinum resistance scale Absolute zero Lower fixed point Temperature gradient Thermodynamic ID: 783196
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Slide1
Thermodynamics 2
Slide2PH: 104: Heat and Thermodynamic Course Outline (Continued)
Thermometry
Concept of temperature
Boyle’s LawCharles’ LawThermometersThermocoupleCalorimetryPlatinum resistance scaleAbsolute zeroLower fixed pointTemperature gradientThermodynamic laws and systemsSystem state propertiesEquations of stateThermodynamic processesWorkFirst Law of ThermodynamicsOpen and closed systemsInternal energy Standard Temperature and PressureConductive heat transferConductivity and resistance
Slide3Objectives
Understand equilibrium and quasi-equilibrium
Define the laws of thermodynamics
Define internal energy Use definitions of work for various closed system cases
Slide4Equilibrium and Equilibrium Assumptions
To do analysis of system the system needs to be at equilibrium (e.g. no system variables are changing)
Analysis can be done at Quasi-equilibrium, which is to say the system is changing in increments at which analysis can be done. If the time change at which the analysis is done is small enough, the system can be assumed to be in Quasi-equilibrium. ALSOFor open systems (e.g. control volumes) the rate of change (mass flow) is assumed to be constant over a very short period of time.
Slide5The Four L
aws of Thermodynamics
0
th Law: if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.1st Law: is the law of conservation of energy. Energy can to be created or destroyed. There is function called internal energy that relates work and heat transfer.2nd Law: Entropy is a function of the state of the system and cannot be reduced. Entropy is a measure of disorder or the possibility of a process occurring in nature.3rd Law: It is impossible by any procedure, no matter how idealized, to reduce any system to the absolute zero temperature in a finite number of operations.
Slide6General Idealisms of the Laws
0
th
Law: temperatures are relatable1st Law: deals with quantity of energy2nd Law: deals with quality of energy and irreversibility3rd Law: everything ceases to exist at absolute zero
Slide7Zeroth Law of Thermodynamics
If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
That is to say, if Ta =
Tc and Tb = Tc then Ta = Tb Are the internal energies of CO2 and H20 equal if they have the same temperature?
A
C
B
Slide8Mechanical energy and heat equivalency
Joules Experiment (adiabatic)
Method
Dropped a weight to mix waterMany other similar experimentsResultSlight temperature riseConclusions4.186 J = 1 cal4186 J = 1 kcal4186 J = 1 Cal (food)
Slide9Mechanical energy and heat equivalency
Joules Experiment (adiabatic)
Further conclusion
Internal Energy is a measurable quantity and is relatable to work
Slide10Internal energy (
U)
is vibration and rotation energy in fluids
Sometimes loosely called heat, but this introduces a confusion between it and the energy transferred by heating Q. Heating is simply the process by which energy is transferred from hot to cold.For ideal gas and closed systems
For real gases and closed system
For open systems
Where,
H
is entropy <- we won’t define this
The First Law of Thermodynamics
(Conservation of Energy)
Where
Q = net heat addedW = work done by the system
= change in internal energy
Internal Energy
Is a property of the system, describes
vibrational/rotational
energy in a fluid
Work
Is not a property, describes
mechanical
energy
entering (-) or leaving the system (+)
Heat
Is not a property, describes
thermal
energy entering (+) or leaving the system (-)
Mass Balance (closed system)
A mass balance for a closed system is very easy.
Mass in = Mass out = 0
Energy Balance (closed system)
Slide14Time dependent Energy Balance:
Closed System
Slide15Work relationships for a closed system
Adiabatic
Isothermal
IsovolumetricIsobaricPolytrophic process
Slide16Polytrophic Process
pV
n
= constant
n
= 1
n
≠
1
Adiabatic Process
Q
= 0
Thus, ΔU = -W
U
1
U
2
Slide18Equations of state for the adiabatic process for an ideal gas (Syllabus)
Isobaric Process
W
=
f x dW = PAdThus, W = PΔVOR
P
1
= P
2
Slide20Isovolumetric
Process
Volume
is constantW = PΔVThus, W = 0
V
1
= V
2
Slide21Isothermal Process
(for
ideal gasses
in closed systems)
Thus,
Thus,
T
1
= T
2
Slide22Isothermal Process
n
= 1
pVn = constant
p
T
1
= T
2
Slide23Cycles
Are just a set of processes in series 1-2-3-4-1-2-3-4-1-2-3-4,
ect
.Two major types: Power Cycle and refrigeration and heat pump cyclesEngines operate on Power CycleCarnot cycleOtto cycle
Slide24Examples of why aren’t processes or cycles 100% efficient (irreversibility)
Heat transfer through a finite temperature difference
Unrestrained expansion of a gas or liquid to a lower pressure
Spontaneous chemical reactionSpontaneous mixing of matter at different compositions or statesFriction- sliding friction as well as friction in the flow of fluidsElectric current flow through a resistanceMagnetization of polarization with hysteresisInelastic deformation
Slide25Reversible cycle (internally)
1 ↔ 2
No irreversibility's in the system (friction,
ect
.)
Slide26Self study syllabus topic
The relationship between the principal specific heat capacities
Conductive heat transfer
Thermal conductivity and resistance
Slide27Second law not on syllabus
But, it is the law that proves no REAL process or cycle is 100% efficient
Slide28Second Law of Thermodynamics
Heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.
Some processes adhere to the First Law of Thermodynamics but do not happen in both directions (i.e. a cup hitting the ground and breaking or mixing salt and pepper)
Things tend to spontaneously go from a state of order to disorder (increasing entropy) but not the other direction.
Slide29Example Problem
How much work is done by the expanding gas?
W
=
f
x
d
= ??? J
P
atm
= 1
atm
or 1.013 x 10⁵ N/m²
A
piston
=
0.02 m²
d = 0.1 m
Slide30Example problem
An
ideal gas expands isothermally, performing 3.40 x 10
3 J of work. Calculate the change in internal energy and the heat absorbed during this expansion.
Slide31An ideal gas expands isothermally, performing 3.40 x 10
3
J of work. Calculate the change in internal energy and the heat absorbed during this expansion.
Do I remember the qualifications of isothermal expansion?
T
1
= x
V
1
= y
P
1
=
z
T
2
= x
V
2
= y + a
P
2
=
???
W = f x d =
3.40 x 10
3
J
An ideal gas expands isothermally, performing 3.40 x 10
3
J of work. Calculate the change in internal energy and the heat absorbed during this expansion.
How do gases expand? How do gases expand without changing temp?
T
1
= x
V
1
= y
P
1
=
z
T
2
= x
V
2
= y + a
P
2
=
???
W = f x d =
3.40 x 10
3
J
An
ideal gas
expands isothermally, performing 3.40 x 10
3 J of work. Calculate the change in internal energy and the heat absorbed during this expansion.
T
1
= x
V
1
= y
P
1
=
z
T
2
= x
V
2
= y + a
P
2
=
z - b
W = f x d =
3.40 x 10
3
J
Problems with mass and energy balances
Slide35= ?
We will use a Mass Balance and
Energy Balance (first law)
and apply the equations
to solve for tank pressure at time (
t
2
) if pump power, flow rate,
P
t1
and surrounding variables were known.
Slide36Polytrophic example
Four kilograms of a gas is contained within a piston-cylinder assembly. The gas undergoes a process for which the pressure-volume relationship is
The initial pressure is 3 bar, the initial volume is 0.1
. And the final volume is 0.2
. The change in specific internal energy of the gas in the process is
. There are no significant changes in the kinetic or potential energy. Determine the net heat transfer for the process, in kJ.
Summary
Slide39Class Questions
Does a ingot of iron that is twice the size of another but at the same temperature have the same internal energy? (yes or no)
When 2 masses of water are mixed
(1) 300 g @ 50 °C(2) 3,000 g @ 45 °C Does heat flow from the water with higher internal energy (2) or temperature (1)?
Slide40Second Law of Thermodynamics
Heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.
Some processes adhere to the First Law of Thermodynamics but do not happen in both directions (i.e. a cup hitting the ground and breaking or mixing salt and pepper)
Things tend to spontaneously go from a state of order to disorder (increasing entropy) but not the other direction.
Slide41Reversible thermodynamic process
Slide42Thermodynamic intensive
and
extensive
properties, variablesIntensive do not change when identical systems are addedExtensive depend on the amount of the substance in the systemIntensiveExtensiveTemperature, TVolume, VPressure, PMass, m
Density,
ρ
Area,
A
Composition
Slide43Group Class Homework
Which is larger 1
°F or 1 C°?
The units for the coefficient of linear expansion α are (°C)-1 and there is no mention of a length unit such as meters. Would the expansion coefficient change if we used feet or millimeters instead of meters? Long steam pipes that are fixed at the ends often have a section in the shape of a U. Why?
Slide44Superheating
When a gas is heated to above the critical temperature the vapor, gas or fluid is said to be super heated. This a metastable phase that usually has a transient existence (e.g. short life).
Slide45Supersaturation
When a gas has more vapor than the
SVP
do to high pressures. This processes is also unstable and often short lived.What does supersaturation look like in our environment?What is the dew point?
Slide46Group homework
15 below zero on the Celsius scale is what temperature Fahrenheit and kelvin?
15 below zero on the
Fahrenheit scale is what temperature in Celsius and kelvin?The Eiffel Tower is 300 m tall and made of wrought iron. Estimate how much the height changes from July (ave. 25 °C) to January (ave. 2 °C).The density of water at 4 °C is 1000 kg/m³. What is water’s density at 94 °C?Absolute zero is what temperature on the Fahrenheit scale?
At (a) atmospheric pressure, in what phases can CO
2
exist? (b) For what range of pressure and temperatures can CO
2
be a liquid?
Extra credit
What is the approximate temperature inside a pressure cooker if the water is boiling at a temperature of 120
°
C? Assume no air escaped during the heating process, which started at 20
°
C.