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Level 143

Thermodynamics IV


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Property
a macroscopic characteristic of a system that is independent of path (or process)
Intensive properties
pressure, temperature, density
State
the condition of a system as described by its properties
Process
the path of succession of states through which the system passes; more simply, the path of transformation from one state to another.
Steady state
the state for which the system's properties do not change with TIME
Thermodynamic cycles
a sequence of processes that begins and ends at the same state
phase
_______ is the degree portion of a cycle at any given time
Pure substance
element (atoms) or compound (molecules)
Thermodynamic equilibrium state
the state for which the properties do not have a tendency to change, in time and space, as long as the external conditions remain
Quasi-equilibrium state
the state for which the deviation from thermodynamic equilibrium is infinitesimal
Quasi-equilibrium process
the path of the succession of quasi-equilibrium states; an idealized process valid for many practical problems
E=KE+PE+U
Energy of a system
Internal energy, U
due to translational, rotational, vibrational motions, chemical bonds of molecules, and other like chemical energy in a battery
Work, W
A form of energy transfer of which the sole effect "could have been" the raising of a weight
Path function
A function that depends, not only on the properties of the two end states, but also on the path of the process; ex. W
Point function
A function that depends only on the properties of the two end states; ex. W of a polytropic process
Heat Transfer neglect criteria
good insulator, temp diff is small, surface area is too small to allow significant transfer, not enough time
Simple Compressible Substance
a substance whose magnetic and electrical effects are insignificant but changes in volume are very important
Simple compressible system
a system consisting of a simple compressible substance
State principal
the state of a simple compressible system at equilibrium can be uniquely determined by two independent intensive thermodynamic properties
Single-phase regions
solid, liquid, vapor
Two-phase regions
liquid-vapor, solid-liquid, solid-vapor
Triple line
the line along which three phases can exist in equilibrium
Saturation temperature
the temperature at which a phase change takes place at a given pressure
Saturation pressure
the pressure at which a phase change takes place at a given temperature
Subcooled liquid
the liquid of which temperature is lower than the saturation temperature; also called compressed liquid (pressure is higher than the saturation pressure)
Saturated liquid
a substance that exists as liquid at the saturation condition
Quality, x
the ratio of the mass of vapor present to the total mass of the liquid-vapor mixture; intensive property
Saturated vapor
a substance that exists as a vapor at the saturation condition
Dry saturated vapor
x=1; saturated vapor with no liquid, still in liquid-vapor dome; vf<=v<=vg
Superheated vapor
a vapor of which temperature is higher than the saturation temperature; substances called gases are generally highly superheated vapors
Vaporization and Condensation Conditions
can only occur when the pressure is lower than the critical pressure
Compressibility factor, Z
ratio of the product of the pressure and specific volume to the product of the gas constant and the temperature
Reduced pressure and temperature
ratio of the pressure/temperature to the critical pressure/temperature
Ideal Gas criteria
T>= 2*Tc && p<=.5*pc
pv=RT, pV=mRT, p=(density)RT
Ideal gas state eqn
Internal energy and enthalpy (I.G.M.)
thermally perfect; dependent ONLY on temperature
Calorically perfect
cp and cv are constant
Isobaric process
pressure is constant
Isometric process
volume is constant; also isovolumetric or isochoric
Reynolds Transport Theorem
coverts a closed system analysis into a control volume analysis and relates the time derivative of a system property to the rate of change of that property within a control volume at the instant …
Mass flow rate
mass flow through a system over time
Conservation of mass
the time rate of change of the mass in a control volume is equal to the sum of the mass flow rates at inlets minus the sum of the mass flow rates at exits
Mass flux
mass flow through an area
Volume flow rate
time rate of change of the volume
One-dimensional flow
flow that satisfies m(dot)=(density)(velocity)(area)
energy conservation
A principle stating that the total energy of an isolated system remains constant regardless of changes within the system.
Thermal reservoir
a body of a system of which temperature remains constant even when heat is added or removed
Clausius statement
it is impossible to construct a system that yields heat transfer from a cooler body to a hotter body without an input of work
Kelvin-Plank statement
it is impossible to construct a device that operates in a cycle, receives heat energy, and does work equal to or greater than the amount of heat energy received
Sources of irreversibilities
friction, heat transfer through a finite temperature difference, unrestrained expansion, mixing of substance at difference compositions or states, chemical reaction, electric current flow
Internal irreversibilities
irreversibilities within a system
External irreversibilities
irreversibilities outside the system (within the surroundings)
Internally reversible process
a process in which there are no internal irreversibilities
Thermal efficiency
ratio of energy sought to the energy cost; eta is independent of substance and relies only on Qh and Qc
Carnot corollaries 1
eta for a reversible cycle will always be greater than eta for an irreversible cycle when each cycle operates between the same two thermal reservoir
Carnot corollaries 2
eta's of all the reversible cycles are the same if they operate between the same two thermal reservoirs
Isentropic process
Reversible and adiabatic
Graph of Pressure against 1/Volume? Conditions?
Fixed temperature and amount of gas.
A constant.
What does pV equal for a fixed temperature and amount of gas?
Constant pressure.
Graph of temperature against Volume? Conditions?
A constant.
What does V/T equal at constant pressure?
p V = n R T
State the ideal gas law and units?
The gas constant.
What is R and what is its value?
How to convert bar to N m⁻² to Pa
1 bar = 1 x 10⁵ N m⁻² = 1 x 10⁵ Pa
XA = nA/n
How to work out the molar fraction, XA (XB etx) for a mixture?
What assumptions are made when using the ideal gas law?
The molecules have no volume (the volume of the molecules is negligible compared to the volume of the container)
1
What value does Z take for ideal gases?
When do deviations typically occur?
The volume of gases.
London dispersion forces - most molecules -
What types of intermolecular forces are there? What type of molecule do they occur between? What are their typical energy?
What are their units?
a depends upon the strength of interactions between molecules.