Thermo Definitions Flashcards
Density
Mass per unit volume - must have a continuum to be valid
Continuum
Make sure fluid etc is uniform
Specific quantities
Thermodynamic property per unit mass denoted using lower case letter of the thermodynamic quantity in question
Specific volume
Volume per kg - reciprocal of density
Absolute pressure
Total pressure
Patm
Pressure in the atmosphere
Gauge pressure
Diff between absolute and atmospheric pressure
Vacuum
Negative guage pressure
1bar
10^5 N/m^2
10^5 Pa
Macroscopic energy
E a system possesses as a whole with respect to an outside reference phrame
Microscopic
E relating to a molecular structure of a system and the degree of molecular activity
Chemical energy
The internal energy relating to the atomic bonds in a molecule. Internal energy changes during combustion
Nuclear energy
Fission of uranium
Fusion of hydrogen
Good clean energy but need high temp
Intensive properties and egs
Not affected by quantity
T, P, density
Extensive properties
Depends on how much there is
M,
V
Thermal equilibrium
When the system temp is uniform throughout - temp can change, just must be uniform
Mechanical equilibrium
When the pressure is uniform and there is no pressure gradients
Chemical equilibrium
Where the working substance does not undergo any chemical reactions
Thermodynamic equilibrium
When the system is in thermal, mech, and Chem equilibrium
Simple compressible system
No electrical, magnetic, gravitational motion or surface tension
Homogenous mixture/ system
Uniform and uniformly distributed
Point and path functions
Start and end are point functions - they fix the thermodynamic state
Route between is a path function
Process
Any state change a system undergoes from one state of equilibrium to another
- the series of states the system passes through during the process is called the path
Quasi equilibrium/ Quadistatic process
When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state (eg slow compression of air in a piston cylinder device )
Non- equilibrium process
One that occurs very fast and deviates from an equilibrium process path (eg puncturing helium balloon )
Isobaric
Constant pressure process- where the system pressure is held constant (eg expansion of gas in a frictionless piston cylinder device)
Isothermal process
Temp is constant
Eg phase change process
Isochoric process
Volume is help constant- quite common in thermodynamics
Aka isometric process
0th law of thermodynamics
If 2 bodies are in equilibrium with a 3rd body, they are also in thermal equilibrium with eachother
Sign of work done BY a system?
Positive
Work of a rotating shaft equation
2pie x n of revs per second x Torque
Heat
A transfer of energy across a system boundary due to a temp difference between the system and the surroundings
Sign of heat received by a system?
Positive
Adiabatic
A process in which there is no energy transfers by the heat transfer mechanism
1st law of thermodynamics for closed systems
Energy can neither be created or destroyed- can only change forms
Qnet?
= Qin - Qout
Wnet?
Wout- Win
Delta Esystem?
Qnet- Wnet
What is saturation temp?
The temp at which vaporisation takes place at a given pressure
What is saturation pressure?
The pressure at which saturation takes place
What is a saturated liquid?
Totally in liquid phase at sat temp and pressure
Left flat bit
Little f is used to denote Saturday’s properties
What is a compressed liquid?
Actual temp is lower than saturated temp for the system pressure before sat bit on graph
What’s a saturated vapour?
Totally vapour- is at sat pressure and temp
Right flat bit
g used to denote properties
What’s a superheated vapour?
Superheated= when the system temp is greater than the saturated temp for a given pressure
Region to the right of the flat bit
What type of vapour are gases?
Super-heated vapour
What’s quality/dryness?
What’s the equation for it?
Used to describe a substance that is in the two phase region and is a measure of the amount of the substance that is in the vapour phase
X= total mass of vapour/ total mass
= mg/(mf+mg)
What’s x for saturated liquids?
X=0
What’s x for saturated vapours?
X=1
When heating lots of pure fluids at constant pressure what Graph is formed when it’s repeated?
A vapour dome- inside it is a mixture
When can it be assumed that steam is an ideal gas?
At low temps
What’s the triple point on a P vs T diagram?
Where theres solid, liquid, and gas present
Whyre there 2 melting limes in a P vs T diagram?
Some substances contract on freezing (like to higher temp- next to vapour)
And some expand on freezing (line to lower temp- next to solid)
How can all properties of a pure substance be defined?
By knowing 2 independent Properties
Eg, Pv. Tv, Px, Tx
PRESSURE AND TEMP AREN’T INDEPENDENT IN THE TWO PHASE REGION
What Substances is the ideal gas equation limited to?
Gases and very super heated vapours
What’s internal energy?
The energy of mass composition - can be one of the 2 independent properties to fully define the state of a pure substance
What’s the equation for internal energy in the saturated region?
u = uf + x ufg
What’s hf?
Enthalpy of saturated liquid
What’s sf?
Entropy of saturated liquid
What’s hg?
Enthalpy of saturated vapour
What’s the equation for enthalpy?
H= U + PV
What’s the unit for enthalpy?
Joule
How do you know if smt is super heated?
The temp on the table is too high for the pressure given
How do you know of smt is sun cooled?
If temp is too low for given pressure
What’s the equation to get the diff between the sat liquid and vapour properties where the substance is between the two states and is in the 2 phase region?
Vfg = Vg - Vf
Why should the compressed liquid generally be treated as a saturated liquid at a given temp?
Properties of compressed liquids aren’t very pressure sensitive
T-V plot would be pretty much be a vertical line for saturated liquids
Which property can yield greater error?
Enthalpy
Define specific heat capacity
Energy required to raise the temp of a substance to one degree
What do Cv and Cp stand for?
Specific heat capacity at constant volume = cv
Specific heat capacity at constant pressure = cp
These are for gases and vapours
Why is cp> cv?
Some of the heat being used to produce the work under constant pressure conditions
What’s Boyles law?
PV = constant
What’s charles’ law?
V1/T1 = V2/T2
ie V/T = constant
When can we assume we have an ideal gas?
When temp is greater than 2x critical temp except where pressure is much greater than the critical pressure
When pressure is much lower than 1bar and the substance is in the vapour phase
What’re the formulae for reduced pressure and temp?
Tr= T/ Tcrit
Pr= P/ Pcrit
