Thermo Definitions Flashcards by Elysia Dooley

Density

Mass per unit volume - must have a continuum to be valid

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Continuum

Make sure fluid etc is uniform

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Specific quantities

Thermodynamic property per unit mass denoted using lower case letter of the thermodynamic quantity in question

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Specific volume

Volume per kg - reciprocal of density

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Absolute pressure

Total pressure

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Patm

Pressure in the atmosphere

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Gauge pressure

Diff between absolute and atmospheric pressure

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Vacuum

Negative guage pressure

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1bar

10^5 N/m^2

10^5 Pa

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Macroscopic energy

E a system possesses as a whole with respect to an outside reference phrame

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Microscopic

E relating to a molecular structure of a system and the degree of molecular activity

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Chemical energy

The internal energy relating to the atomic bonds in a molecule. Internal energy changes during combustion

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Nuclear energy

Fission of uranium
Fusion of hydrogen
Good clean energy but need high temp

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Intensive properties and egs

Not affected by quantity

T, P, density

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Extensive properties

Depends on how much there is

M,
V

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Thermal equilibrium

When the system temp is uniform throughout - temp can change, just must be uniform

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Mechanical equilibrium

When the pressure is uniform and there is no pressure gradients

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Chemical equilibrium

Where the working substance does not undergo any chemical reactions

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Thermodynamic equilibrium

When the system is in thermal, mech, and Chem equilibrium

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Simple compressible system

No electrical, magnetic, gravitational motion or surface tension

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Homogenous mixture/ system

Uniform and uniformly distributed

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Point and path functions

Start and end are point functions - they fix the thermodynamic state

Route between is a path function

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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

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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 )

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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

Thermo Definitions Flashcards

(73 cards)