Lectures 3 and 4 Flashcards
What is a pure substance?
A substance that is homogeneous (i.e. uniform thermodynamic property throughout). The chemical composition is the same in all phases.
What are the three phases of matter?
Solid
Liquid
Gas
Describe the molecules in solids, liquids and gases respectively.
- solid: molecules oscillate about their “fixed” positions
- liquid: groups of molecules float about each other
- gas: gas molecules move randomly
The transformation from
- solid to liquid
- liquid to gas
- gas to liquid
- liquid to solid
- melting
- evaporation
- condensation
- freezing
Describe the refrigeration process
- Refrigerant gas compressed to high P and T through a compressor
- Gas travels through condenser coils and becomes a liquid. Heat is rejected to the room.
- Refrigerant liquid travels through expansion valve, decreasing T
- Cold liquid removes heat from refrigerator. Liquid refrigerant receives heat and liquid evaporates to a gas, and this then goes back into the compressor.
Describe the steam power plant
- Liquid water is pumped from near atmospheric P to a higher P
- Compressed water is heated in a boiler. Water is evaporated into steam
- High T, P steam is sent to turbine. Steam expands and used to turn turbine blades. Turbine connected to electric generator and this produces electricity to grid. T, P of steam reduces significantly and condensation of steam to water
- Steam leaves turbine and sent to condenser where heat is removed and remaining steam condenses to have 100% liquid which is sent back to pumps
What must be added or removed for a phase change?
Energy
What is Gibb’s Phase Rule
F = C - phi + 2
What is F in Gibb’s Phase Ruler
The number of intensive parameters that must be specified in order to completely define the system or number of variables that can be changed independently and still maintain equilibrium
What is phi in Gibb’s phase rule?
The number of phases. Phases are mechanically separable constituents
What is C in Gibb’s phase rule?
The minimum number of components.
What does the number 2 specify in Gibb’s Phase Rule?
Two intensive parameters - usually temperature and pressure
Which systems does Gibb’s Phase rule apply to?
Systems in equilibrium
What are the regions of interest on P-v diagrams?
Compressed liquid, saturated liq-vap mixture, superheated vapour
What is the dome on a P-v diagram?
It is the lines separating these phases.
What is the quality of a two phase liquid-vapour mixture?
x = mass (vapour) / mass (total) x = mass (vapour)/ (mass(liquid) + mass (vapour))
What is the critical point?
It is the point beyond which the liquid and vapour phases are indistinguishable. They have the same density.
What is the triple point?
The co-existence of solid, liquid and vapour.
What is sublimation?
Transformation from vapour to solid.
What is the definition of enthalpy?
H = u + Pv
How does the specific volume of liquid compare to that of vapour?
The specific volume of vapour is always greater than the specific volume of liquid
What is the saturation pressure?
Pressure at which the liquid and vapour phases are in equilibrium at a given temperature
What is the saturation temperature?
The temperature at which the liquid and vapour phases are in equilibrium at a given pressure
Alternative equations for the quality involving variable such as u and h
u = uf + x*(ug-uf) h = hf + x*(hg-hf)
Define compressed liquid
State with the pressure greater than the saturated pressure. Or state with lower temperature than the saturation temperature
Define superheated vapour
State with the pressure lower than the saturated pressure. Or state with higher temperature than saturation temperature
What is required if the exact conditions are not in the table?
Linear interpolation
Equation for linear interpolation
x desired = x1 + Tdesired - T1/ T2 - T1 * (x2 - x1)
What are the ideal gas equations of state?
Pv = RT, PV = mRT, P = rhoRT
When can a gas be considered ideal?
When the gas pressure is very small compared to the critical pressure. Or the gas temperature is twice the critical temperature and the pressure is less than 10 times the critical pressure.
What common vapour is often considered an ideal gas but it is not?
Water vapour
Common gases that can be treated as ideal gases
Air, N2, H2, CO2, CO, Ar
What temperature range is the ideal gas law valid for?
Less than 1000 K
What is the specific heat?
It is the amount of energy required to increase the temperature of a unit mass by a single degree
What is the specific heat capacity at constant volume?
Energy required to raise the temperature of a unit mass of a substance by one degree as the volume is kept constant
What is the specific heat capacity at constant pressure?
Energy required to raise the temperature of a unit mass of substance by one degree as the pressure is maintained at constant
Relation of Cv and dT
Cv = du/ dt
Relation of Cp and dT
Cp = dh/ dT
For sensible heat what are the equations?
Cv ( T2 - T1)
Cp (T2 - T1)
Relation between Cp, Cv and R
Cp = Cv + R
The ratio of specific heat capacities
k = Cp/ Cv
