Lecture 1 Flashcards
only gases have different
cp and cv values as solids and liquids are incompressible so cp and cv are the same
extensive vs intensive
intensive means independent of mass
open system
can exchange mass and energy
closed system
can exchange energy with surroundings but not mass
isolated system
cannot exchange mass or energy
Cp
heat capacity at constant pressure (doesnt actually vary much with pressure)
Cv
heat capacity at constant volume doesnt vary much with volume
change in enthalpy =
Cp*changein temperature
change in internal energy
Cv * change in temperature
closed system important variable
internal energy
to calculate use Cv for U
dont worry about change in volume
open system important variable
enthalpy
calculate use Cp for H
dont worry about change in pressure
zeroth law of thermodynamics
If A is in thermal equilibrium with B and B is in thermal equilibrium with C then A and C are in thermal equilibrium.
all same temp no temp flows between them
adiabatic process
Adiabatic means no heat flows into or out of the system.
adiabatic practical examples
situation occurs so fast that heat cannot escape fast enough to make a difference ie piston cylinder
adiabatic options
perfectly insulated system (never happens)
boundary at same temp as the system
internal energy
total energy of an ideal gas molecule
equation of state
pV = nRuT
portion of internal energy associated with kinetic energy of the molecules called
sensible energy
heat is
form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
work is
Work is the energy transfer associated with a force acting through a distance
work is path dependent
see powerpoint
heat flowing from the surroundings into the system is taken as
positive
work done by the system on the surroundings is taken as
positive
change in energy
heat in - work out
enthalpy =
internal energy + work
viscosity
shear stress/shear strain Pa s or kg m^-1 s^-1
internal energy
sums of molecular energy, rotational, translational, vibrational energy of molecules; extensive energy, J or kJ
enthalpy
H=U+PV (internal energy+ pressure x volume ); kJ, extensive property
What is meant by the phase of a system
A phase is where the properties of a system vary smoothly (with temperature for example), or they are essentially uniform. Water as a liquid is one phase, ice another phase. At the transition from liquid to ice the properties change.
What is meant by “the state” of a system?
“At a given state, all the properties of a system have fixed values.”
The state is essentially a measure of the specific energy of a system. A system can change state, without changing phase. Heating water (at 1 bar) from 10 to 30°C changes its state as the thermodynamic variables describing it change but it remains a liquid. Heating to 120°C changes both phase (liquid to gas) and state.
c. What is the minimum number of independent properties required to completely describe the state of a system? Are they intensive or extensive or either type of properties
Two intensive properties is the minimum required. For example for an idea gas, the equation of state is pV=nRuT :p is intensive V/n is intensive or p and T etc.
difference between heat and work
Heat is the form of energy that is transferred between two systems or a system and its surroundings by virtue of a temperature difference.
Work is the energy transfer associated with a force acting at a distance.
Both have the same units J but more commonly due to the sizes of the changes we will discussing we will use kJ
energy to vaporise =
hf - hg (must be in the same state)
thermodynamic equilibrium
a system that maintains thermal, mechanical, phase, and chemical equilibrium.
Isothermal process
constant temp
Incompressible flow
flow with insignificant density changes
