Thermodynamics 1 Flashcards
Define a closed system
A system whose mass remains constant but energy can be transferred to or from it.
What is the change in enthalpy of a system at constant pressure equal to?
The heat addition.
Define the non-flow energy equation (NFEE)
dU = Q + W
The difference in internal energy of a closed system is equal to the work done and the heat transfer on the system AKA the 1st Law.
Under what condition is the change in internal energy of a system equal to it’s heat addition?
When it’s at constant pressure.
Define the first law of thermodynamics
The change in internal energy of a closed system is equal to the sum of of its heat addition and work done by the surroundings. Net work delivered to surroundings is proportional to the heat taken.
Define a system
A quantity of mass around which we draw a boundary to examine its property changed.
Define an intensive propert
Properties that do not depend on the quantity of matter such as temperature and pressure.
Define an extensive property
A property of the system that does depend on its quantity such as volume.
Define a quasi-equilibrium process
A process in which only infinitesimally small temperature gradients and unbalanced forces exist.
Define an adiabatic process
A process in which NO HEAT TRANSFER occurs. This means energy is only transferred by work.
What is the polytropic index of an adiabatic system?
γ, where γ = Cp/Cv. It comes from the heat transfer being 0, dU= -PdV, the specific heat for constant volume equation and using R= Cp - Cv.
Define internal energy
The sum of all microscopic energies in a body.
What is an isochoric process?
One where the system is kept at constant VOLUME. Therefore no p-v work is done and all energy added converts to internal energy, raising temperature and pressure.
What is an isobaric process?
One where the system is kept at a constant PRESSURE. So the heat addition here is the change in enthalpy.
What is an isothermal process?
One where the system is kept at constant TEMPERATURE. The p-v graph for this is reciprocal so the to find the work done we integrate pressure from the ideal gas law with respect to volume.
Define an isothermal system?
It is a REVERSIBLE ADIABATIC process - ENTROPY REMAINS CONSTANT because all work transfers are frictionless.
In an isochoric process what is equal to the work done?
Isochoric = constant volume, so no pdV (moving boundary) work is done, leaving dQ = dU.
in an isobaric process what is equal to the work done?
Isobaric = constant pressure, so the work done is a square pdV graph where W = -p(V2-V1).
What is the work done in an isothermal process?
Using the pV=nRT graph it will be reciprocal, so integrate it wrt V to find the moving boundary work.
Define a polytropic process?
One that obeys the relation pV^n=C. This covers all of the idealised processes (isobaric n=0, isothermal n=1, isochoric n=inf.)
Define the second law of thermodynamics?
It is impossible to convert all heat addition to work on the surroundings in a system - so it must operate between at least 2 reservoirs.
what is the function of a nozzle?
A nozzle decreases the fluid’s pressure to accelerate its flow (eg convergent pipe). At these very high speeds there is little time for it to transfer heat to/from surroundings so we always assume it to be adiabatic, and no work crosses the boundary either, meaning CHANGE IN VELOCITY = CHANGE IN ENTHALPY
whats the function of a diffuser?
Just the opposite of a nozzle; increases the fluid’s pressure to decelerate its flow (eg divergent pipe). At these very high speeds there is little time for it to transfer heat to/from surroundings so we always assume it to be adiabatic, and no work crosses the boundary either, meaning CHANGE IN VELOCITY = CHANGE IN ENTHALPY
whats the function of a turbine?
To extract work from a flow of fluid expanding from high pressure to low pressure. Its adiabatic and the change in velocity is negligible, but the fluid exerts torque on the shaft. So the work is equal to the change in enthalpy (negative cos work on surroundings).
What is the function of a compressor?
The opposite of a turbine; work is done on the fluid to raise its pressure. Still we assume velocity doesn’t change because before and after there are nozzles/diffusers so that work is still equal to the change in enthalpy.
What is the function of a throttle?
They restrict flow in an adiabatic steady flow process such that the inlet enthalpy is the same as the outlet enthalpy.
What is the Kelvin-Plank statement?
It is impossible for any cyclical device to receive heat from a single reservoir and produce work (there must be a cold reservoir).
Define the Clausius statement
Its impossible for a cyclical device to transfer heat from a cooler body to a hotter one without work being done on the system.
