1st Law of Thermodynamics?
Any closed system working in a cycle has equal work done entering and leaving the system.
2nd Law of Thermodynamics?
- Total entropy of an isolated system will always increase over time or remains constant if a process is reversible. This leads on to the Kelvin-Planck Statement.
- All engines must reject heat. Nothing is 100% efficient.
What is the Kelvin-Planck Statement?
It is impossible for any device that operates on a cycle to receive heat from a single reservoir and do work.
What is the Clausius Statement?
It is impossible to construct a device that operates in a cycle and transfers heat from a cooler body to a hotter body without work being done on the system from the surroundings.
What is a closed system?
A system where fluid doesn't cross the system boundary to or from its surroundings.
What is a state postulate?
Two or more fixed independent properties that define a simple system (a.k.a. a single chemical component e.g. air)
What is Gauge Pressure?
Gauge pressure reads zero in atmospheric pressure or has a datum of 1 atm. Therefore the read value is not absolute/true pressure.
What is Absolute Pressure?
Reads zero when in a vacuum or has a datum of 0 atm/Pa/bar.
What is the 0th Law of Thermodynamics?
Thermally, if A = B and A = C then it follows that B = C.
What does quasi-equilibrium mean? (Reversible means the same thing)
- A system is very close to equilibrium as it moves between start and end points. E.g. a piston moving in infinitesimally small steps.
- The process is frictionless.
What is a Stationary System?
Any change in kinetic or potential energy is very small/negligible.
What is meant by a system being in equilibrium?
All driving forces acting on the system are balanced.
What can be said about a closed, isolated system working in a cycle?
Change in internal energy is zero. A.k.a. all heat added to the system is converted to work done to the surroundings.
When is a system defined to be working in a cycle?
If its start and end states are identical, then a system is working in a cycle.
What are the six properties that can be used to define a system?
- Internal Energy
What is a Property? Why isn't heat or work a property?
- A property describes a state of equilibrium that is independent of path taken.
- Heat/work aren't properties because they can cross the system boundary and are therefore counted as forms of energy. They don't define a system.
What is an Ideal gas?
- A gas which can be defined by simple relationships (namely pV=mRT).
- Their internal energies are also a function of just their temperatures [U=f(T)]. So, U=mCv(T-T0)
How is Cv related to Cp?
Cv + R = Cp
What is a specific heat capacity?
Quantity of energy needed to raise the temperature of one unit mass by one degree. Bear in mind there are two versions of this depending on whether volume or pressure is constant. Usually measured in kJ kg-1 K-1 so be careful of units.
How is Enthalpy defined?
- A combination property of internal energy and boundary work. (H = U + pV)
- Also ΔH=mCpΔT if an ideal gas is used.
When can we use the Steady Flow Energy Equation? (SFEE)
When the system is open, a steady process occurs (inflow of fluid = outflow) and there is no friction.
A polytropic expansion is defined by pVn = constant. What does it mean if n = 0,∞,1 or Cp/Cv?
- n=0, constant pressure process (Isobaric)
- n=∞, constant volume process (Isochoric)
- n=1, constant temperature process (Isothermal)
- n≃1.4, Isentropic process
What is the equation for boundary work?
- δWb = -Fδs = -pδV
- Wb = -p(V2-V1) in a constant pressure process
- in a more general case