Thermodynamics Final Exam Review
Define thermodynamics
'heat in motion' study of energy (how it is transferred, its interactions etc)
What are the three ways to determine changes in internal energy?
1. Property tables 2. By using average specific heats 3. By using cp and cv as functions of temperature and performing the integrations
What are the 4 characteristics of heat engines?
1. They recieve heat from Th source Qh. 2. Operate on a cycle. 3. Reject Ql into Tl. 4. Convert part of Qh into work.
What are the 2 Carnot Principles?
1. n of a real heat engine is always less than n of a reversible one operating between the same T limits. 2. n of all reversible heat engines operating between the same T limits are the same.
Define a closed system.
A closed system is one where no mass is exchanged across the boundary separating the system and its surroundings. Energy (in the form of heat transfer and work) can cross the boundaries of a closed system.
What is a heat engine?
A device that converts heat to work.
What is a steady-flow process?
A process during which a fluid flows through a CV steadily, that is its properties don't change with time.
What is a pure substance?
A pure substance is a substance of fixed and uniform chemical composition throughout
What is a quasi-equilibrium process?
A quasi-equilibrium process is a process occurring so slowly that the system remains infinitesimally close to an equilibrium state at all times (for example slow compression). It is an ideal process.
What is a steady-flow process?
A steady-flow process is one where the fluid flows through a control volume (open system, since there is no flow in closed systems) steadily - fluid properties do not change with time (but they may change with position).
Define an extensive property and give two examples.
An extensive property is a property that depends on the size or amount of the system. For example, volume, total mass.
What does it mean for a process to be isochoric?
An isochoric process occurs at constant specific volume.
What is the first law of thermodynamics and what equation does it follow?
Conservation of energy. Energy can neither be created nor destroyed during a process, it can only change forms. Ein - Eout = ΔEsystem
What is the difference between a refrigerator and a heat pump?
Different Objective, refrigerator removes Ql from Tl and a heat pump rejects Qh into Th.
What is the total energy of a flowing fluid?
E=U+PE+KE+PV
What is flow work(flow energy)?
Energy/Work requirred to move/push the fluid into or out of the control volume (necessary to maintain a continuous flow).
Define an equilibrium state
Equilibrium is a state of balance, there are no unbalances potentials or driving forces
Define an equilibrium state.
Equilibrium is a state of balance. There are no unbalanced potentials or driving forces.
For what type of substance are the specific heats (cv and cp) equal to each other?
For an incompressible substance
State the Clausius statement of the 2nd law of thermodynamics.
H is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from TL to TH.
What does enthalpy physically represent?
H=U+PV, it represents the capacity of the substance to release heat and to do non-mechanical work.
How does the heat transfer into the system affect the entropy of the system?
Heat transfer into the system will increase its entropy.
What is the zeroth law of thermodynamics?
If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
Will the entropy of the working fluid of the ideal Carnot cycle increase, decrease, or remain the same during the isothermal heat addition process?
Increase (heat ADDITION process)
Define an intensive property and give two examples.
Intensive property does not depend on the amount of mass in the system (doesn't depend on the size of the system). Temperature, pressure, density.
Is the relation ∆𝑢 = 𝑚𝑐𝑣,𝑎𝑣𝑔∆𝑇 restricted to constant-volume processes only, or can it be used for any kind of process of an ideal gas?
It can be used for any process for an ideal gas.
What is a polytropic process?
It is a process during which the product of the pressure of the gas and the volume raised to the nth power remains constant.
What does it mean for a process to be isothermal?
It is a process during which the temperature remains constant.
Is it possible to design a heat engine that does not reject any waste heat? Why?
It is not possible. This would violate the 2 nd law of thermodynamics (Kelvin-Planck statement).
What is 'sensible energy'?
It is the portion of the internal energy of a system associated with the kinetic energies of the molecules.
What does it mean when we say that heat and work are path functions?
It means that they depend on the path the system took to get from state 1 to state 2.
What is kinetic energy and how is it calculated?
Kinetic energy is the energy the system possess as a result of its motion relative to some outside reference frame. KE=1/2 mv^2
What is a compressed liquid?
Liquid that is not close to vaporization.
What is an internally reversible process?
No irreversibilities exist within the boundaries of the system.
Is it possible for a cup of hot coffee to get even hotter in a cooler room? Why?
No, it is not possible, it would violate the second law of thermodynamics.
Is there any entropy generation during a reversible process?
No, the entropy does not change during a reversible process; therefore, there is no entropy generation
Is it possible to develop an actual heat engine cycle that is more efficient than a Carnot cycle operating between the same temperature limits?
No. This would violate the 1st Carnot Principle.
What is an open system?
Open system is a selected region of space (control volume) where both mass and energy (heat transfer, work) can cross the boundary of the control volume.
What is the difference between gage pressure and absolute pressure?
Pgage = Pabs -Patm
What is the triple point?
Point at which all 3 phases: Liquid, solid, and gas coexist at equilibrium.
What is potential energy and how is it calculated?
Potential energy is the energy that a system possesses as a result of its elevation in a gravitational field. PE=mgz
What is the difference between pound-mass and pound-force?
Pound-mass is a unit of mass, and pound-force is a unit of weight (force).
DELETE: What is a polytropic process?
Process during which P and V for the substance are related by PV
What is the quality of a pure substance?
Quality, x, is the ratio of mass of saturated vapor in the saturated vapor mixture to the total mass of the mixture.
A pure substance has the quality of 0. What state is this substance in?
Saturated liquid
Give the definition of cp in equation and words.
Specific heat at constant pressure is the amount of energy required to raise the temperature of a unit mass of a substance by one degree while keeping the pressure constant.
What is static energy?
Static energy is energy stored in the system. It is the total energy of a system that is stored.
Most of the energy generated in the engine of a car is rejected to the air by the radiator through the circulating water. Should the radiator be analyzed as a closed system or as an open system?
System = radiator. It is an open system, since we have mass crossing the boundary. (define your system - if you include the tubing and engine in the system, you could consider it to be closed. In this case, the questions is specifically about the radiator, so the system is open).
Under what conditions can we treat a gas as an ideal gas?
Tgas>Tgiven, Pgas<Pgiven
Is it possible for a heat engine to operate without rejecting any waste heat to a low T reservoir? Explain your reasoning.
That is not possible. H violates the Kelvin-Plack statement of the 2nd law of thermodynamics.
Give the definition of cv.
The energy required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant.
What is the increase of entropy principle?
The entropy of an isolated system during a process always increases (or remains constant for a reversible process), it never decreases.
What is the state postulate?
The state of a simple compressible system is completely specified by two independent, intensive properties.
Why are throttling valves considered adiabatic?
There is not enough time for the heat exchange.
What is a superheated vapor?
Vapor that is not about to condense.
Provide the energy equation for an adiabatic compressor with one inlet and two outlets. Neglect kinetic and potential energies.
Win+m1h1=m2h2+m3h3
What is the boundary work for a constant volume process?
Zero
Name the specific forms of energy associated with a flowing fluid.
h(u+PV), ke, pe
What is quality? Does it have any meaning in the superheated vapor region?
x = fraction of saturated vapor in the saturated liquid-vapor mixture. X has no meaning in the superheated vapor region.