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Thermo: Chapters 1-3
Terms in this set (60)
the ability to cause change
Conservation of Energy Principle (1st Law of Thermodynamics)
energy can neither be created nor destroyed
second law of thermodynamics
Processes occur in the direction of decreasing quality of energy
(macroscopic) study of thermodynamics that does not require a knowledge of the behavior of individual particles
(Microscopic) based on average behaviors of large groups of individual particles
Energy that can be transferred from one system to another due to temperature difference
Heat Transfer Determines
1. rate of heat transfer
2. times of heating and cooling
3. variation of temperature
First Law of Heat Transfer
Rate of energy transfer into a system is equal to the rate of increase in energy of that system
Second Law of Heat Transfer
Heat is transferred in the direction of decreasing temperature
Rate of Heat Transfer is Dependent of
magnitude of temperature gradient
Heat Transfer only requires
Thermodynamics vs. Heat Transfer
-quantity of energy transferred
-deals w/ equilibrium state
-Length of process
-Systems Lack thermal equilibrium
The study of fluids and how they behave when at rest and when moving
Closed System (Control Mass)
A fixed amount of mass, and no mass can cross its boundary
Open System (Control Volume)
Both mass and energy can cross the boundary of a control volume
The boundaries of a control volume. It can be real or imaginary
Those that are independent of the mass of a system
Ie. temp, pressure, density
a property that depends on the extent or size of a system
Ie. Mass and Volume
The State Postulate
The state of a simple compressible system is completely specified by two independent, intensive properties.
Quasistatic or quasi-equilibrium process
When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times.
a process that occurs at constant temperature
A process that occurs at a constant pressure
Isochoric or Isometric Process
specific volume remains constant
Process which the initial and final states are identical
A process during which a fluid flows through a control volume steadily
Ie. boilers, refrigerators, pumps, turbines
Zeroth Law of Thermodynamics
objects are in thermal equilibrium only when their temperatures are equal, even if they are not in contact
The temperature at which ice melts or water freezes (0c or 32f)
boiling point of water - liquid and vapor are in equilibrium (100c or 212f)
Pressure to a confined fluid increases the pressure throughout by the same amount
The energy that a system possesses as a result of its motion
Energy that a system possesses as a result of its elevation in a gravitational field
Sum of all microscopic forms of energy
closed systems whose velocity and elevation remain constant
the portion of the internal energy of a system associated with the kinetic energies of the molecules
The internal energy associated with the phase of a system
Phase change & Moisture
2 Forms of Energy transfer
No heat transfer (well insulated)
3 mode of heat transfer
3. thermal radiation
transfer of internal energy by microscopic diffusion & particle collision
transfer of internal energy by conduction and advection (fluid movement)
electromagnetic radiation emitted by matter at temperature greater than absolute zero
energy transfer associated with a force acting through a distance
Work done BY a system
Work done ON a system
Both heat and work are recognized at the boundary of a system as they cross the boundary
Why is heat transfer a non-equilibrium phenomenon?
Heat transfer is a non-equilibrium phenomena since in a system that is in equilibrium there can be no temperature differences and thus no heat flow.
Can there be any heat transfer between two bodies that are at the same pressure but at different temperatures?
Yes there is a temperature difference (driving force of heat transfer)
A large fraction of the thermal energy generated in the engine of a car is rejected to the air by the radiator through circulating water. Should the radiator be analyzed as a closed system or as an open system? Explain.
Open system since mass is entering and leaving the system.
A can of soda at room temperature is put into the refrigerator so that it will cool. Would you model the can of soda as a closed system or open system? Explain.
Closed system since no mass is crossing the boundaries.
Is the weight of a system an extensive or intensive property?
When analyzing the acceleration of gases as they flow through a nozzle, what would you choose as your system? What type of system is this?
When analyzing the acceleration of gases as they flow through a nozzle, the proper choice for the system is the volume within the nozzle, bounded by the entire inner surface of the nozzle and the inlet and outlet cross-sections. This is a control volume since mass crosses the boundary.
What is a quasi-equilibrium's importance in engineering?
Many engineering processes can be approximated as being quasi-equilibrium. The work output of a device is maximum and the work input to a device is minimum when quasi-equilibrium processes are used instead of non-quasiequilibrium processes.
You are trying to understand how a reciprocating air compressor (a piston-cylinder device) works. What system would you use? What type of system is this?
The system is taken as the air contained in the piston-cylinder device. This system is a closed or fixed mass system since no mass enters or leaves it.
How are heat, internal energy, and thermal energy related to each other?
Heat is thermal energy crossing the boundary. Thermal energy is internal energy due to a temperature difference. (Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life.)
When is the energy crossing the boundaries of a closed system heat and when is it work?
Heat if energy crossing is due to a temperature difference. Work is for all other energy crossing the boundary.
A gas in a piston-cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?
A room is heated by an iron that is left plugged in. Is this a heat or work interaction? Take the entire room, including the iron, as the system
A room is heated as a result of solar radiation coming in through the windows. Is this a heat or work interaction for the room?
Heat due to temperature difference between sun and room
What are the different mechanisms for transferring energy to or from a control volume?
Heat, Work, Mass transport
Lifting a weight to a height of 20 m takes 20 s for one crane and 10 s for another. Is there any difference in the amount of work done on the weight by each crane?
Work is the same but power is different
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