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22 Cards in this Set
- Front
- Back
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When an imbalance exists between 2 systems, there is an opportunity for developing work that would be irrevocable lost if the systems were allowed to come into equilibrium in an uncontrolled way. |
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Primary Aspects of the 2nd Law |
Provides the means to 1) Predict the direction of a process 2) Establish Conditions for Equilibrium 3) Determine the best theoretical performance of a cycle, engine, or other device 4) Evaluate quantitatively the factors that preclude the attainment of the best theoretical performance. |
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Additional Aspects of the 2nd Law |
5) Define a temperature scale independant of the properties of and thermometric susbstance
6) Develop the means for evaluating properties such as u and h in terms of more readily obtainable properties. |
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Clausius Statement |
"It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body." |
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Thermal Reservoir |
A special kind of system that always remains at constant temperature even though energy is added or removed by heat transfer. |
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Irreversible process |
A process where the system and all parts of its surrounding cannot be exactly restored to their respective initial states after the process has occured. |
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Reversible Process |
A process where both the system and surrounding can be returned to their initial states. |
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The 2nd law can be used to determin whether a given process is reversible or irreversible. |
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What are common irreversibilities? |
1) Heat transfer through a finite temperature difference (Q & ΔT) 2) Unrestrained expansion of a gas or liguid to a lower pressure (-ΔP with no control) 3) Spontaneous checmical reaction 4) Spontaneous mixing of matter at different compositions or states 5) Friction - sliding friction as well as friction in the flow of fluids 6) Electric current flow through a resistance 7) Magnetization or polarization with hysteresis 8) Inelastic deformation |
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Internal Irreversibilities |
Irreversibilities within a system |
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External Irreversibilities |
Irreversibilities that occur within the surroundings |
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Heat Engine |
Device designed for the purpose of converting other forms of energy to work |
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What can a heat engine be charaterized by? |
1) Recieve heat from high temperature source Q+ = QH comes from TH 2) Convert part of this heat to work W+ = Wout = Wnet 3) Reject the remaining waste heat to a low temperature sink Q- = QL goes to TL 4) They operate on a cycle inital conditions = final conditions |
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Sink |
A thermal reservoir that ABSORBS thermal energy |
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Source |
A thermal reservoir that SUPPLIES thermal energy |
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Combustion Engines |
Engines that operate a mechanical cycle but not a thermodynamic cycle because the working fluid does not undergo a complete cycle |
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Internally Reversible Process |
No Internal Irreversibilities Consists of a series of equilibrium states |
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Assumption applied to Thermal Reservoirs |
That there are NO internal irreversibilities present within a thermal resevoir.
Therefore, every process of a thermal reservoir is an internally reversible process. |
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Working Fluid |
The fluid to and from which heat and work are transferred while undergoing a cycle in heat engines and other cyclic devices. |
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Thermal Efficiency of a Heat Engine |
Nth = [Net Work Output] / [Total Heat Input]
= Wnet,out / Qin |
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Relationship between Work and Thermal Energy of a cycle |
W = QH-QL
The thermal energy from the hot reservoir enables work to be produced while spitting out any unuseable thermal energy. |
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Heat Pump |
A device that transfers heat from a low temperature medium to a high temperature medium. The focus is on the heat being delivered and this process requires work to be inputted. |
