Heat Engines and Refridgerators Flashcards
What (in simple terms) is a heat engine?
A device that converts heat into work
What are the three components of a steam engine?
Boiler (heat in), engine (work out), condenser (heat out).
What is a heat reservoir?
A system with a very large heat capacity so it can absorb or release hear without its own temperature changing.
What is the efficiency of a heat engine?
The ratio of the work done by the engine to the heat absorbed from the high temperature reservoir.
π =π/πβ
What is the efficiency of a heat engine given by in terms of Qc and Qh?
1-(Qc/Qh)
What does the second law of thermodynamics state for heat engines?
It is impossible for a heat engine working in a cycle to produce only the effect of absorbing heat from a single reservoir and performing an equivalent amount of work.
What (in simple terms) is a refridgerator?
A device that uses work to absorb heat from a cold reservoir and release heat to a hot reservoir. (Essentially a heat engine in the opposite direction).
What is the coefficient of performance of a refrigerator?
The ratio of the heat absorbed from the cold reservoir to the work done on the refrigerator.
COP = Qc/W
What does the second law of thermodynamics state for refridgerators?
It is impossible for a refrigerator working in a cycle to produce only the effect of absorbing heat from a cold object and releasing the same amount of heat to a hot object.
What are the steps of The Stirling Cycle?
1 β 2 isometric (constant π) heating at ππππ€
2 β 3 isothermal expansion at πβ
3 β 4 isometric cooling at πβππβ
4 β 1 isothermal compression at πc
What are the steps of the Stirling Cycle as a refrigerator?
Heat is absorbed from the cold reservoir in stage 1 β 4 and released to it in
stage 2 β 1. Heat is released to the hot reservoir in stage 3 β 2 and absorbed
from it in stage 4 β 3. Work is done on the system in stage 3 β 2 and by the
system in stage 1 β 4.
Is the Stirling Cycle reversible?
It is not reversible because the values f Qh and Qc are not the same when operating as a heat engine and as a refrigerator
What are the steps of the Carnot Cycle?
1 β 2 quasi-static adiabatic compression
2 β 3 isothermal expansion at πβ
3 β 4 quasi-static adiabatic expansion
4 β 1 isothermal compression at πc
What are the steps of the Carnot Cycle if it is used in heat engine?
heat is absorbed from the hot reservoir in stage 2 β 3 and released to the cold reservoir in stage 4 β 1. Work is done by the system in stages 2 β 3 and 3 β 4, and work is done on the system in stages 4 β 1 and 1β2.
What are the steps of the Carnot Cycle if it is used in Refrigerator?
Heat is absorbed from the cold reservoir in stage 1 β 4 and released to the hot reservoir in stage 3 β 2. Work is done on the system in stages 4 β 3 and 3 β 2, and work is done by the system in stages 2 β1 and 1 β 4.
Is the Carnot cycle reversible?
Yes, because each step is reversible.
What are the conditions for a process to be reversible?
1) No mechanical energy is transformed into internal thermal energy by friction or other dissipative forces
2) Heat transfer only occurs between objects with an infinitesimal difference in temperature
3) The process must be quasi-static
What does Carnotβs theorem state?
No engine operating between two given heat reservoirs can be more efficient than a reversible engine working between these two reservoirs.
What is the equation that relates Qc/Qh and Tc/Th?
Qc/Qh = Tc/Th
What does the third law of thermodynamics state?
It is impossible to reduce the temperature of any system or part of a system to absolute zero in a finite number of operations.
What equation defines the change in entropy of a system?
ds = dQrev/T
Where is the entropy of an isolated system at maximum?
At equilibrium
What is the equation of entropy as a function of the state?
ΞS = Cvln(T2/T1) + nRln(V2/V1)
