Week 4 Lecture 6 Flashcards
imposes restrictions on the direction of actual processes
second law of thermodynamics
Statements of the second law
- No apparatus can operate in such a way that it only effect is to convert heat absorbed by a system. It is impossible by a cyclic process to convert the heat absorbed by a system completely into work done by the system
- No process is possible which consists sole in the transfer of heat from one temperature level to a higher one
device that takes in energy by heat and operating in a cyclic process, expels a fraction of that energy by means of work
heat engine
A heat engine carries some working substance through a ____ process
cyclical
heat engine steps
- Working substance absorbs energy by heat from a higher temperature reservoir
- Work is done by the engine (Weng)
- Energy is expelled as heat to lower temperature reservoir (Qc)
First law equation
Qh =
Qh = Weng + Qc
efficiency equation
η =
net work output/heat input
efficiency equation
η =
|W eng|/|Qh|
efficiency equation
η =
|Qh|- |Qc|/|Qh|
efficiency equation
η =
1- (|Qc|/|Qh|)
High value of η meanings good conversion of …
Qh to Weng
Qc = 0
heat engine in inverse steps
- Energy is extracted from the cold reservoir (Qc) by work done on the heat pump (W)
- Energy is transferred to the hot reservoir (Qh)
- This process means the heat engine is running as a heat pump
Measured the performance of the heat pump running in cooling mode
Coefficient of performance (COP)
COP equation for cooling mode
|Qc|/W
COP equation for heating mode
|Qh|/W
depends only on the temperature between which it operates and is independent of the nature of the cyclic process
maximum efficiency
|Qh|/|Qc|=
Th / Tc
1 - Tc/Th =
η
|Qh|/ Th =|Qc|/ Tc
Reversible heat engine
|Qh|/ Th <|Qc|/ Tc
Irreversible heat engine
|Qh|/ Th >|Qc|/ Tc
Impossible heat engine
Entropy ____ with temperature
increases
amount or degree of disorder or randomness of a system
Entropy
If a system moves from a state of non equilibrium toward equilibrium, entropy is ____
created
Systems will spontaneously move from non equilibrium states to ________ but NOT VICE VERSA
equilibrium states
System will only spontaneously move toward states that ________ the total entropy in the universe
maximise
2nd Law of Thermodynamics
Entropy can be created by never destroyed
This makes entropy fundamentally different from energy and mass, which must always be conserved
A system undergoing a ____ process is always at equilibrium, and therefore does not create entropy
reversible
A system undergoing a reversible process is always at ____, and therefore does not create entropy
equilibrium
A system undergoing a reversible process is always at equilibrium, and therefore does not create ______
entropy
All ____ processes are driven by a system moving from a non-equilibrium state toward equilibrium and therefore all real processes create entropy and are irreversible
real
All real processes are driven by a system moving from a________ state toward equilibrium and therefore all real processes create entropy and are irreversible
non-equilibrium
All real processes are driven by a system moving from a non-equilibrium state toward equilibrium and therefore all real processes create ____ and are irreversible
entropy
All real processes are driven by a system moving from a non-equilibrium state toward equilibrium and therefore all real processes create entropy and are ______
irreversible
The entropy of the Universe should ultimately reach a maximum. At this time the universe will be at a state of uniform temperature and density. This state of perfect disorder implies no energy will be available for doing work.
Heat death of the universe
When energy is ____, Q is positive and entropy increases
absorbed
When energy is absorbed, Q is ____ and entropy increases
positive
When energy is absorbed, Q is positive and entropy ____
increases
When energy is ____, Q is negative and entropy decreases
expelled
When energy is expelled, Q is ____ and entropy decreases
negative
When energy is expelled, Q is negative and entropy _____
decreases
