Week 4 (Heat Engines, Heat Pumps and the Second Law) Flashcards
Heat Engines, Heat Pumps and the Second Law of Thermodynamics
How does heat engines work
- They receive heat fro a high-temperature source
-They convert part of his heat to work
-They reject the remaining waste heat to a low-temperature sink
-They operate on a cycle
Heat Engine Equation:
Wnet,out = Qin - Qout
(first law of thermodynamics)
Thermal Efficiency equation:
Net work output / Total heat input
According to the 2nd Law of Thermodynamics
A heat-engine cycle cannot be…
A heat-engine cycle cannot be completed without rejecting some heat to a low-temperature sink
The Second Law of Thermodynamics:
Kelvin–Planck Statement
It is impossible to construct a machine which will operate in a
cycle, extract heat from a reservoir, and do an equivalent
amount of work on the surroundings.
To summarise what does the first and second law say, that disproves the 2 types of Perpetual-motion machine
`First Law says
“You can never get out more than you put in”
Second Law says
“You will always get out less”
The Second Law of Thermodynamics:
Clausius Statement
It is impossible to construct a device that operates in a cycle and
produces no effect other than the transfer of heat from a lower temperature body to a higher-temperature body.
What does COP stand for
Coefficient of Performance
How does a refrigerator or heat pump work?
Have cold (refrigerated) space,T sub L, that you want to keep cool
Q sub L is the desired output
Compressor
Wnet,in. the required input
Q sub H the rejected (extracted heat) to the already warmer environment. T sub H > T sub L
Reversible Process
a process that can be reversed without leaving any
trace on the surroundings. That is, both the system and the surroundings are returned to
their initial states at the end of the reversible process. This is possible only if the net heat
and net work exchange between the system and the surroundings is zero for the
combined (original and reverse) process. Processes that are not reversible are called
irreversible processes
Examples of reversible processes (2)
Frictionless pendulum
Quasi-equilibrium expansion and compression of a gas
Examples of irreversible processes (3)
expansion and compression of a gas with friction
Soda can warming up at room temp after being taken out refrigerator
Higher pressure gas diffusing in a chamber (not gonna reverse back to its original state)
Carnot principles meaning
and what are they (2) in relation to the second law of thermodynamics
That the following principles are true based of the assumption that something else is true (in this case second law of thermodynamics).
1) the efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.
2) The efficiencies of all reversible heat engines operating between the same two reservoirs are the same
