Chapter 6: Second Law of Thermodynamics Flashcards
what are the major uses of 2nd law of thermo?
- identify the direction of processes
- asserts that energy has quality as well as quantity
- used in determining the theoretical limits for the performance of commonly used engineering systems
- process must satisfy both laws of thermo to proceed
what is a thermal energy reservoir?
body with large thermal energy capacity that can supply or absorb heat with no noticeable change in temperature
what is a source reservoir?
reservoir that supplies energy as heat
what is a sink reservoir?
reservoir that absorbs energy as heat
what is a heat engine?
device that converts heat to work. receives heat from high-temp source, converts part of heat to mechanical work, rejects the remaining waste heat to a low-temp sink
- operates on a cycle
- work can always be converted to heat directly and completely, but the reverse is not true
- heat can never be converted to work (WITHOUT special conditions)
what is a closed cycle heat engine?
Ex: steam power plant
Due to energy source, heat enters system through boiler, the the water vapour will go into a turbine where part of energy will leave system as electrical work, then the remaining liquid-vapour water will go into a condenser where part of the energy will leave as heat, lastly the liquid water will go into a pump with mechanical work input where it will turn into compressed water
- cycle continues
what is an open heat engine?
Ex: jet engine
Air goes into a compressor, where work will be required as input, then it goes into a combustion chamber where fuel is require as input (this acts as heat entering), then it will go in a turbine where part of energy will leave as heat and the other part as work.
what is heat engine thermal efficiency?
net work output over total heat input
where QH = heat transfer between cyclic device and high temp reservoir at high temp
QL = heat transfer between cyclic device and low temp reservoir at low temp
what is the kelvin-plank statement?
it is impossible for a cyclic device to exchange heat with a single reservoir and produce a net amount of work. Hence, no heat engine can have a thermal efficiency of 100%.
A heat engine cycle cannot be completed without rejecting some heat to a low-temperature sink
what are refrigerators?
device that transfers heat from low-temp medium to a high-temperature (reverse of natural process)
- cyclic device
what is a refrigerant?
working fluid used in the refrigeration cycle
what is the coefficient of performance (COP)?
efficiency of a refrigerator is expressed in terms of COP
COP = desired output / required input
what is clauses statement?
it is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from lower-temp body to a higher-temp body
- hence, a refrigerator cannot operate unless it is driven by an external power source (electric motor)
what is a reversible process?
an ideal process that can be completely reversed with no net effect on its surroundings
- final state = initial state of system & surroundings
- maximum efficiency corresponds to a cycle consisting of a series of idealized reversible processes
- easy to analyze & serve as idealized models
what is an irreversible process?
real process where if the system and surrounding cannot be returned to their respective original states without leaving any trace on the surroundings
- final state = initial state of system
- final state does not equal initial state of surroundings
- all processes in nature are irreversible
what are irreversibilities?
factors that cause a process to be irreversible
- friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temp difference, electric resistance, inelastic deformation of solids, and chemical reactions
what is an internally reversible process?
no irreversibilities occur within the system boundary during the process
- quasi-equilibrium process
what is an externally reversible process?
no irreversibilities occur outside the system boundary
- heat transfer across infinitely small temp difference
what is a totally reversible or simply reversible process?
no irreversibilities within the system or its surroundings
- a totally reversible process involves no heat transfer through a finite temperature difference, no non quasi-equilibrium changes, and no friction or other dissipative effects
what is the Carnot cycle?
- best known reversible cycle for a closed system heat engine (such as piston cylinder device)
4 reversible processes:
1) reversible isothermal expansion (heat transfer from energy source)
2) reversible adiabatic expansion
3) reversible isothermal compression. (heat transfer to energy sink)
4) reversible adiabatic compression
what is the reverse Carnot cycle?
Carnot Refrigeration Cycle (totally reversible cycle)
4 reversible processes:
1) reversible adiabatic expansion
2) reversible isothermal expansion (add heat)
3) reversible adiabatic compression
4) reversible isothermal compression (remove heat)
what is the Carnot heat engine?
the most efficient heat engines operating between the same high and low temperature reservoirs
