2nd Law Flashcards
(21 cards)
Which of the following renders a process irreversible?
Unrestrained expansion
Mixing of two fluids
Friction
Heat transfer across a finite temperature difference
A refrigerator has a COP of 1.5. That is, the refrigerator removes 1.5 kWh of energy from the refrigerated space for each 1 kWh of electricity it consumes. Is this a violation of the first law of thermodynamics?
No. The refrigerator captures energy from a cold medium and carries it to a warm medium. It does not create it.
A heat engine is said to absorb heat from a source at 1000 K at a rate of 250 kW and rejects 50 kW of it to a sink at 300 K. This heat engine is:
Impossible according to the second law of thermodynamics, which states that no heat engine can be more efficient than a Carnot engine operating between the same two temperatures.
What is a carnot cycle
The Carnot cycle is made up of reversible processes. The Carnot cycle is the most efficient cycle operating between two specified temperature limits.
A process of a closed system that violates the second law of thermodynamics necessarily violates the first law of thermodynamics.
False because First Law of Thermodynamics:
energy cannot be created or destroyed, only transformed from one form to another. Second Law of Thermodynamics: the entropy of an isolated system can never decrease over time. It governs the direction of processes and implies that natural processes tend to move towards a state of maximum entropy.
A process can violate the second law without violating the first law. For example, if a process decreases the entropy of a closed system (violating the second law), it does not necessarily mean that energy is not conserved.
Baseboard heaters are basically electric resistance heaters and are frequently used in space heating. A homeowner claims that their 7-year-old baseboard heaters have a conversion efficiency of 100 percent. Is this claim in violation of the Second Law of Thermodynamics?
No. Because 100% of the work can be converted to heat.
Consider the air-conditioning process during which heat is absorbed from the indoors at 23degreeC and is discarded to the outdoors at 40degree C. Identify the heat source and the heat sink for this process.
Indoors: Source, Outdoors: Sink
A refrigerator is claimed to absorb 5 kJ of heat from the refrigerated space at 200 K and rejects 10 kJ of heat to a medium at 300 K. This refrigerator is:
Irreversible because it operates with a COP lower than the theoretical maximum (Carnot COP) and involves irreversibilities that prevent it from achieving ideal performance.
An experimentalist claims to have raised the temperature of a small amount of water to 150°C by transferring heat from high pressure steam at 120°C. Is this a reasonable claim?
No because Heat naturally flows from a higher temperature to a lower temperature. Heat cannot flow from the steam at 120°C to the water at 150°C because the water is at a higher temperature than the steam.
Facts about the second law of thermodynamics
The second law is used in determining the theoretical limits for the performance of engineering systems.
The second law asserts that energy has quality.
A process cannot occur unless it satisfies both the first and the second laws of thermodynamics.
Every process consistent with the conservation of energy and conservation of mass principles can actually occur in nature. Why is this statement false
Even if a process conserves energy and mass, it may not be feasible if it decreases entropy or violates the directionality imposed by the second law
The Kelvin–Planck and Clausius statements of the second law of thermodynamics are what?
Equivalent, because a violation of one statement implies the violation of the other.
Consider the energy generated by a TV set. Can the surrounding air in the room that houses the TV set be a suitable choice for a thermal energy reservoir?
Yes
What things can be modelled as a thermal energy reservoir?
Atmospheric air
Large rivers
Large lakes
Industrial furnaces
Can a heat engine convert all the heat it receives to useful work
No because the second law of thermodynamics and practical inefficiencies ensure that no heat engine can convert all the heat it receives into useful work. Some energy will always be lost as waste heat, making 100% conversion impossible.
Facts about reversible processes
Reversible processes can be viewed as theoretical limits for the corresponding irreversible ones.
Reversible processes serve as idealized models to which actual processes can be compared.
When does a turbine deliver the most power?
when the process is reversible.
A pump consumes the least power when…
The process is reversible.
Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir?
Yes. Because the temperature of the oven remains constant no matter how much heat is transferred to the potatoes
Facts about irreversible processes
Expansion of a fluid in a throttling valve is an irreversible process.
The larger the irreversibilities in a power plant, the smaller the thermal efficiency.
The smaller the electrical resistance during current flow in a wire, the smaller the irreversibility.
Facts regarding the Kelvin scale?
On the Kelvin scale, the temperature ratios are related to the heat transfer ratios of reversible heat engines.
On the Kelvin scale, temperatures vary between zero and infinity.
The magnitudes of temperature change units on the Kelvin and Celsius scales are identical.
