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What is the thermal efficiency of a reversible heat engine?

  • The ratio of work output to heat input (correct)
  • The ratio of heat rejected to work input
  • The ratio of energy lost to energy gained
  • The ratio of heat absorbed to temperature difference

    • What characterizes the performance of heat pumps?

  • They transfer heat using work input (correct)
  • They produce heat without energy input
  • They always perform better than refrigerators
  • They cannot operate in reverse

    • Which of the following is true for refrigerators?

  • They convert heat from a hot reservoir to a cold one
  • Their efficiency is measured by the ratio of heat removed to work input (correct)
  • They can produce more heat than they remove
  • They require no work to operate

    • In the context of thermodynamics, which statement is true regarding the first law?

    <p>Energy is conserved but processes can be unfeasible</p> Signup and view all the answers

    What is a common misconception regarding the cooling of hot coffee in a cooler room?

    <p>Heat flows from the coffee to the cooler air spontaneously</p> Signup and view all the answers

    Which formula is typically used to calculate the coefficient of performance for refrigerators?

    <p>$COP = \frac{Q_{c}}{W}$</p> Signup and view all the answers

    What ensures that a process can occur according to thermodynamic principles?

    <p>Satisfaction of both the first and second laws</p> Signup and view all the answers

    What happens when heat is transferred from a cooler room to hot coffee?

    <p>It could happen with external work input</p> Signup and view all the answers

    What is the thermal efficiency of the heat engine when it violates the Kelvin–Planck statement?

    <p>100%</p> Signup and view all the answers

    What does the refrigerator do with the heat removed from the low-temperature reservoir?

    <p>It rejects it to the high-temperature reservoir.</p> Signup and view all the answers

    Which statement describes the violation of the Clausius statement?

    <p>Heat is transferred from a colder body to a warmer one without external input.</p> Signup and view all the answers

    What is the net amount of heat received by the high-temperature reservoir during this process?

    <p>QL</p> Signup and view all the answers

    How does the combination of the heat engine and the refrigerator function in this scenario?

    <p>The heat engine powers the refrigerator to perform work.</p> Signup and view all the answers

    What results from a violation of the Kelvin–Planck statement?

    <p>The Clausius statement is violated.</p> Signup and view all the answers

    In the described heat engine-refrigerator combination, what is the role of the work W?

    <p>To power the refrigerator using energy extracted.</p> Signup and view all the answers

    What is the main implication of the combination of the heat engine and refrigerator as described?

    <p>It illustrates the second law of thermodynamics being violated.</p> Signup and view all the answers

    What does the Kelvin–Planck statement of the second law of thermodynamics imply about heat engines?

    <p>They must exchange heat with both high and low-temperature sources.</p> Signup and view all the answers

    Why is it impossible for a heat engine to achieve 100 percent efficiency?

    <p>It is a fundamental limitation of thermodynamic principles.</p> Signup and view all the answers

    What must happen for a power plant to operate effectively?

    <p>The working fluid must exchange heat with both a source and a sink.</p> Signup and view all the answers

    Which of the following statements accurately reflects the limitations of heat engines?

    <p>Heat engines cannot operate without transferring heat to a low-temperature sink.</p> Signup and view all the answers

    What is the primary reason heat is transferred from hot to cold mediums?

    <p>It's a natural process governed by the second law of thermodynamics.</p> Signup and view all the answers

    What occurs when a heat engine exchanges heat with its environment?

    <p>It must release some heat to a lower temperature sink.</p> Signup and view all the answers

    How is the maximum thermal efficiency of a heat engine determined?

    <p>It varies based on the temperatures of the heat reservoirs.</p> Signup and view all the answers

    Which of the following is true regarding the operation of refrigerators and heat pumps?

    <p>They require work to transfer heat against its natural direction.</p> Signup and view all the answers

    What is the COP (Coefficient of Performance) for air-source heat pumps at design conditions?

    <p>3.0</p> Signup and view all the answers

    In which climate are air-source heat pumps considered inappropriate?

    <p>Cold climates</p> Signup and view all the answers

    How much more efficient are geothermal heat pumps compared to air-source heat pumps?

    <p>45 percent</p> Signup and view all the answers

    What is the depth requirement for burying pipes for geothermal heat pumps?

    <p>1 to 2 m</p> Signup and view all the answers

    What mode allows a window air-conditioning unit to function as a heat pump?

    <p>By reversing its operation</p> Signup and view all the answers

    What is the COP of ground-source heat pumps?

    <p>4.0</p> Signup and view all the answers

    What is a key disadvantage of geothermal heat pumps compared to air-source heat pumps?

    <p>Higher installation cost</p> Signup and view all the answers

    What common function do air conditioners and heat pumps share?

    <p>They both discharge heat to the outside.</p> Signup and view all the answers

    What is the primary function of a steam power plant in terms of energy conversion?

    <p>To convert thermal energy into work</p> Signup and view all the answers

    Which process occurs outside the engine in an external-combustion engine?

    <p>Combustion of fuel</p> Signup and view all the answers

    What does Qin represent in a steam power plant?

    <p>The heat absorbed by the steam from the boiler</p> Signup and view all the answers

    What happens to the heat rejected (Qout) in a steam power plant?

    <p>It is expelled to the atmosphere or a body of water</p> Signup and view all the answers

    Which of the following quantities is considered a positive value in the energy interactions of a steam power plant?

    <p>Work output from steam expansion</p> Signup and view all the answers

    Which type of engine provides a clearer example of an external-combustion engine?

    <p>Steam power plant</p> Signup and view all the answers

    What does Wout signify in the context of a steam power plant?

    <p>Work done by the steam as it expands</p> Signup and view all the answers

    What is the role of Win in the steam power plant process?

    <p>Energy needed to compress water to boiler pressure</p> Signup and view all the answers

    What does a closed system undergoing a cycle indicate about its internal energy change?

    <p>It remains constant throughout the cycle.</p> Signup and view all the answers

    What is the relationship between net work output and heat input in a heat engine?

    <p>Net work output is less than heat input.</p> Signup and view all the answers

    In the formula for thermal efficiency, what does Qin represent?

    <p>Heat input to the system.</p> Signup and view all the answers

    What aspect of a heat engine does thermal efficiency measure?

    <p>Fraction of heat converted to work.</p> Signup and view all the answers

    If Qout is the energy wasted during the cycle, which statement is true?

    <p>Qout is never zero in real heat engines.</p> Signup and view all the answers

    Considering ηth,1 = 20% and ηth,2 = 30%, what can be inferred about the performance of these heat engines?

    <p>ηth,2 is more efficient than ηth,1.</p> Signup and view all the answers

    What is the primary input needed for a heat engine to produce work?

    <p>Amount of heat supplied.</p> Signup and view all the answers

    Why is some of the heat input converted to work in a heat engine and some is wasted?

    <p>Both A and B are correct.</p> Signup and view all the answers

    Study Notes

    Introduction to Thermodynamics

    • Thermodynamics is the study of energy and its transformations.
    • It deals with the relationships between heat, work, and other forms of energy.

    First Law of Thermodynamics

    • Energy is conserved.
    • The total energy of an isolated system remains constant.
    • Energy can be transferred as heat or work.
    • The first law of thermodynamics does not specify the direction of heat transfer, and it gives no indication about the spontaneity or feasibility of a process.

    Second Law of Thermodynamics

    • Processes occur in a certain direction, not in any direction.
    • Energy has quality as well as quantity. The first law is concerned with the quantity, and the second law focuses on the quality.
    • The second law defines the concept of entropy and irreversibilities.
    • Useful work output is obtained from a system using two different reservoirs.

    Kelvin-Planck statement

    • It is impossible to construct a device that operates on a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body.
    • In other words, a heat engine cannot have an efficiency of 100 percent with a single heat reservoir.

    Clausius Statement

    • It is impossible to construct a device that operates on a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body.
    • In other words, a refrigerator cannot operate unless its compressor is driven by an external power.

    Reversible Processes

    • A reversible process is a process that can be reversed without leaving any trace on the surroundings.
    • The system and the surroundings are returned to their initial states after the reverse process.
    • There is no irreversibilities.

    Irreversible Processes

    • A process that is not reversible.
    • All processes occurring in nature are irreversible.
    • Irreversibilities include:
    • Friction
    • Unrestrained expansion
    • Mixing of two fluids
    • Heat transfer through a finite temperature difference
    • Electrical resistance
    • Inelastic deformation of solids
    • Chemical reactions.

    Carnot Cycle

    • The Carnot cycle is a theoretical cycle that consists of four reversible processes.
    • The Carnot cycle is the most efficient cycle between two specified temperature limits.

    Carnot Theorem

    • The efficiency of a reversible engine is always greater than that of a non-reversible engine.
    • The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.

    Concepts of Heat Engines, Refrigerators, and Heat Pumps

    • Heat engines are cyclic devices that convert heat to work.
    • Refrigerators remove heat from a cold medium to a warmer medium using work.
    • Heat pumps do the opposite of a refrigerator by transferring heat from a cold medium to a warmer medium.

    Coefficient of Performance

    • The efficiency of a refrigerator or heat pump is expressed in terms of a 'coefficient of performance'.

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