Thermodynamics Chapter 6: Second Law
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Questions and Answers

What is the thermal efficiency of ordinary spark-ignition automobile engines?

  • 20 percent
  • 35 percent
  • 25 percent (correct)
  • 30 percent

    • Which type of engine has the highest thermal efficiency mentioned?

  • Diesel engine
  • Large combined gas-steam power plant (correct)
  • Large gas-turbine plant
  • Ordinary spark-ignition engine

    • What percentage of energy supplied to the most efficient heat engines ends up as waste?

  • One-quarter
  • One-third
  • Two-thirds
  • One-half (correct)

    • What does QH represent in the context of heat engines?

    <p>Heat transfer to high temperature medium</p> Signup and view all the answers

    What is a primary function of the condenser in a steam power plant?

    <p>To reject waste heat</p> Signup and view all the answers

    Why is it not feasible to eliminate the condenser from a steam power plant?

    <p>It is essential for cooling the system</p> Signup and view all the answers

    Which factor is most likely to limit the thermal efficiency of work-producing devices?

    <p>Energy conversion processes</p> Signup and view all the answers

    Which of the following statements about thermal efficiencies of heat engines is correct?

    <p>They vary significantly with design and usage.</p> Signup and view all the answers

    What limitation does the First Law of Thermodynamics have compared to the Second Law?

    <p>It cannot identify the direction of processes.</p> Signup and view all the answers

    What does the Second Law of Thermodynamics assert about energy?

    <p>Energy has both quality and quantity.</p> Signup and view all the answers

    Which of the following is a major use of the Second Law of Thermodynamics?

    <p>Identifying the direction of processes.</p> Signup and view all the answers

    How does the Second Law of Thermodynamics relate to chemical reactions?

    <p>It predicts the degree of completion of chemical reactions.</p> Signup and view all the answers

    What is a thermal energy reservoir?

    <p>A hypothetical body with large thermal energy capacity.</p> Signup and view all the answers

    What are heat engines and refrigerators designed with respect to?

    <p>The theoretical limits as per the Second Law of Thermodynamics.</p> Signup and view all the answers

    Which statement differentiates the focus of the First Law from the Second Law of Thermodynamics?

    <p>The First Law disregards energy degradation.</p> Signup and view all the answers

    Which of the following descriptions is true regarding the role of the Second Law?

    <p>It incorporates the predictability of energy quality.</p> Signup and view all the answers

    What does the Clausius Statement of the Second Law of Thermodynamics assert?

    <p>It is impossible to transfer heat from a lower-temperature body to a higher-temperature body without work.</p> Signup and view all the answers

    Why must a refrigerator's compressor have an external power source?

    <p>To enable it to reverse the natural direction of heat flow.</p> Signup and view all the answers

    What conclusion can be drawn from a device that operates without external work as described?

    <p>It violates the laws of classical thermodynamics.</p> Signup and view all the answers

    Which of the following describes the relationship between the Kelvin–Planck and Clausius statements?

    <p>They are equivalent in their consequences.</p> Signup and view all the answers

    What must a heat engine achieve to violate the Kelvin–Planck statement?

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

    What happens during the operation of a refrigerator according to the Clausius Statement?

    <p>Work is done by the refrigerator to exhaust heat to a low-temperature reservoir.</p> Signup and view all the answers

    What is implied if a refrigerator successfully removes heat from a low-temperature reservoir without an external energy source?

    <p>It contradicts the Clausius Statement.</p> Signup and view all the answers

    What is the net effect of a refrigerator on the surroundings?

    <p>It consumes work and expels heat to the surroundings.</p> Signup and view all the answers

    What type of perpetual-motion machine violates the First Law of Thermodynamics?

    <p>Perpetual-motion Machine of the first kind (PMM1)</p> Signup and view all the answers

    Which statement accurately reflects the efficiency of a proposed power plant eliminating the condenser?

    <p>It could theoretically achieve 100% efficiency.</p> Signup and view all the answers

    What does a perpetual-motion machine of the second kind (PMM2) do?

    <p>Works while exchanging heat with a single reservoir only.</p> Signup and view all the answers

    Which law is violated by a perpetual-motion machine that claims to create energy at a certain rate?

    <p>The First Law of Thermodynamics</p> Signup and view all the answers

    Why will the suggested power plant design not work?

    <p>It violates the Second Law of Thermodynamics.</p> Signup and view all the answers

    A system that provides energy to the outside at a rate without receiving any energy exemplifies which concept?

    <p>Perpetual-motion machine of the First kind</p> Signup and view all the answers

    Which component is suggested to be removed in order to improve the theoretical efficiency of a power plant?

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

    What is the main limitation for any perpetual-motion machine?

    <p>They cannot create or destroy energy.</p> Signup and view all the answers

    What is the primary function of a heat engine?

    <p>To convert part of the heat received from a high-temperature source into work</p> Signup and view all the answers

    In the context of thermal energy reservoirs, what does a source do?

    <p>Supplies energy in the form of heat</p> Signup and view all the answers

    Which of the following cannot be reversed in a heat engine according to the second law of thermodynamics?

    <p>The conversion of heat to work</p> Signup and view all the answers

    What happens to the remaining waste heat in a heat engine?

    <p>It is rejected to a low-temperature sink</p> Signup and view all the answers

    Which system can be modeled as a thermal energy reservoir?

    <p>An ocean or a lake</p> Signup and view all the answers

    What type of devices are necessary to convert heat to work?

    <p>Heat engines</p> Signup and view all the answers

    What happens to the energy of a hot cup of coffee when it cools down?

    <p>Energy is lost by the coffee and gained by the surroundings.</p> Signup and view all the answers

    Which of the following best describes a sink in thermodynamic terms?

    <p>It absorbs energy in the form of heat</p> Signup and view all the answers

    In the scenario of heating a room with an electric heater, what is true about the relationship between electrical energy and heat transferred to the room air?

    <p>Electrical energy supplied does not equal heat transferred to the room.</p> Signup and view all the answers

    When a falling mass operates a paddle wheel in a fluid, which of the following is true?

    <p>The decrease in potential energy of the mass results in an increase in internal energy of the fluid.</p> Signup and view all the answers

    What does the First Law of Thermodynamics state in relation to energy transfer?

    <p>Energy cannot be created or destroyed, only transformed.</p> Signup and view all the answers

    Which of the following statements reflects the limitations of the First Law of Thermodynamics?

    <p>The First Law does not dictate how energy transformations occur.</p> Signup and view all the answers

    What can be inferred if a car drives uphill using gasoline and cannot restore the fuel to its original level when coasting down?

    <p>The process illustrates the directionality of energy transformations.</p> Signup and view all the answers

    What is a significant implication of the Second Law of Thermodynamics regarding energy conversions?

    <p>Some energy is always lost in the form of heat in transformations.</p> Signup and view all the answers

    Which scenario illustrates a failure to convert heat energy into useful work?

    <p>Using gasoline to power a car and not restoring fuel while coasting.</p> Signup and view all the answers

    Study Notes

    Second Law of Thermodynamics

    • This law deals with the direction of processes.
    • Thermodynamics, an Engineering Approach by Yunus A. Cengel, Michael A. Boles, 5th Edition (Chapter 6) is a relevant textbook.
    • Other helpful books include Applied Thermodynamics by Eastop, A. McConkey, 5th Edition and Fundamentals of Thermodynamics by Borgnakke and Sontag, 8th Edition (Chapter 5).
    • A hot cup of coffee cools down when exposed to a cooler room. The energy lost by the coffee equals the energy gained by the surroundings. This is consistent with the First Law of Thermodynamics.
    • However, the opposite is not true. Taking heat energy from the surroundings does not automatically heat up the coffee.
    • The First Law of Thermodynamics is satisfied in both cases.
    • The Second Law shows that heat flows from higher-temperature to lower-temperature systems.
    • Heat engines, such as steam power plants, convert heat into work. However, some heat is inevitably lost as waste.
    • The maximum possible thermal efficiency for a heat engine is given by the Carnot cycle.
    • The efficiency of heat engines is always less than 100%.
    • Refrigerators and heat pumps also involve the transfer of heat, but in the opposite direction requiring work input.
    • The coefficient of performance (COP) is a measure of the efficiency of refrigerators and heat pumps.

    Heat Engines

    • Heat engines are devices that convert heat into work.
    • They take heat from a high-temperature source, do work, and reject heat to a low-temperature sink
    • Several types of heat engines exist including internal combustion engines (IC engines), external combustion engines, and turbofan engines.
    • Efficiency is measured by the ratio of work output to heat input
    • Processes in heat engines are often cyclical meaning the start and end point are the same.
    • Heat engines produce work requiring special devices.
    • Work can be converted into heat
    • Heat cannot be completely converted into work.

    Perpetual Motion Machines

    • Perpetual-motion machines of the first kind (PMM1) are impossible.
    • It is impossible to create energy from nothing.
    • Perpetual-motion machines of the second kind (PMM2) are also impossible.
    • It is impossible to create a cyclical device that transfers heat from a colder body to a hotter body without work input.

    Reversible and Irreversible Processes

    • A reversible process is one that can return to its initial state without leaving any effect on its surroundings.
    • Irreversible processes, like friction, cannot be reversed easily.
    • Irreversibilities include friction, unrestrained expansion, mixing of fluids and temperature difference.
    • There is always some loss or dissipation of energy due to irreversibilities.
    • The flow of heat between bodies with a temperature difference is irreversible.
    • Processes occurring in nature are typically irreversible.
    • The Second Law of Thermodynamics defines heat engines and refrigerators.
    • The Second Law tells us that certain processes are not reversible without input energy

    The Carnot Cycle

    • A Carnot cycle is a theoretical cycle that represents the most efficient heat engine possible between two heat reservoirs.
    • It consists of four reversible processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression
    • The efficiency of a Carnot cycle depends only on the temperatures of the hot and cold reservoirs.
    • The efficiency of the Carnot cycle is always less than 100%.

    Additional Notes

    • Temperature scale is thermodynamically independent of substance properties
    • Heat Engines and Refrigerators are compared to reversible processes showing that the former cannot have high efficiency.

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    Ch-6: The Second Law of Thermodynamics PDF

    Description

    Explore the Second Law of Thermodynamics as discussed in Chapter 6 of 'Thermodynamics, an Engineering Approach' by Cengel and Boles. Understand the concepts of heat flow, energy transfer, and the implications for heat engines. This quiz will test your grasp of the fundamental principles that govern thermodynamic processes.

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