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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 (B)</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 (D)</p> Signup and view all the answers

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

<p>$COP = \frac{Q_{c}}{W}$ (D)</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 (D)</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 (C)</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% (D)</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. (B)</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. (A)</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 (B)</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. (A)</p> Signup and view all the answers

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

<p>The Clausius statement is violated. (B)</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. (D)</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. (C)</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. (B)</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. (B)</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. (A)</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. (C)</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. (D)</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. (C)</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. (C)</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. (D)</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 (B)</p> Signup and view all the answers

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

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

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

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

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

<p>1 to 2 m (A)</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 (B)</p> Signup and view all the answers

What is the COP of ground-source heat pumps?

<p>4.0 (C)</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 (A)</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. (D)</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 (B)</p> Signup and view all the answers

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

<p>Combustion of fuel (C)</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 (B)</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 (B)</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 (C)</p> Signup and view all the answers

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

<p>Steam power plant (B)</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 (D)</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 (A)</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. (B)</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. (C)</p> Signup and view all the answers

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

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

What aspect of a heat engine does thermal efficiency measure?

<p>Fraction of heat converted to work. (D)</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. (A)</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. (A)</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. (C)</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. (B)</p> Signup and view all the answers

Flashcards

First Law of Thermodynamics

States that energy is conserved in a process. No energy is created or destroyed.

Reversible Heat Engine

A hypothetical engine that can be reversed and return to its initial state without any net change in the surroundings.

Heat Pump

A device that transfers heat from a cold reservoir to a hot one.

Refrigerator

A device that transfers heat from a cold reservoir to a hot reservoir.

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Thermal Efficiency

The ratio of the useful work output to the heat input in a thermodynamic cycle.

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Coefficient of Performance (COP)

A measure of the effectiveness of a heat pump or refrigerator.

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Closed System

A system where no mass crosses its boundaries.

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Open System

A system where mass can cross its boundaries.

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Second Law of Thermodynamics (Kelvin-Planck)

It's impossible for a cyclic device to receive heat from only one thermal reservoir and produce a corresponding amount of net usable work.

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Heat Engine

A device converting heat energy into work by transferring heat between reservoirs of different temperatures.

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Thermal Efficiency

Ratio of net work output to input heat from a reservoir.

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Thermal Reservoir

A heat source or sink with a large thermal capacity, whose temperature doesn't change significantly during its interaction with something else.

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Maximum Thermal Efficiency

Highest possible efficiency a heat engine can achieve, limited by the reservoir temperatures.

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Refrigerators and Heat Pumps

Work-driven devices used to transfer heat from a cold reservoir to a hot reservoir. Reverse the natural heat flow.

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Heat Transfer

Movement of thermal energy between bodies due to a temperature difference.

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100% Efficient Heat Engine

An impossible heat engine that violates the second law of thermodynamics, converting all input heat into work.

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High-temperature reservoir

A heat reservoir at a higher temperature than the working substance.

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Low-temperature reservoir

A heat reservoir at a lower temperature than the working substance.

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Heat Engine

A device that converts heat into work.

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Refrigerator

A device that moves heat from a cold reservoir to a hot reservoir.

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Kelvin-Planck Statement

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

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Clausius Statement

It is impossible to construct a device that operates on a cycle and produces no other effect than the transfer of heat from a cooler body to a warmer body.

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100% efficient heat engine

A heat engine that converts all the heat it receives into work.

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Thermal Efficiency

The ratio of the net work output to the heat input.

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Heat Engine

A device that converts heat energy into mechanical work.

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Steam Power Plant

An external combustion engine that uses steam to produce work.

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Qin

Heat supplied to the system/steam in boiler.

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Qout

Heat rejected from the system/steam in condenser.

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Wout

Work delivered by the system (expansion).

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Win

Work required to compress water (to boiler pressure).

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External Combustion Engine

Combustion happens outside the working part of the engine.

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High-Temperature Source

The source that provides heat to the engine (like a furnace).

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Closed System

A system where no mass enters or leaves.

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Net Work Output

The total work done by a system in a cycle.

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Net Heat Transfer

The total heat gained or lost by a system in a cycle.

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Thermal Efficiency

Fraction of heat input converted to net work output.

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Heat Input

Energy added (heat) to a system.

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Waste Heat

Heat energy not converted to useful work.

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Work Output equation

Net work out = heat in - heat out.

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Thermal Efficiency Example

Efficiency values represent heat conversion into work.

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Air-source heat pumps

Heat pumps that use outside air as the heat source in winter.

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COP (Coefficient of Performance)

A measure of how efficiently a heat pump or refrigerator works.

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Geothermal heat pumps

Heat pumps that use the ground as the heat source.

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Air conditioners as heat pumps

Air conditioners can be used as heat pumps by reversing their operation.

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Design conditions (COP)

Standard conditions for evaluating the performance of heat pumps.

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Ground source heat pumps efficiency

More efficient than air-source heat pumps (up to 45% more).

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Refrigerated space (in air conditioners)

The room or building that's being cooled by an air conditioner.

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Reversing valve (in air conditioners)

A valve that changes the direction of refrigerant flow in an AC unit, converting it to a heat pump function.

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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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