Questions and Answers
The refrigerant absorbs heat from the air in the food compartment.
False
The compressor compresses the vapor refrigerant isothermally.
False
The vapor refrigerant enters the evaporator and becomes a hot, high-pressure liquid.
False
The vapor refrigerant leaves the compressor at a lower pressure and temperature.
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Positive work is done by the vapor on the compressor.
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The air in the food compartment is colder than the refrigerant.
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A heat engine can run in the reverse direction, transferring energy from the hot reservoir to the cold reservoir.
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An air conditioner transfers energy from the warm air outside to the cool room in the home.
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Work must be done by the engine in a heat pump or refrigerator.
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The efficiency of a heat pump or refrigerator is given by the ratio of the energy transferred to the hot reservoir to the work done on the engine.
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A heat pump includes a circulating fluid that passes through three sets of metal coils.
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The fluid in a heat pump is hot and at high pressure when it is in the coils located in a cool environment.
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In a refrigerator, the external coils are located in the walls of the refrigerator.
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The liquid in a refrigerator expands isothermally at a rate controlled by the expansion valve.
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Heat pumps and refrigerators are devices that transfer energy from a hotter body to a colder body.
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The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.
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The thermal efficiency of a heat engine is defined as the ratio of the energy input to the net work done by the engine during one cycle.
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All heat engines have an efficiency of 100%.
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A heat engine with perfect efficiency would have to expel all the input energy by work.
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The Second Law of Thermodynamics states that it is possible to construct a heat engine that produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work.
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The first law of thermodynamics makes a distinction between spontaneous and non-spontaneous processes.
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The net work done by a heat engine equals the energy transferred to it.
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The second law of thermodynamics establishes which processes do and do not occur in nature.
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Diesel engines have an efficiency of 100%.
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Lord Kelvin was the first to propose the use of a relative scale of temperature.
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The efficiency of a heat engine is defined as the ratio of what you give to what you gain.
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A heat engine can achieve 100% efficiency if it expels all the input energy to the cold reservoir.
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A heat engine is a device that takes in energy by work and expels a fraction of that energy by means of heat.
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The Second Law of Thermodynamics states that some energy must be expelled to the environment during the operation of a heat engine.
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The efficiency of a heat engine is always 100%.
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The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.
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An irreversible process is one that occurs naturally in both directions.
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The internal energy of a heat engine remains the same at the beginning and end of a cycle.
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Energy always transfers from a cold object to a hot object.
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William Thomson, Lord Kelvin, was a French physicist and mathematician.
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A refrigerator is an example of a heat engine.
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A perfect heat pump can be constructed with no input of energy by work.
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The coefficient of performance (COP) is similar to thermal efficiency for a heat engine.
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A good refrigerator should have a low COP.
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In heating mode, the COP is the ratio of the work required to the heat transferred.
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Qh is typically lower than W in a heat pump.
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Heat pumps are not suitable for use in very low winter temperatures.
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The COP of a refrigerator can be calculated by dividing the energy transferred from the cold reservoir by the work done on the pump.
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A COP of 4 for a heat pump means that the amount of energy transferred is four times less than the work done.
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Heat pumps are only used for cooling purposes.
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A refrigerator with a COP of 5.00 is less efficient than one with a COP of 3.00.
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Study Notes
The Second Law of Thermodynamics
- States that most natural processes occur in one direction and are irreversible
- Energy always transfers from a hot object to a cold object with which it is in contact
- This directionality is governed by the second law
Lord Kelvin
- William Thomson, Lord Kelvin, a British physicist and mathematician
- Proposed the use of an absolute scale of temperature
- Led to the idea that energy cannot pass spontaneously from a colder object to a hotter object
Heat Engines and the Second Law of Thermodynamics
- A heat engine is a device that takes in energy by heat and operates in a cyclic process, expelling a fraction of that energy by means of work
- Examples: power plant producing electricity, internal combustion engine of an automobile
- A heat engine carries a working substance through a cyclic process during which it absorbs energy by heat from a high-temperature energy reservoir, does work, and expels energy as heat to a lower-temperature reservoir
Components of a Simplified Steam Engine
- A steam engine may be used to turn a turbine
Thermal Efficiency of a Heat Engine
- Defined as the ratio of the net work done by the engine during one cycle to the energy input at the higher temperature
- The efficiency of a heat engine is always less than 100%
- Example: diesel engines have efficiencies ranging from 35% to 40%
Perfect Heat Engine
- A heat engine with perfect efficiency would have to expel all the input energy by work
- Impossible to construct such an engine
The Second Law of Thermodynamics (Kelvin-Planck Form)
- It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work
Heat Pumps and Refrigerators
- Devices that transfer energy from a cold reservoir to a hot reservoir
- Example: air conditioner, refrigerator
- A heat pump includes a circulating fluid that passes through two sets of metal coils that exchange energy with the surroundings
Heat Pump Process
- Energy is extracted from the cold reservoir
- Energy is transferred to the hot reservoir
- Work must be done on the engine
Coefficient of Performance (COP)
- Describes the effectiveness of a heat pump
- Ratio of what you gain (energy transferred to or from a reservoir) to what you give (work input)
- A good refrigerator should have a high COP, typically 5 or 6
- In heating mode, the COP is the ratio of the heat transferred to the work required
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Description
Test your understanding of the Second Law of Thermodynamics, heat engines, and heat pumps and refrigerators in this thermodynamics and fluid mechanics quiz.
