Thermodynamics

Thermodynamics

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The word thermodynamics is derived from the Greek words therme, meaning heat and dunamis, meaning power. Thermodynamics is a branch of physics that studies the effects of changes in temperature, pressure, and volume on systems at the macroscopic scale by studying the motion of their particles. A system is the subject of study. Heat means energy in transit and dynamics relates to movement of particles; thus, in essence thermodynamics studies the movement of energy and how energy instills movement. Thermodynamics describes how systems respond to changes in their surroundings. This can be applied to a wide variety of topics in science (physics and chemistry) and engineering, such as engines, phase transitions of matter, chemical reactions, and transportation.

The study of thermodynamics is separated into two branches: the classical and the statistical thermodynamics.

Classical thermodynamics was the original study of thermodynamics in early 1800s. It was concerned with thermodynamic states, and properties as energy, work, and heat, and with the two laws of thermodynamics. However, classical thermodynamics lacked an atomic interpretation of the processes. Classical thermodynamics derives from the research done by physicist Robert Boyle. He developed the concept that the pressure P of a given quantity of gas varies inversely to its volume V at constant temperature. In other words this equation was derived: PV = k, a constant. From here, the thermo-science began to develop with the construction of the first successful atmospheric steam engines.

The first and second laws of thermodynamics emerged simultaneously in the 1850s.

With the development of atomic and molecular theories in the late 19th century, thermodynamics was given a molecular interpretation, which the classical thermodynamics lacked. This field is called statistical thermodynamics, which can be thought of as a bridge between macroscopic and microscopic properties of systems. Statistical thermodynamics is focused around the macroscopic results. The statistical approach is to derive all macroscopic properties (temperature, volume, pressure, energy, entropy, etc.) from the properties of moving particles and the interactions between them in the given system. Statistical thermodynamics was found to be very accurate and successful; therefore it is widely used by scientists around the world.

Thermodynamics is a branch of physics which deals with the energy and work of a system. It was born in the 19th century as scientists were first discovering how to build and operate steam engines. The term thermodynamics was first used by James Joule to express the relationship between heat and power.

The history of thermodynamics begins with a German scientist who designed and built the first vacuum pump.

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Shortly after that development, Robert Boyle built an air pump. Using this pump, Boyle noticed the pressure-temperature-volume proportionality. With this new development Boyle's Law was formulated, which states that pressure and volume are inversely proportional.

The little developments in pumps and piston engines lead to the invention of an engine. The first engine was invented by Thomas Savery. Although these early engines were crude and inefficient, they attracted the attention of the leading scientists of the time. One such scientist was Sadi Carnot, the "father of thermodynamics” (Carnot Engine). He marks the beginning of thermodynamics as a branch of science. He created the Carnot Cycle and designed an ideal engine called the Carnot engine. This engine has the highest achievable efficiency an engine can have. Sadi Carnot, in 1824, published “Reflections on the Motive Power of Fire”, a research paper on heat, power, and engine efficiency. The paper outlined the basic relations between the Carnot engine, the Carnot cycle, and locomotive power (“Thermodynamics”). The Heat Engine and the Carnot inventions are discussed further on.

Before anything else is discussed about thermodynamics, it is important to define what a thermodynamic system and process is.

An important concept in thermodynamics is the system. A system is the region being studied. A system is separated from the remainder of the universe by a boundary which may be imaginary or not, but which limits the volume of region being studied. The possible exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. There are five dominant classes of systems:

1.

Isolated Systems – matter and energy may not cross the boundary.
2.

Adiabatic Systems – heat may not cross the boundary.
3.

Diathermic Systems - heat may cross boundary.
4.

Closed Systems – matter may not cross the boundary.
5.

Open Systems – heat, work, and matter may cross the boundary.


A thermodynamic process is the difference between the initial state and the final states within a thermodynamic system. Typically, each thermodynamic process is distinguished from other processes according to what parameters (as in temperature, pressure, or volume) are held fixed.

The six most common thermodynamic processes are described below:

1.

An isothermal process occurs at a constant temperature.
2.

An isobaric process occurs at constant pressure.
3.

An isometric or isovolumetric process occurs at constant volume.
4.

An adiabatic process occurs without loss or gain of heat.
5.

An isentropic process occurs at constant entropy.
6.

An isenthalpic process occurs at a constant enthalpy.

(Serway)

The first three processes are the most common process used to describe systems.

Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.

A heat engine acts by transferring energy from a warm region to a cool region and, in the process, converting energy to mechanical work. A heat engine is a reversible process. However, it would take an outside force to reverse any process and will not be an efficient process at all. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one. One such reversible process is a refrigerator; however it takes a lot of power and energy to keep things cooled down. Efficiency becomes very important in such an instance.

A special case of a thermodynamic cycle is the Carnot cycle. The Carnot Engine is the most efficient heat engine possible in nature.

The Carnot cycle is a special type of thermodynamic cycle. It is special because it is the most efficient cycle possible for converting a given amount of energy into work or for using a given amount of work for refrigeration purposes.

Heat flows from a high temperature TH furnace through the working body (working substance) and into the cold reservoir TC. This process forces the working substance to do work W on the surroundings.

No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs.

No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs. A Carnot Cycle is the most efficient cycle possible in nature. The efficiency η is defined to be:

η = ΔW/ΔQH = 1 – TC/TH

where:

ΔW is the work done by the system (energy exiting the system as work)
ΔQH is the heat put into the system (heat energy entering the system)
TC is the absolute temperature of the cold reservoir
TH is the temperature of the hot reservoir.

The above equation gives the maximum efficiency possible for any engine using the corresponding temperatures. The efficiency can never equal one however. It is not possible as defined by the laws of thermodynamics.

The above diagram represents a Carnot cycle acting as a heat engine. The process is illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature TH and a cold reservoir at temperature TC.



The Carnot cycle when acting as a heat engine (in a piston and cylinder system) consists of the following steps:

1.

Isothermal expansion of the gas at the hot temperature, TH:. Step 2 on the diagram - the expanding gases cause the piston to do work on the surroundings. The gas expansion is propelled by absorption of heat from the high temperature reservoir.
2.

Adiabatic expansion of the gas: Step 3 on the diagram - no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the cold temperature, TC.
3.

Isothermal compression of the gas at the cold temperature, TC. Step 4 on the diagram. Now the surroundings do work on the gas, causing heat to flow out of the gas to the low temperature reservoir.
4.

Adiabatic compression of the gas: Step 1 on the diagram. The surroundings do work on the gas, compressing it and causing the temperature to rise to TH. At this point the gas is in the same state as at the start of the isothermal expansion.

(Boles)


References:


Boles, Cengel.Thermodynamics an Engineering Approach. Sixth edition. McGraw Hill Inc. New York, NY. 2007

“Carnot Cycle”. Wikipedia, the free encyclopedia. 22 March 2007, 30 March 2007.

“Carnot Engine”. Wikipedia, the free encyclopedia. 15 March 2007. 30 March 2007.

Serway Jewett. Physics for Scientists and Engineers. Sixth edition. Thomson Brooks/Cole. Belmont CA. 2004

“System”. Wikipedia, the free encyclopedia. 30 March 2007. 30 March 2007.

“Thermodynamics”. Wikipedia, the free encyclopedia. 29 March 2007. 30 March 2007.

“What is Thermodynamics?”. Tom Benson. Glenn Research Center. 19 March 2007. 30 March 2007.
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