thermodynamics1

Thermodynamics

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Iannclark Antolen Allan Baterna

Edwarf Castil Fedil Curilan

Arnel Cale

Mark anthony Andebur

Erian Cusap

Junhil Alba

Thermodynamics

• Thermodynamics is a science and, more importantly, an engineering tool used to describe processes that involve changes in temperature, transformation of energy, and the relationships between heat and work. It can be regarded as a generalization of an enormous body of empirical evidence1.1. It is extremely general: there are no hypotheses made concerning the structure and type of matter that we deal with. It is used to describe the performance of propulsion systems, power generation systems, and refrigerators, and to describe fluid flow, combustion, and many other phenomena.

• A thermodynamic system is a quantity of matter of fixed identity, around which we can draw a boundary (see Figure 1.3 for an example). The boundaries may be fixed or moveable. Work or heat can be transferred across the system boundary. Everything outside the boundary is the surroundings.

• When working with devices such as engines it is often useful to define the system to be an identifiable volume with flow in and out. This is termed a control volume. An example is shown in Figure 1.5.

• A closed system is a special class of system with boundaries that matter cannot cross. Hence the principle of the conservation of mass is automatically satisfied whenever we employ a closed system analysis. This type of system is sometimes termed a control mass.

• Figure 1.3: Piston (boundary) and gas (system) • Figure 1.4: Boundary around electric motor (system) • • Figure 1.5: Sample control volume

Figure 1.3: Piston (boundary) and gas (system)

Figure 1.4: Boundary around electric motor (system)

Figure 1.5: Sample control volume

• The thermodynamic state of a system is defined by specifying values of a set of measurable properties sufficient to determine all other properties. For fluid systems, typical properties are pressure, volume and temperature. More complex systems may require the specification of more unusual properties. As an example, the state of an electric battery requires the specification of the amount of electric charge it contains.

• Properties may be extensive or intensive. Extensive properties are additive. Thus, if the system is divided into a number of sub-systems, the value of the property for the whole system is equal to the sum of the values for the parts. Volume is an extensive property. Intensive properties do not depend on the quantity of matter present. Temperature and pressure are intensive properties.

• The state of a system in which properties have definite, unchanged values as long as external conditions are unchanged is called an equilibrium state.

• Figure 1.6: Equilibrium [Mechanical Equilibrium] [Thermal Equilibrium]

• A system in thermodynamic equilibrium satisfies:

• mechanical equilibrium (no unbalanced forces)

• thermal equilibrium (no temperature differences)

• chemical equilibrium.

Specific properties are extensive properties per unit mass and

are denoted by lower case letters. For example:

Figure 1.6: Equilibrium

Mechanical Equilibrium

Thermal Equilibrium

The fathers of thermodynamics. Sadi Carnot (1796-1832).Analizing the steam engines Carnot became aware that exists a "inefficiency" changing the heat in mechanical work an was, thus, the discoverer of the second law of thermodynamics.Formulating the concept of a steam engine efficiency, Carnot established the basics concepts for any transformation.In the year 1824 he published his " Reflexions sur la Puissance Motrice du Feu," in which he made a first attempt to express the principles involved in the application of heat to the production of mechanical effect.Carnot had accepted the "caloric theory" in conformity with it the heat was a non-material fluid which can pass from a body to another.As a water wheel worked by the water descent on the paddles, so the steam engine worked by the heat descent from a hot region (boiler) to a cooler region (condenser).As all the water returned to the river also the "caloric" (Carnot thinked) keeps after to have worked in the steam engine.

James Prescott Joule (1818-1889).Joule was son of a beer-grower in Manchester and could made his experiments in the laboratory in the father's manufactury. In the forties of 1800 he proved that the heat was a form of energy and also proved the equivalence of mechanical energy and heat.First consequence: the heat was'nt a non material fluid. The concept of "caloric" was wrong.

Diagram of the most famous Joule experiment: the conversion of mechanical energy in heat.The descent of the weight run the turbine-paddle in the water. The water temperature increases by the friction.Comparing the mechanical work made by the descent of the weight with the temperature increase of water,Joule proved the equivalence of mechanical energy and heat.

William Thomson (Lord Kelvin) - 1824 - 1870 -

In the year 1846 Kelvin became professor of Natural Philosophy at The University of Glasgow.Here, in 1847, meets Joule. This event was very interesting an Kelvin began to study the heat and his conversion in mechanical work.Joule had the opinion that the Carnot idea on the heat preservation was wrong.In 1851 Kelvin published the article "On the Dynamical Theory of Heat" that advanced the hypothesis that the theory of Carnot was not contray to the work of Joule.

Rudolf Clausius (1822 - 1888).Rudolph Gottlieb, after named Clausius, continued the study initiaded by Carnot, pursued by Joule and Lord Kelvin.Clausius understood that the divergences between Carnot and Joule could be overcome. He got rid of the cocncept of "caloric" and he assumed that the heat could be explained by the conduct of the elementary particles of matter.Clausius also gave us the extremely important principle: It is impossible for a self acting machine, unaided, to transfer heat from one body at a low temperature to another having a higher temperature.

Ludwig Boltzman (1844 - 1906).Boltzman was the first man to understand the true nature of naturals transformations and he made this before the general acceptance of the atoms existence.His work wouldn't understood. Boltzman overwhelmed with grief killed oneself.Carnot reached to the thermodynamics studying the steam engine, Boltzman made out the thermodynamics from the opposite side i.e. from the atoms.Following research and experiments attested that Boltzman was right and now he is universally acknowledged as one of greatest physicists in the history of science.

First Law

• There exists for every system a property called energy, . The system energy can be considered as a sum of internal energy, kinetic energy, potential energy, and chemical energy.

– Like the Zeroth Law, which defined a useful property, ``temperature,'' the First Law defines a useful property called ``energy.''

– The two new terms (compared to what you have seen in physics and dynamics, for example) are the internal energy and the chemical energy. For most situations in this class, we will neglect the chemical energy. We will generally not, however, neglect the internal energy, . It arises from the random or disorganized motion of molecules in the system, as shown in Figure 2.1. Since this molecular motion is primarily a function of temperature, the internal energy is sometimes called ``thermal energy.''

Figure 2.1: Random motion is the physical basis for internal energy

• The internal energy, , is a function of the state of the system. Thus , or , or . Recall that for pure substances the entire state of the system is specified if any two properties are specified. (We will discuss the equations that relate the internal energy to these other variables as the course progresses.)

1. The change in energy of a system is equal to the difference between the heat added to the system and the work done by the system,

Formulas:

Second Law of Thermodynamics The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy.

The maximum efficiency which can be achieved is the Carnot efficiency.

Second Law: Heat Engines

Second Law of Thermodynamics: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.

This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law.

Second Law: Refrigerator Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.

This is the "second form" or Clausius statement of the second law.

Second Law: Entropy Second Law of Thermodynamics: In any cyclic process the entropy will either increase or remain the same.

Entropy:

a state variable whose change is defined for a reversible process at T where Q is the heat absorbed.

Entropy:

a measure of the amount of energy which is unavailable to do work.

Entropy:

a measure of the disorder of a system.

Entropy:

a measure of the multiplicity of a system.

Since entropy gives information about the evolution of an isolated system with time, it is said to give us the direction of "time's arrow" . If snapshots of a system at two different times shows one state which is more disordered, then it could be implied that this state came later in time. For an isolated system, the natural course of events takes the system to a more disordered (higher entropy) state.

Carnot Cycle The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. When the second law of thermodynamics states that not all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used. In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy.

The temperatures in the Carnot efficiency expression must be expressed in Kelvins.

The conceptual value of the Carnot cycle is that it establishes the maximum possible efficiency for an engine cycle operating between TH and TC. It is not a

practical engine cycle because the heat transfer into the engine in the isothermal process is too slow to be of practical value. As Schroeder puts it "So don't bother installing a Carnot engine in your car; while it would increase your gas mileage, you would be passed on the highway by pedestrians."

Entropy and the Carnot Cycle

The efficiency of heat engine cycle is given by

For the ideal case of the Carnot cycle, this efficiency can be written

Using these two expressions together

If we take Q to represent heat added to the system, then heat taken from the system will have a negative value. For the Carnot cycle

which can be generalized as an integral around a reversible cycle

Clausius Theorem

For any part of the heat engine cycle, this can be used to define a change in entropy S for the system

or in differential form at any point in the cycle

For any irreversible process, the efficiency is less than that of the Carnot cycle. This can be associated with less heat flow to the system and/or more heat flow out of the system. The inevitable result is

Clausius Inequality

Any real engine cycle will result in more entropy given to the environment than was taken from it, leading to an overall net increase in entropy.

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