Thermodynamics of open biological environments Heat and Thermodynamics
Thermodynamics of open biological environments.
Heat and Thermodynamics First Law of Thermodynamics Second Law of Thermodynamics Entropy Adiabatic Process Heat Engine Cycle Enthalpy
First Law of Thermodynamics The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:
The first law makes use of the key concepts of internal energy, heat , and system work. It is used extensively in the discussion of heat engines.
When work. System is done by Work a thermodynamic system, it is ususlly a gas that is doing the work. The work done by a gas at constant pressure is:
For non-constant pressure, the work can be visualized as the area under the pressure-volume curve which represents the process taking place. The more general expression for work done is: Work done by a system decreases the internal energyof the system, as indicated in the First Law of Thermodynamics. System work is a major focus in the discussion of heat engines.
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.
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.
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.
Entropy in Terms of Heat and Temperature The macroscopic relationship which was originally used to define entropy S is d. S = Q/T This is often a sufficient definition of entropy if you don't need to know about the microscopic details.
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.
Alternative statements: Second Law of Thermodynamics Biological systems are highly ordered; how does that square with entropy?
Adiabatic Process An adiabatic process is one in which no heat is gained or lost by the system. The first law of thermodynamics with Q=0 shows that all the change in internal energy is in the form of work done. This puts a constraint on the heat engine process leading to the adiabatic condition shown below. This condition can be used to derive the expression for the work done during an adiabatic process.
The ratio of the specific heats g = CP/CV is a factor in determining the speed of sound in a gas and other adiabatic processes as well as this application to heat engines. This ratio g = 1. 66 for an ideal monoatomic gas and g = 1. 4 for air, which is predominantly a diatomic gas.
Heat Transfer The transfer of heat is normally from a high temperature object to a lower temperature object. Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics.
Heat Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:
Q = heat transferred in time = t T = thermal conductivity of the barrier A = area d = thickness of barrier
Heat Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law).
Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy. Convection is thought to play a major role in transporting energy from the center of the Sun to the surface, and in movements of the hot magma beneath the surface of the earth
It is difficult to quantify the effects of convection since it inherently depends upon small nonuniformities in an otherwise fairly homogeneous medium. In modeling things like the cooling of the human body, we usually just lump it in with conductio
Heat Engines A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. Thermodynamics is the study of the relationships between heat and work. The first law and second law of thermodynamics constrain the operation of a heat engine.
The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow.
Enthalpy Four quantities called "thermodynamic potentials" are useful in the chemical thermodynamics of reactions and non-cyclic processes. They are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy. Enthalpy is defined by H = U + PV
where P and V are the pressure and volume, and U is internal energy. Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables. It is somewhat parallel to the first law of thermodynamics fora constant pressure system Q = DU + PDV since in this case Q=DH
It is a useful quantity for tracking chemical reactions. If as a result of an exothermic reaction some energy is released to a system, it has to show up in some measurable form in terms of the state variables. An increase in the enthalpy H = U + PV might be associated with an increase in internal energy which could be measured by calorimetry, or with work done by the system, or a combination of the two.
The internal energy U might be thought of as the energy required to create a system in the absence of changes in temperature or volume. But if the process changes the volume, as in a chemical reaction which produces a gaseous product, then work must be done to produce the change in volume. For a constant pressure process the work you must do to produce a volume change DV is PDV. Then the term PV can be interpreted as the work you must do to "create room" for the system if you presume it started at zero volume.
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