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Second Law of Thermodynamics. The 2 nd law of thermoD. is a general principle which places constraints upon : The direction of heat transfer The attainable efficiencies of heat engines. implications may be visualized in terms of the waterfall analogy.
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The 2nd law of thermoD. is a general principle which places constraints upon : • The direction of heat transfer • The attainable efficiencies of heat engines. implications may be visualized in terms of the waterfall analogy.
Qualitative Statements: Second Law of Thermodynamics • The 2nd law of thermoD is a profound principle of nature which affects the way energy can be used. There are several approaches to state this principle qualitatively. Here are some approaches to giving the basic sense of the principle.
1. Heat will not flow spontaneously from a cold object to a hot object. • 2. Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. • 3. You cannot create a heat engine which extracts heat and converts it all to useful work.
4. There is a thermal bottleneck: • The 2nd Law says that no heat engine can use all the heat produced by a fuel to do work. The Carnot cycle sets the ideal efficiency which can be obtained if there is no friction, mechanical losses, leakage, etc., but real machine efficiencies are much less.
Second Law: Heat Engines • 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 • It is not possible for heat to flow from a colder body to a warmer body without any work having been done • Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator Also true for air conditioners and heat pumps • This is the "second form" or Clausius statement of the second law.
Second Law: Entropy • 2nd Law: In any cyclic process the entropy will either increase or remain the same.
Entropy: S= Q/T, 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 gives 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.
Energy and Order in Biological Systems • The concept of entropy and the 2nd law suggests that systems naturally progress from order to disorder. If so, how do biological systems develop and maintain such a high degree of order? Is this a violation of the 2nd law?
Order can be produced with an expenditure of energy, and the order associated with life on the earth is produced with the aid of energy from the sun
For example, plants use energy from the sun in tiny energy factories called chloroplasts. Using chlorophyll in the process called photosynthesis, they convert the sun's energy into storable form in ordered sugar molecules. In this way, carbon and water in a more disordered state are combined to form the more ordered sugar molecules.
