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Chapter 3 The Laws of Thermodynamics

09/19/2001. Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993). 3-2. The Laws of Thermodynamics. 0th Law --- There is a temperature scale and energy (heat) flows down hill on that scale.1st Law --- Energy is conserved.2nd Law --- Entropy is not conserved. Entropy alw

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Chapter 3 The Laws of Thermodynamics

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    1. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-1 Chapter 3 The Laws of Thermodynamics Notes on Thermodynamics in Materials Science by Robert T. DeHoff (McGraw-Hill, 1993).

    2. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-2 The Laws of Thermodynamics 0th Law --- There is a temperature scale and energy (heat) flows down hill on that scale. 1st Law --- Energy is conserved. 2nd Law --- Entropy is not conserved. Entropy always changes in one direction (increases). 3rd Law --- There is an absolute zero temperature, and the entropy of all substances is the same at absolute zero.

    3. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-3 Temperature Scales

    4. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-4 Energy The ability to do work. Can be converted or transported. Cannot be created or destroyed. Kinetic Energy --- Energy of motion. K.E. = 1/2 mV2 Potential Energy --- Energy of position in a potential field. P.E. = mg.h + ... Internal Energy (U) --- Energy associated with the condition of matter, not its motion or position.

    5. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-5 First Law Energy is a property of the universe which cannot change no matter what process occurs. The change in internal energy of a system must be equal to the sum of all energy transfers across the boundary of the system. DU = increase in the internal energy of the system. Q = quantity of heat that flows into the system. W = mechanical work done on the system by external pressure exerted by the surroundings. W/ = all other kinds of work done on the system. For a process: DU = Q + W + W/ For an infinitesimal step: dU = dQ + dW + dW/

    6. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-6 Work A process variable. Value depends on path. An increment of work (dW) equals an increment of motion (dx) of a point on a body under an applied force (F). dW = F.dx The work done by a process is integrated over the path.

    7. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-7 Second Law Entropy is a property of the universe which always changes in the same direction no matter what process occurs. DSt, DSt/ = entropy transferred across the boundary during the process for system & surroundings, respectively. DSp , DSp/ = entropy production within the system & surroundings, respectively, during the process.

    8. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-8 Entropy --- a State Function Entropy is transferred across a boundary & created within the system & the surroundings. Entropy production results from dissipative processes. Entropy production increases as the rate of the process increases & system is further from equilibrium. Entropy processes carried out infinitesimally close to equilibrium, ideally, have zero entropy production & are reversible. Real processes, carried out at finite rates, are irreversible, create entropy & there is some permanent change in the universe.

    9. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-9 3.6 Give five examples of the operation of the second law of thermodynamics in your daily experience (different from those given in the text). The morning coffee cools with time. Sugar dissolves in hot coffee. Left to itself a pendulum will slow and stop. Organisms die. An expanding gas cools.

    10. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-10 3.6 Why is “irreversible” so appropriate in its application to the description of processes in thermodynamics? Suggest 2 or 3 alternate phrases. Incapable of being reversed. The reverse process cannot happen. Process producing a permanent change. Irretraceable. All real processes are accompanied by permanent change in the universerse. By definition, the permanent changes cannot be undone by reversing the influences driving the system.

    11. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-11 Combined 1st & 2nd Laws Applying, dWrev = -PdV and dQrev = TdS then dU = TdS - PdV + dW/

    12. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-12 The Third Law There is a lower limit to the temperature that can be attained by matter, absolute zero. The entropy of all substances at that temperature, at absolute zero, is the same.

    13. 09/19/2001 Notes from R.T. DeHoff, Thermodynamics in Materials Science (McGraw-Hill, 1993) 3-13 Entropy Change for a Reaction

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