Essence of Thermodynamics in 10 mins. Stefan Bringuier References: Thermodynamics in Materials Science Robert DeHoff

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1 Essence of Thermodynamics in 10 mins Stefan Bringuier References: Thermodynamics in Materials Science Robert DeHoff

2 A Material Scientist Viewpoint Given these conditions (i.e. temperature, pressure, composition) what state of matter may exist. Equilibrium maps provide the answer! (e.g. Phase- Diagrams, Predominance-Diagrams, Pourbaix Diagrams etc.) Example Phase-Diagram for single component system. Adapted from the Wikimedia Commons file "Image:Phase-diag2" org/wikipedia/commons/thumb/3/34/phasediag2.svg/200px-phase-diag2.svg.png

3 How To Make Such Maps Use of experimental (or computational) thermochemical/thermophysical data. The Thermodynamics framework takes in this data and outputs what we want. Data Inputs: Heat capacity Melting point Boiling point etc. Thermodynamics Outputs: Free-Energy Curves Phase-Diagrams Gas Composition Maps etc.

4 Phenomenological Thermodynamics We observe and describe. No effort in understanding a more clear description of matter. This means measured properties like temperature, pressure, composition, etc. is sufficient description of materials.

5 Statistical Thermodynamics Matter consist of atoms in different structures having properties like mass and energy. Bonds between atoms also have properties. Statistical Thermodynamics - A statistical approach to describe properties using large ensembles of fundamental building blocks.

6 Quick and Dirty:The Laws 1st Law - Energy of the Universe is conserved. 2nd Law - We define a quantity of the Universe called Entropy and its value is always increasing ( We will clarify later the physical meaning). 3rd Law - There exist a universal scale where temperature is zero and all matter has the same Entropy.

7 Classification of systems We can think of materials as subsystems of the Universe. We then seek to classify these materials to fit into the thermodynamic framework: Single Component vs. Multi-component Single Phase vs. Multi-phase Chemically Permeability vs. Non-Permeability Chemically reactive vs. Non-Reactive External fields vs. No fields

8 Variables of a system A material system can be described by quantities such as temperature, volume, pressure,number of atoms, etc. These types of dependent variables can be described as state variables. State variables only depend current condition of the system. Thus differences in state variable is simple: dt = Tfinal - Tinitial

9 Continue... State variables are further described as intensive or extensive. Intensive refers to that the quantity can have spatial dependence. Ex. Temperature T(x,y,z) Extensive means that the state variable describes the entire system Ex. Volume and Energy It is possible to represent extensive variables as intensive quantities by choosing finite volumes to represent the quantity.

10 Process Variables Quantities that flow, for example heat flow, depend upon the path for a changing system. From basic mechanics we can demostrate this for work: dw = F.dx To observe the amount of mechanical work we must integrate over dx

11 1st Law of Thermodynamics U - Energy Q - Heat flow W - Mechanical work C - Other work du = dq + dw + dc 1st Law This is for infinitesimal change in U with incremental changes in Q,W,and C. However simple it is not very useful because we have a state function defined by only process variables.

12 2nd Law of Thermodynamics ds = ds_transfered + ds_produced 2nd Law: S_produced >= 0 Reversible process S_produced = 0. This is what Thermodynamics is concerned with. Irreversible process S_produced > 0, real processes

13 Continued S_produced can be thought of as a loss or spreading out of information(i.e. energy). A pebble dropped in a pond will never spontaneously reverse its dissipation of energy. See R. DeHoff pg. 40 for a good description

14 Heat Flow From the 1st law we have the process variable dq dq_reversible. Based on the Carnot Cycle arguement it can be shown that: dq_reversible = TdS_transfered We note that for an irreversible process ds_irreversible,transferred < ds, reversible_transferred

15 Useful 1st Law We can now write the 1st law in a useful form: du = TdS - PdV + dc where -PdV comes from mechanical work on a system.

16 3rd Law of Thermodynamics At 0 Kelvin the change in entropy between two states is zero. In other words, entropy is the same for all matter at 0 Kelvin

Irreversible Processes

Irreversible Processes Examples: Block sliding on table comes to rest due to friction: KE converted to heat. Heat flows from hot object to cold object. Air flows into an evacuated chamber. Reverse process