Basics of Thermodynamics.ppt

1、Basics of Thermodynamics,Four Laws that Drive the UniversePeter Atkins*Oxford University Press, Oxford, 2007,Reading,Some of the material covered here is also covered in the chapter/topic on: Equilibrium,*It is impossible for me to write better than Atkins- his lucid (& humorous) writing style is tr

2、uly impressive- paraphrasing may lead to loss of the beauty of his statements- hence, some parts are quoted directly from his works.,MATERIALS SCIENCE & ENGINEERING,Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: anand

3、hiitk.ac.in, URL: home.iitk.ac.in/anandh,AN INTRODUCTORY E-BOOK,Part of,http:/home.iitk.ac.in/anandh/E-book.htm,A Learners Guide,Physical Chemistry Ira N Levine Tata McGraw Hill Education Pvt. Ltd., New York (2002).,Thermodynamics deals with stability of systems. It tells us what should happen?. Wil

4、l it actually happen(?) is not the domain of thermodynamics and falls under the realm of kinetics. At 5C at 1 atm pressure, ice is more stable then water. Suppose we cool water to 5C. “Will this water freeze?” (& “how long will it take for it to freeze?”) is (are) not questions addressed by thermody

5、namics. Systems can remain in metastable state for a long-time. Window pane glass is metastable but it may take geological time scales for it to crystallize! At room temperature and atmospheric pressure, graphite is more stable then diamond but we may not lose the glitter of diamond practically fore

6、ver!,Thermodynamics versus Kinetics,* The term metastable is defined in the chapter on equilibrium.,One branch of knowledge that all engineers and scientists must have a grasp of (to some extent or the other!) is thermodynamics. In some sense thermodynamics is perhaps the most abstract subject and a

7、 student can often find it very confusing if not motivated strongly enough. Thermodynamics can be considered as a system level science- i.e. it deals with descriptions of the whole system and not with interactions (say) at the level of individual particles. I.e. it deals with quantities (like T,P) a

8、veraged over a large collection of entities (like molecules, atoms)*. This implies that questions like: “What is the temperature or entropy of an atom?”; do not make sense in the context of thermodynamics (at lease in the usual way!). TD puts before us some fundamental laws which are universal* in n

9、ature (and hence applicable to fields across disciplines).,Thermodynamics (TD): perhaps the most basic science,* Thermodynamics deals with spatio-temporally averaged quantities. * They apply to the universe a whole as well! (Though the proof is lacking!).,To understand the laws of thermodynamics and

10、 how they work, first we need to get the terminology right. Some of the terms may look familiar (as they are used in everyday language as well)- but their meanings are more technical and precise, when used in TD and hence we should not use them casually. System is region where we focus our attention

11、 (Au block in figure). Surrounding is the rest of the universe (the water bath at constant temperature). Universe = System + Surrounding (the part that is within the dotted line box in the figure below) More practically, we can consider the Surrounding as the immediate neighbourhood of the system (t

12、he part of the universe at large, with which the system effectively interacts). In this scheme of things we can visualize: a system, the surrounding and the universe at large. Things that matter for the surrounding: (i) T (ii) P (iii) ability to: do work, transfer heat, transfer matter, etc. Paramet

13、ers for the system: (i) Internal energy, (ii) Enthapy, (iii) T, (iv) P, (v) mass, etc.,The language of TD,In TD we usually do not worry about the universe at large!,To a thermodynamic system two things may be added/removed: energy (heat, work) matter. An open system is one to which you can add/remov

14、e matter (e.g. a open beaker to which we can add water). When you add matter- you also end up adding heat (which is contained in that matter). A system to which you cannot add matter is called closed. Though you cannot add/remove matter to a closed system, you can still add/remove heat (you can cool

15、 a closed water bottle in fridge). A system to which neither matter nor heat can be added/removed is called isolated. A closed vacuum thermos flask can be considered as isolated.,Open, closed and isolated systems,* By or on the system * Mass, Heat or Work,Matter is easy to understand and includes at

16、oms, ions, electrons, etc. Energy may be transferred (added) to the system as heat, electromagnetic radiation etc. In TD the two modes of transfer of energy to the system considered are Heat and Work. Heat and work are modes of transfer of energy and not energy itself. Once inside the system, the pa

17、rt which came via work and the part which came via heat, cannot be distinguished*. More sooner on this! Before the start of the process and after the process is completed, the terms heat and work are not relevant. From the above it is clear that, bodies contain internal energy and not heat (nor work

18、!). Matter when added to a system brings along with it some energy. The energy density (energy per unit mass or energy per unit volume) in the incoming matter may be higher or lower than the matter already present in the system.,* The analogy usually given is that of depositing a cheque versus a dra

19、ft in a bank. Once credited to an account, cheque and draft have no meaning. (Also reiterated later).,Here is a brief listing of a few kinds of processes, which we will encounter in TD: Isothermal process the process takes place at constant temperature (e.g. freezing of water to ice at 10C) Isobaric

20、 constant pressure (e.g. heating of water in open air under atmospheric pressure) Isochoric constant volume (e.g. heating of gas in a sealed metal container) Reversible process the system is close to equilibrium at all times (and infinitesimal alteration of the conditions can restore the universe (s

21、ystem + surrounding) to the original state. Cyclic process the final and initial state are the same. However, q and w need not be zero. Adiabatic process dq is zero during the process (no heat is added/removed to/from the system) A combination of the above are also possible: e.g. reversible adiabati

22、c process.,Processes in TD,We will deal with some of them in detail later on,Though we all have a feel for temperature (like when we are feeling hot); in the context of TD temperature is technical term with deep meaning. As we know (from a commons sense perspective) that temperature is a measure of

23、the intensity of heat. Heat flows (energy is transferred as heat) from a body at higher temperature to one at lower temperature. (Like pressure is a measure of the intensity of force applied by matter matter (for now a fluid) flows from region of higher pressure to lower pressure). That implies (to

24、reiterate the obvious!) if I connect two bodies (A)-one weighing 100kg at 10C and the other (B) weighing 1 kg at 500C, then the heat will flow from the hotter body to the colder body (i.e. the weight or volume of the body does not matter). But, temperature comes in two important technical contexts i

25、n TD: 1 it is a measure of the average kinetic energy (or velocity) of the constituent entities (say molecules) 2 it is the parameter which determines the distribution of species (say molecules) across various energy states available.,Temperature,500C,Heat flow direction,10C,A,B,Let us consider vari

26、ous energy levels available for molecules in a system to be promoted to. At low temperatures the lower energy levels are expected to be populated more, as compared to higher energy levels. As we heat the system, more and more molecules will be promoted to higher energy levels. The distribution of mo

27、lecules across these energy levels is given by:,Temperature as a parameter determining the distribution of species across energy levels,do,Celsius (Farenheit, etc.) are relative scales of temperature and zero of these scales do not have a fundamental significance. Kelvin scale is a absolute scale. Z

28、ero Kelvin and temperatures below that are not obtainable in the classical sense. Classically, at 0K a perfect crystalline system has zero entropy (i.e. system attains its minimum entropy state). However, in some cases there could be some residual entropy due to degeneracy of states. At 0K the kinet

29、ic energy of the system is not zero. There exists some zero point energy.,Few points about temperature scales and their properties,Pressure* is force per unit area (usually exerted by a fluid on a wall*). It is the momentum transferred (say on a flat wall by molecules of a gas) per unit area, per un

30、it time. (In the case of gas molecules it is the average momentum transferred per unit area per unit time on to the flat wall). P = momentum transferred/area/time. Pressure is related to momentum, while temperature is related to kinetic energy.,Pressure,Wall of a container,Crude schematic of particl

31、es impinging on a wall.,* Normal pressure is also referred to as hydrostatic pressure. * Other agents causing pressure could be radiation, macroscopic objects impinging on a wall, etc.,Work (W) in mechanics is displacement (d) against a resisting force (F). W = F d Work has units of energy (Joule, J

32、). Work can be expansion work (PV), electrical work, magnetic work etc. (many sets of stimuli and their responses). Heat as used in TD is a tricky term (yes, it is a very technical term as used in TD). The transfer of energy as a result of a temperature difference is called heat. “In TD heat is NOT

33、an entity or even a form of energy; heat is a mode of transfer of energy” 1. “Heat is the transfer of energy by virtue of a temperature difference” 1. “Heat is the name of a process, not the name of an entity” 1. “Bodies contain internal energy (U) and not heat” 2. The flow of energy down a temperat

34、ure gradient can be treated mathematically by considering heat as a mass-less fluid 1 this does not make heat a fluid!,Heat and Work,1 Four Laws that Drive the Universe, Peter Atkins, Oxford University Press, Oxford, 2007. 2 Physical Chemistry, Ira N Levine, Tata McGraw Hill Education Pvt. Ltd., New

35、 York (2002).,To give an example (inspired by 1): assume that you start a rumour that there is lot of gold under the class room floor. This rumour may spread when persons talk to each other. The spread of rumor with time may be treated mathematically by equations, which have a form similar to the di

36、ffusion equations (or heat transfer equations). This does not make rumour a fluid!,Expansion work,Work is coordinated flow of matter. Lowering of a weight can do work Motion of piston can do work Flow of electrons in conductor can do work. Heat involves random motion of matter (or the constituent en

37、tities of matter). Like gas molecules in a gas cylinder Water molecules in a cup of water Atoms vibrating in a block of Cu.,Energy may enter the system as heat or work. Once inside the system: it does not matter how the energy entered the system* (i.e. work and heat are terms associated with the sur

38、rounding and once inside the system there is no memory of how the input was received and the energy is stored as potential energy (PE) and kinetic energy (KE). This energy can be withdrawn as work or heat from the system.,* As Aktins put it: “money may enter a back as cheque or cash but once inside

39、the bank there is no difference”.,A reversible process is one where an infinitesimal change in the conditions of the surroundings leads to a reversal of the process. (The system is very close to equilibrium and infinitesimal changes can restore the system and surroundings to the original state). If

40、a block of material (at T) is in contact with surrounding at (TT), then heat will flow into the surrounding. Now if the temperature of the surrounding is increased to (T+T), then the direction of heat flow will be reversed. If a block of material (at 40C) is contact with surrounding at 80C then the

41、heat transfer with takes place is not reversible. Though the above example uses temperature differences to illustrate the point, the situation with other stimuli like pressure (differences) is also identical. Consider a piston with gas in it a pressure P. If the external pressure is (P+P), then the

42、gas (in the piston) will be compressed (slightly). The reverse process will occur if the external (surrounding pressure is slightly lower). Maximum work will be done if the compression (or expansion) is carried out in a reversible manner.,Reversible process,T,Heat flow direction,T+T,T,Heat flow dire

43、ction,TT,Reversible process,40C,Heat flow direction,80C,NOT a Reversible process,Reversible is a technical term (like many others) in the context of TD.,Let us keep one example in mind as to how we can (sometimes) construct a reversible equivalent to a irreversible processes. Let us consider the exa

44、mple of the freezing of undercooled water* at 5C (at 1 atm pressure). This freezing of undercooled water is irreversible (P1 below). We can visualize this process as taking place in three reversible steps hence making the entire process reversible (P2 below).,How to visualize a reversible equivalent

45、 to a irreversible processes?,* Undercooled implies that the water is held in the liquid state below the bulk freezing point! How is this possible? read chapter on phase transformations,Water at 5C,Ice at 5C,Irreversible,Water at 5C,Water at 0C,Reversible,Ice at 0C,Ice at 5C,Heat,Cool,P2,P1,Ultimate

46、ly, all forms of energy will be converted to heat! One nice example given by Atkins: consider a current through a heating wire of a resistor. There is a net flow of electrons down the wire (in the direction of the potential gradient) i.e. work is being done. Now the electron collisions with various

47、scattering centres leading to heating of the wire i.e. work has been converted into heat.,P,In a closed system (piston in the example figure below), if infinitesimal pressure increase causes the volume to decrease by V, then the work done on the system is: The system is close to equilibrium during t

48、he whole process thus making the process reversible. As V is negative, while the work done is positive (work done on the system is positive, work done by the system is negative). If the piston moves outward under influence of P (i.e. P and V are in opposite directions, then work done is negative.,Re

49、versible P-V work on a closed system,1,2,A property which depends only on the current state of the system (as defined by T, P, V etc.) is called a state function. This does not depend on the path used to reach a particular state. Analogy: one is climbing a hill- the potential energy of the person is

50、 measured by the height of his CG from say the ground level. If the person is at a height of h (at point P), then his potential energy will be mgh, irrespective of the path used by the person to reach the height (paths C1 & C2 will give the same increase in potential energy of mgh- in figure below).

51、 In TD this state function is the internal energy (U or E). (Every state of the system can be ascribed to a unique U). Hence, the work needed to move a system from a state of lower internal energy (=UL) to a state of higher internal energy (UH) is (UH) (UL). W = (UH) (UL) The internal energy of an isolated system (which exchages neither heat nor mass) is constant this is one formulation of the first law of TD. A process for which the final and initial states are same is called a cyclic process. For a cyclic process change in a state function is zero. E.g. U(cyclic process) = 0.,