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Note for Thermodynamics - TD By Mahabub Alom

  • Thermodynamics - TD
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Thermodynamic Properties (TP) Thermodynamic Properties • ◦ ◦ ◦ ◦ ◦ ◦ ◦ • ◦ ◦ ◦ ◦ ◦ • ◦ ◦ ◦ Define and articulate some of the critical language and concepts of Thermodynamics Distinguish between the universe, system, surroundings, and boundary Define open system, closed system, and isolated system Define adiabatic, isothermal, isobaric, and isochoric processes Distinguish between extensive and intensive thermodynamic properties Explain the difference between state and path variables Distinguish between equilibrium and steady state Define the term phase and explain what it means for phases to be in equilibrium Relate properties to phase behavior Relate the measured thermodynamic properties of temperature and pressure to molecular behavior Describe phase and chemical reaction equilibrium in terms of dynamic molecular processes Apply the state postulate and the Gibbs phase rule to determine the number of required independent properties needed to constrain the state of a system (pure species) Identify the phases present on a PT and/or Pv diagram as well as the critical point and triple point Describe the difference between the saturation and vapor pressures Determine thermodynamic properties using both calculations and tabulated data Read desired thermodynamic properties from steam tables Using linear, sometimes double, interpolation to calculate property values from sparse tabular data Use equations of state to calculate unknown properties from measured properties TP: The origins of Thermodynamics The origins of Thermodynamics Thermodynamics began as a way to evaluate the potential of steam engines to provide work.

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It is based on "laws" that are simple generalized statements that are consistent with all known observations. NOTE: It tells us nothing about rates. As we will see in the Transport Pillar, rates depend on driving forces and resistances. While Thermo tells us about driving forces, it tells us nothing about resistances. NOTE: Also, Thermo is a "macroscopic" science in that it cannot describe the molecular mechanisms of events (although a molecular viewpoint can help us understand Thermodynamic properties, as we will see). TP: Thermodynamic Properties Thermodynamic Properties We will distinguish between the properties of a material in several different ways: DEFINITION: Measured properties are properties that are directly accessible in the laboratory. Examples include: temperature, pressure, volume DEFINITION: Fundamental properties are properties that are directly related to the fundamental laws of thermodynamics. These are internal energy and entropy (both will be discussed later). DEFINITION: Derived properties are specific relations that include combinations of measured and derived properties. Examples include enthalpy, Gibbs free energy. NOTE: Both fundamental and derived properties are unmeasurable TP: Extensive vs Intensive Properties Properties may be further classified in the following way: DEFINITION: An extensive variable is one which depends on system size. Examples include mass, volume. While extensive variables are useful for characterizing the specific system being analyzed.

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DEFINITION: An intensive variable is one which does not depend on system size. Examples include temperature, pressure, density. While it may not be immediately obvious, intensive variables tell us more about the system than extensive variables. In particular, the temperature and pressure are two of the most critical intensive variables. NOTE: A ratio of extensive variables will yield an intensive variable! (For example, mass/volume -- two extensive variables -- gives density, an intensive variable) This is one way to understand why intensive variables "tell you more". OUTCOME Distinguish between extensive and intensive thermodynamic properties TP: Scales of scrutiny Scales of scrutiny We will discuss three scales of scrutiny: Macroscopic, microscopic, and molecular. Macroscopic refers to bulk systems that are readily observable. Microscopic refers to regions that are smaller than those observable by the naked eye, but which contain sufficient numbers of molecules so that they may be considered "continuous". The molecular scale refers to a region that is small enough that it includes individual atoms/molecules (so that thermodynamic properties are no longer continuous). TP: Measured Properties Measured Properties • Amount: Volume V [=] LxLxL extensive Mass/moles M [=] M or N [=] moles extensive Intensive "volume" (often called specific volume,) or • Temperature: Always intensive. Refers to the degree of hotness. On a molecular scale it is a measure of the average kinetic energy of the molecules/atoms in a system (that is, = (3/2) kT). Obviously, because it is an average, there will be a distribution of kinetic energies of the molecules. The distribution that is observed in gases is called the Maxwell-Boltzmann distribution and is how one determines when there are enough gas molecules in a sample to consider the continuum hypothesis (i.e., a macroscopic description of the material) to be valid. • Pressure: Always intensive. Refers to the normal force per unit area exerted on a "surface". Pressure itself has no direction (it is a scalar quantity), but it can obtain a

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direction from the surface. The pressure in a continuous medium is the force exerted on a hypothetical surface. From a molecular viewpoint, this force is exerted due to the exchange of momentum between molecules and the surface during collisions with that surface (note that the units of force is the rate of change of momentum). Converting between temperature scales: • T(K) = T(C) + 273.15 • T(R) = T(F) + 459.67 • T(R) = 1.8*T(K) • T(F) = 1.8*T(C) + 32 NOTE: Converting temperature differences is slightly different (the additive corrections cancel out!). TEST YOURSELF: Convert 20C to F. How about R? Convert -40C to F :) TP: Work and Energy Work and Energy Recall that Thermo began as a way to evaluate the potential of steam engines. In other words, the goal was to ascertain how much work could be extracted from a given system. DEFINITION: Work is the action of a force over a distance. In other words, moving stuff is doing work. NOTE: The pressure has no direction thing is the origin of the - sign; specifically, since moving something requires *input* of work we need to calculate the work to compress something (which would cause a dV<0) in such a way as to give us a positive work. Energy has the same units as work and can be thought of as somethings's ability to perform work (if you like). DEFINITION: Kinetic energy follows naturally from Newton's Law and can be thought of as energy related to motion: integrating gives: DEFINITION: Potential energy can be thought of as energy related to position

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