Thermodynamics: Understanding Systems, Properties, and Equilibrium, Exercises of Thermodynamics

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Presented by: John Lester D. Mones

Objectives

At the end of this course, the learners should be able to:

• explain the basic concepts of thermodynamics such as system, state, equilibrium, process, and cycle;
• discuss the steady-flow process of fluid in control volume; and
• solve thermodynamic problems involving temperature scales and pressure

• Open system (also known as a control volume)
  • mass or energy can cross the boundary
• Closed system (also known as a control mass)
  • no mass can enter or leave but energy, in the form of heat or work, can cross the boundary
• Isolated system
  • even energy is not allowed to cross the boundary

• mass cannot cross the boundaries of a closed system, but energy can
• a closed system with a moving boundary

• An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition

• Density is defined as mass per unit volume

• Specific volume is defined as volume per unit mass

• For a differential volume element of mass dm and volume dV, density can be expressed as

(kg/m 3 )

• Specific gravity (or relative density) is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature

𝟐𝐎

• In SI units, the numerical value of the specific gravity of a substance is exactly equal to its density in g/cm 3 or kg/L
• Specific weight is the weight of a unit volume of a substance

𝛄𝐬 = 𝛒𝐠 (N/m

3 )

There are many types of equilibrium:

• Thermal equilibrium – temperature is the same throughout the entire system
• Mechanical equilibrium – there is no change in pressure at any point of the system with time phase (a) Before (b) After

There are many types of equilibrium:

• Phase Equilibrium - when the mass of each phase reaches an equilibrium level and stays there
• Chemical equilibrium - if the chemical composition does not change with time (no chemical reactions occur)
• Thermodynamic equilibrium – the conditions of all the relevant types of equilibrium are satisfied

• Quasi-static (or quasi-equilibrium, process) – any a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts. (a) Slow compression (quasi-equilibrium) (b) Very fast compression (nonquasi-equilibrium)

• Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes

• Steady implies no change with time. The opposite of steady is unsteady/transient.
• Uniform implies no change with location over a specified region
• Steady-flow process is a process during which a fluid flows through a control volume steadily
• Therefore, the volume V, the mass m, and the total energy content E of the control volume remain constant during a steady-flow process T ime: 1 PM T ime: 3 PM

• Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as: