Engineering Fundamentals II Thermodynamics: Units and Dimensions, Problem Solving, and Systems

Units and Dimensions Basic Dimensions Derived Dimensions Length [L] meter foot mass [m] kilogram pound-mass Time [t] second second Temperature [T] K or °C °R or °F Derived Dimensions Force [F] Newton pound-force Energy [E] joule foot-pound

Example: Units and Dimensions Always include units in calculations E.g. Dimensional analysis

Problem Solving Known properties Find Schematic and Data (from tables, etc.) Assumptions (e.g. ideal gas) Properties Analysis Comments

Macroscopic and Microscopic The everyday experience of smoothness of matter is an illusion. Microscopic – statistical thermodynamics Explains mechanics of temperature, pressure and latent heats. Macroscopic – classical thermodynamics Based on volumes large enough that statistical deviation is not measurable A limit of statistical thermodynamics where properties are understood as averages.

Systems Closed, Isolated, Open Systems Properties and States Processes and Cycles Extensive and Intensive Properties Equilibrium Temperature

Identify the System

Defining the boundary is critical! ENGM 295 Spring 2008 Defining the boundary is critical!

Closed: no mass crosses boundary

Isolated: no mass, energy (via work or heat) or entropy crosses boundary ….

Open: mass, energy and entropy cross boundary

Properties Intensive – independent of “amount of system” Density (specific volume) Temperature Pressure Also: velocity, voltage Extensive – dependent on “amount of system” Weight and mass Volume Energy Entropy Also: momentum, charge

Extensive and Intensive Properties The sum of its parts Can have a density attributed to it e.g. momentum, mass, charge, entropy Intensive Remains the same when body is divided Can vary within a body e.g. velocity, pressure, voltage, temperature

Example: Properties Weight (W) and mass (m) Volume (V) and specific volume (v = V/m) Density (ρ = m/V) Specific weight (γ = W/V) Specific gravity (SG = ρ/ρ water) Pressure (p = Force/Area) Atmospheric pressure = 101 kPa Pressure Head Pascal’s law

Measuring Pressure

Example 2-4

States A collection of properties. Some properties are “state variables” You can integrate between two states to determine the property’s value Steady state –properties constant in time

Process: change of state The change in value of a property that is altered is determined solely by the end states. If the value of a quantity depends on the process, it is not a property.

Cycle: Series of processes that return to the initial state

Zeroth law of Thermodynamics A is in thermal equilibrium with C, and B is in thermal equilibrium with C, then A is in thermal equilibrium with B.

Thermal equilibrium means: Temperature is equalized. Energy is not necessarily equalized.

Temperature and Thermometers Thermometer in thermal equilibrium with substance being measured.

Temperature Ideal gas temperature: p(T) = p0(1+βT) → p(T) = p0βTK i.e. pV = mRT → p = (constant)T Absolute Zero – 0 K (no degree symbol)

Quasi-equilibrium Processes Systems are not always in thermal equilibrium during a process. Non-equilibrium states exhibit spatial variations of intensive properties. Quasi-equilibrium An idealized process Departure from equilibrium is infinitesimal