Engineering Fundamentals II

Thermodynamics: Units and Dimensions, Problem

Solving, and Systems

Units and Dimensions

• Basic 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

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

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

• Extensive– 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, andB 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