Thermodynamics: Laws and Principles

Thermodynamics Review

• Thermodynamics is the science of energy, where 'therme' means heat and 'dynamic' means power.

Conservation of Energy Principle

• States that during an interaction, energy can change form, but the total amount of energy remains constant i.e. energy cannot be created or destroyed.

First Law of Thermodynamics

• It is an expression of the conservation of energy principle.
• Asserts that energy is a thermodynamic property.

Second Law of Thermodynamics

• Energy has quality as well as quantity.
• Actual processes occur in the direction of decreasing quality of energy.

Historical Context

• Thomas Savery (1697) and Thomas Newcomen (1712) created the first successful atmospheric steam engines.
• First and second laws emerged primarily out of the works of William Rankine, Rudolph Clausius.
• Lord Kelvin also contributed to the field.

Molecules

• A substance consists of a large number of particles.

Classical Thermodynamics

• It uses a macroscopic approach that does not require knowledge of the behavior of individual particles.
• It offers a direct and easy way to find solutions.

Statistical Thermodynamics

• It uses a more elaborate approach.
• It builds on the average behavior of large groups of individual particles.

Application Areas of Thermodynamics

• Application areas of Thermodynamics include daily life and nature.
• It also encompasses temperature and heat transfer, as well as industrial and mechanical systems.

Dimensions

• Dimensions can be any physical quantity.

Units

• Units are magnitudes assigned to dimensions.
• Primary or fundamental dimensions include mass, length, time, and temperature.
• Secondary or derived dimensions include velocity, energy and volume.

English System

• The English System refers to the United States Customary System (USCS).

Metric SI

• The Metric SI refers to Le Système International d'Unités

Libra

• Libra is a pound symbol Ib.
• It refers to an ancient Roman unit of weight.

Force Unit: Newton (SI)

• Force required to accelerate a mass of 1 kg at a rate of 1 m/s².

Pound-Force (English)

• Force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s².

Weight

• Weight is a force, which is gravitational force applied to a body.
• Its magnitude is determined from Newton's 2nd Law.

Specific Weight

• Weight of a unit volume of a substance is specific weight.
• γ = ρg

Work

• Work is a form of energy defined as force times distance.
• N⋅m = Joule

System

• A system is defined as a quantity of matter or a region in space chosen for study.

Surroundings

• Surroundings are described as the mass or region outside the system.

Boundary

• The boundary is a real or imaginary surface that separates the system from its surroundings.
• It has zero thickness and can neither contain any mass nor occupy any volume in space.

Closed System (Control Mass)

• Consists of a fixed amount of mass, and no mass can cross its boundary.
• Energy, in the form of heat or work, can cross the boundary.

Isolated System

• Even energy is not allowed to cross the boundary.

Open System (Control Volume)

• It is a properly selected region in space.
• It involves mass flow, such as in a compressor, turbine or nozzle.
• Both mass and energy can cross the boundary.
• It involves fixed, moving, real, imaginary boundaries.
• The control surface is the boundary of the control volume.

Property

• Property is any characteristic of a system.

Intensive Properties

• Properties that are independent of the mass of a system.
• T, P, p

Extensive Properties

• Properties whose values depend on the size or extent of the system.
• Total mass, volume, and momentum.

Specific Properties

• Extensive properties per unit mass.

Density

• Density is mass per unit volume.
• ρ = m/V (kg/m³)

Specific Volume

• Volume per unit mass.
• v = V/m = 1/ρ (m³/kg)

Specific Gravity / Relative Density

• Ratio of the density of a substance to the density of some standard substance at a specified temperature.
• SG = ρ/ρH2O

Specific Weight

• Weight of a unit volume of a substance.

Equilibrium

• State of balance.
• No changes occur when the system is isolated from its surroundings.

Thermal Equilibrium

• Temperature is the same throughout the entire system.

Mechanical Equilibrium

• Related to pressure.
• No change in pressure at any point of the system with time.
• Pressure may vary with elevation.

Phase Equilibrium

• System involves two phases.
• Mass of each phase reaches an equilibrium level and stays there.

Chemical Equilibrium

• Chemical composition does not change with time.
• No chemical reactions occur.

The State Postulate

• The state of a simple compressible system is determined by two independent, intensive properties.

Process

• Any change that a system undergoes from one equilibrium state to another.

Path

• A series of states through which a system passes during a process.

Quasistatic / Quasi-Equilibrium Process

• Sufficiently slow process that allows the system to adjust itself internally.
• System remains infinitesimally close to an equilibrium state at all times.

Non-quasi-Equilibrium

• Very fast compression.

Iso-

• Used to designate a process for which a particular property remains constant.

Isothermal Process

• Temperature remains constant.

Isobaric Process

• Pressure remains constant.

Isochoric (Isometric) Process

• Specific volume remains constant.

Zeroth Law of Thermodynamics

• If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

Ice Point

• Mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm.

Steam Point

• Mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm.

Celsius Scale

• Centigrade scale.

Fahrenheit Scale

Thermodynamic Temperature Scale

• Temperature scale that is independent of properties of any substance.
• K = °C + 273.15
• R = °F + 460
• R = 1.8K
• °F = 1.8°C + 32

Pressure

• Normal force exerted by a fluid per unit area (gas or liquid).
• Counterpart is normal stress.
• P = F/A (N/m²) = Pa

Absolute Pressure

• Actual pressure at a given point.

Gage Pressure

• Difference between absolute pressure and local atmospheric pressure.
• Pabs = Pgage + Patm

Vacuum Pressure

• Pressure below atmospheric pressure.
• Pabs = Patm – Pvac

Manometer

• Fluid column used to measure pressure difference.

Chapter 2

Steady Flow

• Neither the mass nor energy within the control volume change with respect to time.

Mass Flow Rate (ṁ)

• Amount of mass flowing through a cross-section per unit time.

Volume Flow Rate (V)

• Volume of fluid flowing through a cross-section per unit time.
• ṁ = ρAV
• V = AV

Conservation of Mass / Continuity Equation

• Mass cannot be created nor destroyed.

Conservation of Energy Principle

• First Law of Thermodynamics.
• Energy can neither be created nor destroyed; it can only be converted from one form of energy to another.

Macroscopic

• A system possesses with respect to some outside reference frame.

Microscopic

• Related to the molecular structure of a system and degree of molecular activity.
• Independent of the outside reference frame.

Internal Energy

• Sum of all microscopic forms of energy.

Forms of Energy

• Ability of a substance or medium to release power when its stored energy is used up.

Kinetic Energy

• A system possesses energy as a result of its motion relative to a reference frame.
• KE = (1/2)mv² (J)

Potential Energy

• A system possesses energy due to its elevation.
• PE = mgh

Internal Energy

• Developed through the movement of molecules within substances brought about by a change in temperature.
• U = mC(T₂ - T₁) (J).

Flow Work

• Work due to the movement of volume brought about by the displacement of boundaries.

Enthalpy

• Total amount of heat present in a system where pressure is constant.
• H = mC(T₂ - T₁) (J)

Entropy

• Randomness of a system.
• S = Q/T (J/K)

Exchange of Energy between Control Volume and Surroundings

• Heat transfer
• Work
• Mass transfer

Energy Balance Equation

• ΣEin = ΣEout
• PE1 + KE1 + U1 + P1V1 + QA = PE2 + KE2 + U2 + P2V2 + 𝑊 + Eloss

Sign Convention

• Work done by the system (-)
• Heat added to the system (+)
• Heat rejected by the system (-)
• Work done on the system (+)

Energy Transfer

• Types of Heat.

Sensible Heat

• Causes an object’s temperature to change.

Latent Heat

• Heat that corresponds to the heat added or rejected to an object in order for it to change state.

Heat

• Form of energy that is transferred between two systems by virtue of a temperature difference.

Adiabatic Process

• There is no heat transfer.
• Adiabats (not to be crossed).

Conduction

• Heat is transferred through interaction.

Convection

• Transferred through liquid.

Radiation

• Due to emission of electromagnetic waves.

Chapter 3: Ideal Gasses

• It is the product of volume and absolute pressure of a defined amount of gas.
• Shows a direct correlation to absolute temperature
• PV = KT

K (Boltzmann Constant)

• It is a constant
• K = 1.38 * 10-23 J/K

Equation of State

• Relates to pressure, volume and temperature
• PV = nRT
• P = pressure
• Volume (m^3, in^3, ft^3) R = Universal gas constant
• It is constant (k, R) T = absolute temp

Mole

• Relates to mount of substance
• Indicates the amount of atoms as possible or molecules
• PV = mRT
• Related to mass equation

Ideal Gas Laws

• Includes Boyles law, charles, and Gay-Lussac

Boyles Law

• States: Pressure is inversely proportional to the volume
• States: Volume is directly proportional to the temp
• States: Pressure = directly proportional to temperature

Avogadros number

• Na = 6.02 * 10^23 mol^-1
• States: the actual number of atoms and molecules to work