Thermodynamics and Energy: Principles

• Thermodynamics centers around obtaining power from hot bodies and is encountered in daily life.
• Physical processes in nature can be spontaneous, forced, unidirectional, or bidirectional.
• Thermodynamics defines occurrence of physical processes associated with energy transformation.
• Thermodynamics establishes the relationships between different physical properties affected by these processes.
• Thermodynamics guides natural processes involving energy transfer and conversion, with directional and quantitative constraints.

Thermodynamics and Energy

• Energy is the ability to cause changes.
• Thermodynamics is the science of energy.
• The word "thermodynamics" comes from the Greek words "therme" (heat) and "dynamics" (power).
• "Thermo" refers to energy transfer via heat.
• "Dynamics" refers to motion as mechanical work.
• The conservation of energy principle states energy can change forms, but the total amount remains constant.
• Energy cannot be created or destroyed.

Laws of Thermodynamics

• The first law is an expression of the conservation of energy principle.
• Energy is a thermodynamic property.
• dQ = dU + dW represents the relationship between heat input (dQ), internal energy (dU), and work output (dW).
• The second law asserts energy has quality as well as quantity.
• Actual processes occur in the direction of decreasing energy quality.
• The second law defines the efficiency of processes converting heat to work.
• No heat engine or animal has 100% efficiency.

Approaches to Thermodynamic Studies

• Macroscopic Approach (Classical Thermodynamics):
  • Studies thermodynamics without needing to know the behavior of individual particles.
  • "Macro" means big or total.
  • It provides a direct and easy way to solve engineering problems and is used in this text
• Microscopic Approach (Statistical Thermodynamics):
  • It based on the average behavior of large groups of individual particles.
  • "Micro" means small.

Application Areas of Thermodynamics

• Thermodynamics applies to all activities in nature involving energy and matter interaction.
• It is hard to imagine an area that does not relate to thermodynamics.

Importance of Dimensions and Units

• Any physical quantity is characterized by dimensions.
• The magnitudes assigned to dimensions are units.
• Basic dimensions: mass (m), length (L), time (t), and temperature (T).
• These are considered primary or fundamental.
• Secondary (derived) dimensions: velocity (V), energy (E), and volume (V).
• These are expressed using primary dimensions.
• Metric SI system: A simple, logical system based on decimal relationships.
• English system: It features no apparent systematic numerical base, using arbitrary relationships between units.

Systems and Control Volumes

• System: A chosen quantity of matter or space region for study.
• Surroundings: The mass or region outside the system.
• Boundary: The real or imaginary surface separating the system from surroundings.
• It distinguishes the system from its surroundings
• Boundaries can be fixed or movable.
• Systems can be closed or open.

Closed System (Control Mass)

• Characterized by a fixed amount of mass that cannot cross its boundary.

Isolated System

• A special closed system which does not interact with its surroundings.

Open System (Control Volume)

• A selected space region.
• It usually encloses a device with mass flow like a compressor, turbine, or nozzle.
• Both mass and energy can cross the control volume boundary.
• Control Surface: The boundary of a control volume, real or imaginary.

Properties of a System

• Property: Macroscopic quantity that can be measured or calculated.
• It depends on the system state, not the path to that state.
• Properties describe a system only in an equilibrium state.
• Familiar properties: pressure (P), temperature (T), volume (V), and mass (m).

Types of Properties

• Intensive properties: Independent of system mass (temperature, pressure, density).
• Extensive properties: Depend on system size or extent.
  • If you halve the system and its value is halved, the property is extensive.
• Specific properties: Extensive properties per unit mass.
  • (v = V/m)

Density and Specific Gravity

• Density: How much matter occupies a given space amount.
• Specific volume: Volume occupied by a system's unit mass.
• The unit of mass is kilograms, and the unit of volume is cubic meters.
• Specific gravity: Ratio of a substance's density to a standard substance's density at a specified temperature (usually water at 4°C).
• Specific weight: Weight of a unit volume of a substance.

State and Equilibrium

• Thermodynamics deals with equilibrium states.
• Equilibrium: A balance state.
• A system is in equilibrium when no unbalanced potentials or driving forces exist within its system boundary.
• Thermal equilibrium is reached when temperature is uniform throughout the system.
• Mechanical equilibrium occurs when there is no pressure change at any point of the system with time.
• Phase equilibrium is reached in a multi-phase system when the mass of each phase stabilizes.
• Chemical equilibrium is when the chemical composition of a system remains constant, with no chemical reactions occurring.

Properties to Define a State

• State Postulate: Two independent, intensive properties are needed to specify the state of a simple compressible system (P, T, v, density).
• Simple system: Gravitational, electrical, magnetic, motion, and surface tension effects are absent.
• Compressible system: A system whose volume changes with pressure.
• For an incompressible simple system, one intensive property is enough to describe its state.

Processes and Cycles

• Process: Any change a system undergoes from one equilibrium state to another.
• Path: The series of states through which a system passes during a process.
• Complete process description: Initial and final states, path, and interactions with surroundings.

Quasi-Equilibrium Process

• This is a process performed slowly, with infinitesimal deviations from equilibrium.
• Intermediate states are considered equilibrium states.

Process Diagrams

• Represented by plotting thermodynamic properties as coordinates which helps visualize processes.
• Common properties used: temperature (T), pressure (P), volume (V), or specific volume (v).
• The prefix “iso-” designates a process where a particular property remains constant.

Types of Processes Encountered in Thermodynamics

• Isothermal: Constant temperature.
  • Example: Freezing water to ice at -10°C.
• Isobaric: Constant pressure.
  • Example: Heating water in open air.
• Isochoric: Constant volume.
  • Example: Heating gas in a sealed metal container.
• Reversible: The system is close to equilibrium at all times.
  • Infinitesimal changes restore the universe (system + surroundings) to the original state; truly reversible processes do not exist in nature.
• Cyclic: The final and initial states are the same, but q and w need not be zero.
• Adiabatic: No heat is added or removed from the system.
  • dq is zero.
• Combinations: Reversible adiabatic process.

Steady-Flow Process

• Steady implies no change with time. The opposite is unsteady, or transient.
• Many engineering devices operate for long periods under the same conditions.
• These are classified as steady-flow devices.
• Process: A fluid flows steadily through a control volume.
• Good Approximations: Turbines, pumps, boilers, condensers, heat exchangers, power plants, or refrigeration systems.

Temperature

• Temperature is a technical term with deep meaning.
• It is a measure of the intensity of heat.
• Heat flows from higher to lower temperature.
• Temperature shows the average kinetic energy or velocity of entities and determines distribution of species across energy states.

Temperature and Zeroth Law

• Zeroth Law of Thermodynamics: If two bodies are individually in thermal equilibrium with a third body, then they are also in thermal equilibrium with each other.
• Replacing the third body with a thermometer: Two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.

Temperature Scales

• All based on reproducible states like water's freezing and boiling points.
• Ice point: Ice and water mixture in equilibrium with air saturated with vapor at 1 atm (0°C or 32°F).
• Steam point: Liquid water and vapor in equilibrium at 1 atm (100°C or 212°F).
• Celsius scale: SI unit system temperature.
• Fahrenheit scale: English unit system temperature.
• Thermodynamic temperature scale: Independent of substance properties.
  • Kelvin (SI) and Rankine.

Pressure

• Pressure: Normal force exerted by a fluid per unit area.
• 1 Pa = 1 N/m²
• 1 bar = 105 Pa = 0.1 MPa = 100 kPa
• 1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars
• 1 kgf/cm² = 9.807 N/cm² = 9.807 × 104 N/m² = 9.807 × 104 Pa = 0.9807 bar = 0.9679 atm

Absolute Pressure

• The actual pressure at a given position, measured relative to absolute vacuum.
• It references the zero pressure of a complete vacuum.

Gage Pressure

• The difference between absolute pressure and local atmospheric pressure.
• Most devices are calibrated to read zero in the atmosphere: they indicate gage pressure.

Vacuum Pressures

• Pressures are below atmospheric pressure.

Pressure Measurement

• Pressure measures absolute pressure unless stated otherwise.

Variation of Pressure with Depth

• Pressure is the change as depth increases.

Pascal's Law

• The pressure applied to a confined fluid increases throughout the same amount.
• The area ratio is called the ideal mechanical advantage of the hydraulic lift.

Pressure Measurement Devices

• Barometer:
  • Measures Atmospheric pressure
• Manometer:
  • Measures small and moderate pressure differences.
  • Contains one or more fluids such as mercury, water, alcohol, or oil.
• Bourdon tube:
  • A hollow metal tube bent like a hook. The end is closed, then connected to a dial indicator needle.
• Pressure transducers:
  • Techniques to convert pressure effect to an electrical effect, e.g., changes in voltage, resistance, or capacitance.
  • Pressure transducers are fast, small and reliable.
  • These components more sensitive and precise than mechanical counterparts.
• Strain-gage pressure transducers:
  • Work by having a diaphragm deflect between two chambers open to the pressure inputs.
• Piezoelectric transducers:
  • Transducers convert physical pressures like force or acceleration into an electric charge.

Problem-Solving Technique:

• Step 1: Problem Statement
• Step 2: Schematic
• Step 3: Assumptions and Approximations
• Step 4: Physical Laws
• Step 5: Properties
• Step 6: Calculations
• Step 7: Reasoning, Verification, and Discussion