Properties, state, and equilibrium form the foundation of thermodynamics. These concepts help us understand how systems behave and change. By grasping these ideas, you'll be able to analyze energy transfer and transformation in various systems.
Knowing the difference between extensive and intensive properties is crucial. Understanding thermodynamic state, equilibrium, and the state postulate will help you predict system behavior. These concepts are essential for solving real-world engineering problems involving energy systems.
Extensive vs Intensive Properties
Defining Extensive and Intensive Properties
- Extensive properties depend on the size or extent of a system
- Examples of extensive properties include volume, energy, and entropy
- Intensive properties are independent of the system size
- Examples of intensive properties include temperature, pressure, and density
Relationships between Extensive and Intensive Properties
- The ratio of two extensive properties is an intensive property
- For example, the ratio of mass to volume gives density, an intensive property
- During a process, the value of an extensive property changes in proportion to the change in mass
- Intensive properties remain constant if mass is changed while other extensive properties are held constant
- Extensive properties are additive for subsystems, meaning the property for the whole system is the sum of the property for the parts
- For example, the total volume of a system is the sum of the volumes of its subsystems
- Intensive properties are not additive
- For example, the temperature of a system is not the sum of the temperatures of its subsystems
Thermodynamic State and Variables
Understanding Thermodynamic State
- The thermodynamic state refers to the condition of a system as described by its physical properties
- Examples of physical properties include temperature, pressure, volume, and composition
- A state variable is a property that depends only on the current state of the system, not on how the system reached that state
- Common state variables include pressure ($P$), temperature ($T$), volume ($V$), internal energy ($U$), enthalpy ($H$), and entropy ($S$)
- A change in state variables describes a change in the system's thermodynamic state
- For example, if the temperature and volume of a gas change, the system has undergone a change in its thermodynamic state
Thermodynamic Processes and State Postulate
- A thermodynamic process is a change from one state to another
- A process can be described by specifying the initial and final states, along with the path taken between the states
- Examples of thermodynamic processes include isothermal (constant temperature), isobaric (constant pressure), and adiabatic (no heat transfer) processes
- The number of independent properties required to fix the state of a system is given by the state postulate
- For a simple compressible system, the state is fixed by specifying any two independent, intensive properties
- Examples of independent properties include temperature and specific volume, pressure and specific enthalpy, and pressure and specific entropy
Thermodynamic Equilibrium and Types
Defining Thermodynamic Equilibrium
- Thermodynamic equilibrium is a condition in which there are no unbalanced potentials (or driving forces) within the system
- A system in equilibrium experiences no changes in its macroscopic properties over time
- A system can be in one type of equilibrium but not necessarily in other types
- For a system to be in thermodynamic equilibrium, it must be in thermal, mechanical, and chemical equilibrium simultaneously
Types of Thermodynamic Equilibrium
- Thermal equilibrium: A system is in thermal equilibrium if the temperature is uniform throughout the entire system
- In thermal equilibrium, there is no net heat transfer within the system or with the surroundings
- Example: A cup of coffee left at room temperature will eventually reach thermal equilibrium with its surroundings
- Mechanical equilibrium: A system is in mechanical equilibrium if there is no change in pressure at any point of the system with time
- In mechanical equilibrium, there are no unbalanced forces acting within the system or between the system and its surroundings
- Example: A gas in a sealed container at rest is in mechanical equilibrium
- Chemical equilibrium: A system is in chemical equilibrium if there is no change in the chemical composition of the system with time
- In chemical equilibrium, the forward and reverse reactions proceed at the same rate, resulting in no net change in the amounts of reactants and products
- Example: A saturated solution of salt in water is in chemical equilibrium, with the rate of dissolution equal to the rate of crystallization
State Postulate for Simple Systems
Applying the State Postulate
- The state postulate asserts that the thermodynamic state of a simple compressible system is completely specified by two independent, intensive properties
- A simple compressible system is one for which the effects of motion and gravity are negligible
- Common pairs of independent properties used to fix the state of a simple compressible system include:
- Temperature and specific volume
- Pressure and specific enthalpy
- Pressure and specific entropy
- If the state postulate is satisfied, all other intensive properties can be expressed as functions of the two specified independent properties
- For example, if temperature and specific volume are known, pressure can be determined using an equation of state, such as the ideal gas law ($PV = nRT$)
Property Relations and Diagrams
- The state postulate allows the development of property relations, tables, and diagrams that help in analyzing thermodynamic processes
- Examples of property relations and diagrams include:
- P-v-T surface: A three-dimensional surface representing the relationship between pressure, specific volume, and temperature for a substance
- P-v diagram: A two-dimensional diagram showing the relationship between pressure and specific volume for a process
- T-s diagram: A two-dimensional diagram showing the relationship between temperature and specific entropy for a process
- These property relations and diagrams are useful tools for visualizing and analyzing thermodynamic processes and cycles
- For example, the area under a process curve on a P-v diagram represents the work done during the process