The first law of thermodynamics is all about energy conservation. It's the foundation for understanding how energy moves and changes in systems. This law helps us track energy as it flows in and out, whether through heat, work, or mass transfer.

We use energy balance equations to apply the first law to real-world situations. These equations let us analyze both closed systems, which don't exchange mass, and open systems, which do. Understanding these concepts is key to solving thermodynamic problems.

First Law and Energy Balance

Conservation of Energy

• First law of thermodynamics states energy cannot be created or destroyed, only converted from one form to another
• Energy balance is the accounting of energy transfer into and out of a system
  • Based on the principle of conservation of energy
  • Includes heat transfer, work, and changes in internal, kinetic, and potential energy
• Closed system does not exchange mass with its surroundings, only energy (heat and work)
  • Examples include a sealed piston-cylinder device or a rigid, sealed tank
• Open system exchanges both mass and energy with its surroundings
  • Also known as a control volume
  • Examples include a turbine, nozzle, diffuser, or heat exchanger

Energy Balance Equations

• First law for a closed system: $\Delta E_{system} = Q - W$
  • $\Delta E_{system}$ is the change in total energy of the system (internal, kinetic, and potential)
  • $Q$ is the net heat transfer into the system
  • $W$ is the net work done by the system
• Energy balance for an open system: $\Delta E_{CV} = Q - W + \sum\limits_{in} \dot{m}(h + \frac{V^2}{2} + gz) - \sum\limits_{out} \dot{m}(h + \frac{V^2}{2} + gz)$
  • $\Delta E_{CV}$ is the change in total energy of the control volume
  • $\dot{m}$ is the mass flow rate
  • $h$ is the specific enthalpy, $\frac{V^2}{2}$ is the specific kinetic energy, and $gz$ is the specific potential energy

Thermodynamic Processes

Steady-State and Transient Processes

• Steady-state process occurs when a system operates under steady conditions and all properties are independent of time
  • Examples include a power plant operating at constant load or a refrigerator maintaining a constant temperature
• Transient process occurs when a system undergoes changes in its properties with time
  • Also known as an unsteady process
  • Examples include startup or shutdown of a power plant, or a batch chemical reactor

Adiabatic and Isothermal Processes

• Adiabatic process occurs when there is no heat transfer between a system and its surroundings ($Q = 0$)
  • Can be reversible (quasi-equilibrium) or irreversible
  • Examples include a well-insulated piston-cylinder device or a rapid expansion or compression
• Isothermal process occurs at constant temperature
  • Requires heat transfer between the system and surroundings to maintain constant temperature
  • Examples include a piston-cylinder device with a heat reservoir, or a phase change process at constant pressure (boiling or condensation)