🔥Thermodynamics I Unit 3 – First Law of Thermodynamics

Key Concepts and Definitions

• First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another
• Internal energy $(U)$ represents the total energy contained within a system, including kinetic and potential energy of its particles
• Enthalpy $(H)$ is a thermodynamic property defined as the sum of internal energy and the product of pressure and volume $(H = U + PV)$
• Closed system does not exchange matter with its surroundings, but can exchange energy in the form of heat or work
• Open system exchanges both matter and energy with its surroundings
  • Examples of open systems include a steam turbine or a chemical reactor
• Isolated system does not exchange either matter or energy with its surroundings
• Quasi-static process occurs slowly enough that the system remains in thermodynamic equilibrium throughout the process
• Reversible process can be reversed without any net change to the system or its surroundings

Historical Context and Development

• The First Law of Thermodynamics emerged from the work of several scientists in the 19th century, including Julius Robert von Mayer, James Prescott Joule, and Hermann von Helmholtz
• Mayer proposed the conservation of energy principle in 1842, stating that energy cannot be created or destroyed
• Joule demonstrated the equivalence of mechanical work and heat through a series of experiments (Joule's apparatus)
• Helmholtz formulated a mathematical expression of the First Law in 1847, combining the work of Mayer and Joule
• Rudolf Clausius introduced the concept of internal energy in 1850 and refined the mathematical formulation of the First Law
• The development of the First Law laid the foundation for the field of thermodynamics and its applications in various branches of science and engineering
  • It provided a framework for understanding heat engines, refrigeration cycles, and chemical reactions

Energy and Its Forms

• Energy is the capacity to do work or cause change in a system
• Kinetic energy is the energy associated with the motion of an object $(KE = \frac{1}{2}mv^2)$
• Potential energy is the energy stored in an object due to its position or configuration (gravitational, elastic, electric)
• Internal energy includes the kinetic and potential energy of the particles within a system
  • Translational, rotational, and vibrational motions contribute to internal energy
• Chemical energy is stored in the bonds between atoms and can be released or absorbed during chemical reactions
• Thermal energy is the energy associated with the random motion of particles in a substance
• Electrical energy is the energy associated with the flow of electric charges (current)
• Mechanical energy is the sum of kinetic and potential energy in a system

System and Surroundings

• A system is the specific portion of the universe under study, while the surroundings include everything else that can interact with the system
• Boundary separates the system from its surroundings and can be real or imaginary
• State of a system is described by its thermodynamic properties (pressure, volume, temperature)
• State variables depend only on the current state of the system, not on how it reached that state
• Process is a change in the state of a system, characterized by the initial and final states
• Path is the sequence of states through which a system passes during a process
  • Different paths between the same initial and final states can result in different amounts of heat and work exchanged
• Equilibrium state is achieved when the system's properties remain constant over time and there are no net flows of energy or matter

Work and Heat Transfer

• Work $(W)$ is the energy transfer associated with a force acting through a distance
  • Expressed as $W = \int F \cdot ds$ or $W = \int P \cdot dV$ for pressure-volume work
• Heat $(Q)$ is the energy transfer due to a temperature difference between the system and its surroundings
• Sign convention: work done by the system is negative $(W < 0)$, while work done on the system is positive $(W > 0)$
• Heat added to the system is positive $(Q > 0)$, while heat removed from the system is negative $(Q < 0)$
• Adiabatic process occurs without heat transfer between the system and its surroundings $(Q = 0)$
• Isothermal process occurs at constant temperature, with heat transfer balanced by work
• Isobaric process occurs at constant pressure, with changes in volume and heat transfer
• Isochoric (isovolumetric) process occurs at constant volume, with changes in pressure and heat transfer

Mathematical Formulation

• The First Law of Thermodynamics can be expressed as $\Delta U = Q - W$, where $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, and $W$ is the work done by the system
• For a closed system undergoing a reversible process, the First Law can be written as $dU = \delta Q - \delta W$, where $\delta Q$ and $\delta W$ represent infinitesimal amounts of heat and work
• For an ideal gas, the change in internal energy depends only on the change in temperature: $\Delta U = C_v \Delta T$, where $C_v$ is the specific heat at constant volume
• Enthalpy change is given by $\Delta H = \Delta U + \Delta(PV)$, which simplifies to $\Delta H = Q_p$ for a process at constant pressure
• Heat capacity is the amount of heat required to raise the temperature of a substance by one degree
  • Specific heat is the heat capacity per unit mass $(c)$
  • Molar heat capacity is the heat capacity per mole $(C)$
• Work done in a pressure-volume process is calculated using $W = \int P \cdot dV$, where $P$ is the pressure and $dV$ is the change in volume

Applications and Real-World Examples

• Heat engines convert thermal energy into mechanical work, operating between a high-temperature reservoir and a low-temperature reservoir (internal combustion engines, steam turbines)
• Refrigerators and heat pumps transfer heat from a low-temperature reservoir to a high-temperature reservoir, requiring work input (air conditioners, refrigerators)
• Thermochemistry studies the heat absorbed or released during chemical reactions (combustion, neutralization)
• Phase changes involve heat transfer without a change in temperature (melting, vaporization)
  • Latent heat is the energy required for a substance to change phase
• Adiabatic processes occur in rapid compressions and expansions (diesel engines, sound waves)
• Throttling processes involve a sudden pressure drop without significant heat transfer (refrigerant expansion in air conditioners)
• Steady-flow processes occur in open systems with constant flow rates and no accumulation of mass or energy (turbines, compressors, nozzles)

Problem-Solving Strategies

• Identify the system and its boundaries, specifying whether it is open, closed, or isolated
• Determine the initial and final states of the system, as well as the process connecting them
• Apply the First Law of Thermodynamics, considering the appropriate form for the given situation (e.g., $\Delta U = Q - W$ for a closed system)
• Identify the known and unknown variables, using the problem statement and any given assumptions
• Use the appropriate equations and relationships to solve for the unknown variables
  • Ideal gas law: $PV = nRT$
  • Specific heat equations: $Q = mc\Delta T$ or $Q = nC\Delta T$
  • Work equations: $W = \int P \cdot dV$ or $W = -P\Delta V$ (for constant pressure)
• Pay attention to sign conventions for heat and work, and ensure consistent units throughout the problem
• Interpret the results in the context of the problem, considering the physical meaning of the calculated values