3.4 Path dependence and independence

Thermodynamics can be tricky, but understanding path dependence is key. Some quantities, like work and heat, change based on how you get from A to B. Others, like internal energy, only care about where you start and end up.

This matters for solving problems and analyzing cycles. Knowing which quantities are path-dependent helps you calculate efficiently and understand how different processes affect outcomes. It's crucial for optimizing engines, refrigerators, and other thermodynamic systems.

Path Dependence and Independence in Thermodynamic Processes

Path-dependent vs path-independent quantities

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• Path-dependent quantities depend on the specific route taken by the system from initial to final state
  • Different paths between same initial and final states result in different values (work, heat)
• Path-independent quantities depend only on initial and final states, not the path taken
  • Change in these quantities is the same for any path connecting initial and final states (internal energy, enthalpy, entropy, Gibbs free energy)

Examples of thermodynamic quantities

• Work ($W$) is path-dependent
  • Calculated by integrating pressure ($P$) with respect to volume ($V$) along specific path: $W = \int P dV$
  • Different paths between same initial and final states result in different amounts of work done (compression, expansion)
• Heat ($Q$) is path-dependent
  • Calculated by integrating temperature ($T$) with respect to entropy ($S$) along specific path: $Q = \int T dS$
  • Different paths between same initial and final states result in different amounts of heat transferred (isothermal, adiabatic)
• Internal energy ($U$) is path-independent
  • Change in internal energy ($\Delta U$) depends only on initial and final states
  • Calculated using first law of thermodynamics: $\Delta U = Q - W$
• Enthalpy ($H$) is path-independent
  • Change in enthalpy ($\Delta H$) depends only on initial and final states
  • Calculated using definition of enthalpy: $H = U + PV$, and $\Delta H = \Delta U + \Delta(PV)$

Applications of path independence

• When solving problems involving changes in state variables, focus on initial and final states of system
• Use appropriate equations to calculate change in path-independent quantities
  1. For internal energy: $\Delta U = Q - W$
  2. For enthalpy: $\Delta H = \Delta U + \Delta(PV)$
• Path taken between initial and final states does not affect change in path-independent quantities
• Utilize state functions and their relationships to simplify calculations and problem-solving

Significance in thermodynamic cycles

• Thermodynamic cycles involve series of processes that return system to its initial state
  • Path-independent quantities (internal energy, enthalpy) have net change of zero over complete cycle
  • Path-dependent quantities (work, heat) may have non-zero net values over complete cycle
• Efficiency calculations depend on path-dependent quantities
  • Thermal efficiency of heat engine: $\eta = \frac{W_{net}}{Q_{H}}$
    • Depends on net work output ($W_{net}$) and heat input from hot reservoir ($Q_{H}$)
  • Coefficient of performance (COP) of heat pump or refrigerator: $COP_{HP} = \frac{Q_{H}}{W_{net}}$, $COP_{ref} = \frac{Q_{C}}{W_{net}}$
    • Depends on heat transferred to hot reservoir ($Q_{H}$) or from cold reservoir ($Q_{C}$) and net work input ($W_{net}$)
• Understanding path dependence and independence is crucial for analyzing and optimizing thermodynamic cycles and their efficiencies (Carnot, Rankine, Brayton)