Thermodynamics Chapter 12

Chapter Twelve: Thermodynamics

• Thermodynamics is the branch of physics that deals with concepts of heat and temperature, and the conversion of energy.
• It's a macroscopic science, focusing on bulk systems rather than molecular structures.
• Concepts like heat, temperature, work, and internal energy are more precisely defined.

12.1 Introduction

• Thermal energy is converted to work (e.g., rubbing hands together) and vice-versa (e.g., a steam engine).
• Historically, heat was thought of as a fluid, but now understood as a form of energy.
• Rumford's experiment (1798) demonstrated heat as energy, not a fluid. The amount of heat produced relied on the work done, not the sharpness of the drill.

12.2 Thermal Equilibrium

• Thermal equilibrium: a system's macroscopic variables remain constant over time.
• This means properties like pressure, volume, temperature, mass, and composition aren't changing.

12.3 Zeroth Law of Thermodynamics

• Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.
• This implies a shared variable: temperature (T). If Tₐ = Tₓ and Tₓ = Tᵧ, then Tₐ = Tᵧ

12.4 Heat, Internal Energy, and Work

• Internal energy (U) is the sum of molecular kinetic and potential energies within a system.
• It's a state function (value depends only on the current state, not the path to get there).
• Heat (Q) is energy transfer due to temperature difference.
• Work (W) is energy transfer not involving a temperature difference (e.g., pushing a piston).
• First Law of Thermodynamics: ΔQ = ΔU + ΔW (Change in heat equals change in internal energy plus change in work).

12.5 First Law of Thermodynamics

• Internal energy can change through heat flow and work done on or by the system.
• ΔQ = ΔU + ΔW
• ΔQ: heat supplied to the system by the surroundings
• ΔW: work done by the system on the surroundings
• ΔU: change in internal energy of the system

12.6 Specific Heat Capacity

• Specific heat (s): Amount of heat needed to raise the temperature of 1 kg of a substance by 1 K.
• Molar specific heat (C): Amount of heat needed to raise the temperature of 1 mole of a substance by 1 K.

12.7 Thermodynamic State Variables and Equations of State

• State variables: variables that describe a system's state (e.g., pressure, volume, temperature, mass).
• Equations of state: relationships between state variables (e.g., the ideal gas law: PV = nRT).
• Extensive variables: depend on the size of the system (e.g., volume, internal energy, mass).
• Intensive variables: do not depend on the size of the system (e.g., temperature, pressure, density).

12.8 Thermodynamic Processes

• Quasi-static process: a process that happens infinitely slowly, allowing the system to remain in equilibrium at each step.
• Isothermal process: constant temperature.
• Isobaric process: constant pressure.
• Isochoric process: constant volume.
• Adiabatic process: no heat transfer.

12.9 Heat Engines

• Heat Engine: converts heat to work in a cyclic process.
• A working substance undergoes a cycle of processes absorbing heat at a high temperature and rejecting heat at a low temperature, while performing work.
• Efficiency η = (work done) / (heat input) = 1 - (heat rejected) / (heat input).

12.10 Refrigerators and Heat Pumps

• Refrigerators are heat pumps operating in reverse.
• They absorb heat from a cold reservoir, and reject heat to a hot reservoir, requiring work input.
• Coefficient of Performance (COP) = (heat removed) / (work input).

12.11 Second Law of Thermodynamics

• The Second Law of Thermodynamics limits the efficiency of a heat engine.
• Kelvin-Planck statement: It's impossible to construct a cyclic process that absorbs heat from one reservoir and delivers an equal amount of work.

12.12 Reversible and Irreversible Processes

• Reversible process: one in which both the system and surroundings can return to their initial states without any net change elsewhere in the universe.
• Irreversible process: a process that cannot be exactly reversed.

12.13 Carnot Engine

• A theoretical heat engine that operates on a cyclic process involving reversible steps.
• It sets an upper limit on the efficiency of any heat engine between two temperatures.