Thermodynamics | AP Physics 2 (2025) Unit 9 Review

Key Concepts

Thermodynamics

• Thermodynamics is the study of energy, heat, work, and how they interact.
• It deals primarily with macroscopic systems.

Laws of Thermodynamics

1. First Law of Thermodynamics (Law of Energy Conservation)

   • Energy cannot be created or destroyed.
   • The total energy of an isolated system is constant.
   • Change in internal energy (ΔU) = Heat added to the system (Q) - Work done by the system (W).

2. Second Law of Thermodynamics

   • Total entropy of an isolated system can never decrease over time.
   • Heat cannot spontaneously flow from a colder body to a hotter body.
   • Implies the concept of irreversibility in natural processes.

3. Third Law of Thermodynamics

   • As the temperature of a system approaches absolute zero, the entropy of the system approaches a minimum value.

4. Zeroth Law of Thermodynamics

   • If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
   • Basis for the definition of temperature.

Important Concepts

Heat and Work

• Heat (Q): Energy transferred due to temperature difference.
• Work (W): Energy transfer when a force moves an object over a distance.

Internal Energy

• Sum of the kinetic and potential energies of the particles in a system.

Entropy

• Measure of the disorder or randomness in a system.
• Higher entropy means more disorder.

Thermal Efficiency

• Ratio of work output to heat input in a system.
• Important in evaluating engines and refrigerators.

Applications

Heat Engines

• Convert thermal energy into mechanical work.
• Operate in cycles.
  • Carnot Cycle: Idealized cycle with the maximum possible efficiency.

Refrigerators and Heat Pumps

• Transfer heat from a cooler space to a warmer space.
• Require work input.

Calculations and Equations

• First Law Equation: ΔU = Q - W
• Efficiency of Heat Engine: η = (Work output / Heat input) = 1 - (Q_cold/Q_hot)
• Carnot Efficiency: η_carnot = 1 - (T_cold/T_hot)

Study Tips

• Understand and memorize the laws of thermodynamics.
• Practice solving problems related to heat engines and refrigerators.
• Familiarize yourself with key equations and how to apply them.