JEE Physics Notes: Thermodynamics

Introduction

Thermodynamics is the branch of physics that deals with heat, work, and energy transformations. It plays a crucial role in understanding how energy flows in different systems, including engines, refrigerators, and biological processes.

1. Basic Terms in Thermodynamics

• System: The part of the universe under study.
• Surroundings: Everything outside the system.
• Types of Systems:
  • Open System: Exchanges both energy and matter with surroundings.
  • Closed System: Exchanges only energy but not matter.
  • Isolated System: Exchanges neither energy nor matter.
• State Variables: Properties that define the state of a system, such as pressure (P), volume (V), temperature (T), and internal energy (U).

2. Zeroth Law of Thermodynamics

• It states that if two bodies are each in thermal equilibrium with a third body, then they are in thermal equilibrium with each other.
• This law establishes the concept of temperature.

3. First Law of Thermodynamics

• It is the law of conservation of energy applied to thermodynamic systems.
• Mathematical Form: ΔQ = ΔU + ΔW, where:
  • ΔQ = Heat added to the system
  • ΔU = Change in internal energy
  • ΔW = Work done by the system
• The first law explains how heat energy is converted into work and internal energy.

4. Thermodynamic Processes

• Isothermal Process (Constant Temperature, T = constant):
  • Internal energy remains constant (ΔU = 0).
  • Work done: W = nRT ln(Vf / Vi).
• Adiabatic Process (No Heat Exchange, Q = 0):
  • ΔU = -W
  • Equation: PV^γ = constant.
• Isochoric Process (Constant Volume, V = constant):
  • No work is done (W = 0).
  • ΔQ = ΔU
• Isobaric Process (Constant Pressure, P = constant):
  • ΔQ = ΔU + PΔV
  • Work done: W = PΔV.

5. Second Law of Thermodynamics

• It states that heat energy cannot spontaneously flow from a colder body to a hotter body.
• Kelvin-Planck Statement: It is impossible to extract heat from a reservoir and convert it entirely into work without any loss.
• Clausius Statement: Heat cannot flow from a colder body to a hotter body without external aid.
• This law introduces the concept of entropy.

6. Entropy

• Entropy is a measure of disorder in a system.
• For a reversible process: ΔS = Q/T.
• Entropy always increases for an irreversible process.

7. Heat Engines and Refrigerators

• Heat Engine: Converts heat energy into mechanical work.
• Efficiency of a Heat Engine: η = 1 - (Qc / Qh).
• Refrigerator: Transfers heat from a colder body to a hotter body using external work.
• Coefficient of Performance (COP): COP = Qc / W.

Conclusion

Thermodynamics is a fundamental subject in physics that helps understand energy transfer and transformations. Mastering concepts like the laws of thermodynamics, entropy, and heat engines is essential for solving JEE-level problems efficiently.