Lesson 7.2: Laws of Thermodynamics & Applications

The Laws of Thermodynamics form the foundation of energy analysis in engineering systems. GATE and PSU exams often test first and second law applications, efficiencies, and system limitations.

🔹 1. Introduction

• Definition: Thermodynamics laws govern the conversion of energy and its limitations

• Applications: Power plants, refrigeration, HVAC, engines, turbines

• Key Concepts: First law (energy conservation), second law (entropy & irreversibility), third law (absolute zero), zeroth law (temperature equilibrium)

🔹 2. Zeroth Law of Thermodynamics

• Statement: If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other

• Application: Basis for thermometers and temperature measurement

🔹 3. First Law of Thermodynamics

• Closed System:

$$\Delta U=Q-W$$

Where ΔU = change in internal energy, Q = heat added, W = work done by system

• Open System (Steady Flow):

Q˙−W˙=ΔH+ΔKE+ΔPE\dot{Q} – \dot{W} = \Delta H + \Delta KE + \Delta PEQ˙​−W˙=ΔH+ΔKE+ΔPE

• Applications: Piston-cylinder, turbines, compressors, heat exchangers

• Solved Example: Gas expansion in cylinder, calculate ΔU, Q, and W

🔹 4. Second Law of Thermodynamics

• Kelvin-Planck Statement: No heat engine can convert all heat into work without losses

• Clausius Statement: Heat cannot flow from cold to hot without external work

• Entropy (S): Measure of disorder or irreversibility

ΔS=∫dQrevT\Delta S = \int \frac{dQ_\text{rev}}{T}ΔS=∫TdQrev​​

• Applications: Determining maximum efficiency of engines, refrigerators, and heat pumps

• Example: Efficiency of Carnot engine

ηCarnot=1−TCTH\eta_\text{Carnot} = 1 – \frac{T_C}{T_H}ηCarnot​=1−TH​TC​​

🔹 5. Third Law of Thermodynamics

• Statement: Entropy of a pure perfect crystal at absolute zero = 0

• Application: Calculation of absolute entropy, low-temperature processes

🔹 6. Thermodynamic Applications

1. Engines: Otto, Diesel, Rankine, Brayton cycles

2. Refrigeration: Coefficient of performance (COP) using second law

3. Heat Exchangers: Energy balance using first law

4. Power Plants: Efficiency calculation using Carnot or Rankine cycle

5. Entropy Analysis: Determine feasibility and irreversibility

🔹 7. Solved Examples (PYQ Style)

1. First law application to piston-cylinder system

2. Calculate Carnot cycle efficiency between two temperatures

3. Determine entropy change for heating or cooling processes

4. Analyze feasibility of a heat engine using second law

🔹 8. Practice Exercises

1. Apply first law to closed and open systems

2. Calculate entropy change for various processes

3. Determine maximum efficiency using Carnot cycle

4. Solve energy balance problems for engines and refrigeration systems

5. Identify reversible and irreversible processes

🔹 9. Summary

• Zeroth Law: Temperature equilibrium

• First Law: Conservation of energy (ΔU = Q – W)

• Second Law: Entropy & limitations of energy conversion

• Third Law: Absolute zero and entropy

• Applications: Engines, turbines, refrigeration, power plants, heat exchangers