Module 1 · JEE Physics

Thermodynamics

Thermal expansion, calorimetry, laws of thermodynamics, heat engines, kinetic theory.

Thermal Expansion · Calorimetry · TD Laws · Heat Engines · Kinetic Theory

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Learning Objectives

Understand the fundamental concepts of Thermodynamics
Apply key formulas and techniques to solve problems
Practice with exam-level questions to build speed and accuracy

Key Concepts

What is Thermodynamics?

Thermodynamics is the branch of physics that deals with heat, work, temperature, and the conversion of energy between different forms. It explains how thermal energy is transferred and how it affects matter. Key concepts for NEET include the laws of thermodynamics, specific heat capacity, calorimetry, heat engines, and entropy. Thermodynamics is crucial for understanding processes in engines, refrigerators, and natural phenomena.

Key Concepts

Zeroth Law: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. First Law of Thermodynamics: ΔU = Q - W, where ΔU is change in internal energy, Q is heat added, and W is work done by the system. Second Law: Heat cannot spontaneously flow from a colder body to a hotter body. Specific Heat: c = Q/(mΔT). Molar Specific Heats: C_v = (f/2)R and C_p = C_v + R, where f is degrees of freedom. Efficiency of Heat Engine: η = 1 - T_c/T_h (Carnot efficiency).

Solved Examples

Example 1

A gas is compressed by doing 500 J of work on it, and it absorbs 300 J of heat. Find the change in internal energy of the gas.

a) 200 J b) 800 J c) -200 J d) -800 J

Solution: Using first law ΔU = Q - W. Since work is done ON the system, W = -500 J (negative). Heat absorbed Q = +300 J. So ΔU = 300 - (-500) = 300 + 500 = 800 J. Hence option (b) is correct.

Example 2

A Carnot engine operates between 127°C and 27°C. Find its efficiency.

a) 25% b) 50% c) 75% d) 33.3%

Solution: Convert temperatures to Kelvin: T_h = 127 + 273 = 400 K, T_c = 27 + 273 = 300 K. Efficiency η = 1 - T_c/T_h = 1 - 300/400 = 1 - 0.75 = 0.25 = 25%. Hence option (a) is correct.

Example 3

Calculate the work done when 2 moles of an ideal gas expand isothermally at 300 K from a volume of 10 L to 20 L. (R = 8.314 J/mol-K)

a) 3456 J b) 4157 J c) 4988 J d) 2885 J

Solution: For isothermal expansion, W = nRT ln(V₂/V₁). W = 2 × 8.314 × 300 × ln(20/10) = 2 × 8.314 × 300 × ln(2) = 2 × 8.314 × 300 × 0.693 = 3456 J. Hence option (a) is correct.

Shortcut Techniques

For adiabatic processes, remember PV^γ = constant and use γ = C_p/C_v.
In calorimetry problems, apply heat lost = heat gained. Always check the final equilibrium phase (ice/water/steam).
For efficiency problems, always convert Celsius to Kelvin before using formulas.
In cyclic processes, ΔU = 0, so net work = net heat (Q_in - Q_out).

Pro Tip

For NEET, remember that the first law is essentially conservation of energy. The sign convention is crucial: work done BY the system is positive, work done ON the system is negative. Practice with sign conventions to avoid mistakes in exams.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Application-Based Learning

Connect concepts to real-world applications: Why sky is blue (Rayleigh scattering), how pressure cookers work, why apples turn brown (oxidation).

Problem-Solving Methodology

1) Identify given data. 2) Recall formulas. 3) Check unit consistency. 4) Solve step-by-step. 5) Verify through estimation.

Pro Tip

Check if your answer is reasonable through order-of-magnitude estimation to catch gross errors.

Practice Questions

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