Thermodynamics Class 11 Physics Chapter Notes

Key Terminology in Thermodynamics

Thermodynamic System

A specific collection of matter, bounded by a real or imaginary wall, which is under study. E.g., gas in a cylinder.

Surroundings

Everything outside the thermodynamic system that can interact with it.

State Variables

The macroscopic properties that define the state of a system, such as Pressure (P), Volume (V), and Temperature (T).

Internal Energy (U)

The sum of the kinetic and potential energies of the molecules of a system. It is a state function.

Zeroth Law of Thermodynamics

If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law defines temperature.

First Law of Thermodynamics

Energy cannot be created or destroyed. The heat (ΔQ) supplied to a system is equal to the sum of the increase in its internal energy (ΔU) and the work done (ΔW) by the system. Formula: ΔQ = ΔU + ΔW.

Second Law of Thermodynamics

It specifies the direction of spontaneous processes. Key statements include the Kelvin-Planck statement (no heat engine can be 100% efficient) and the Clausius statement (heat cannot spontaneously flow from a colder body to a hotter body).

Entropy (S)

A measure of the molecular disorder or randomness of a system. The second law implies that the entropy of an isolated system always increases or remains constant.

Heat Engine

A device that converts heat energy into mechanical work in a cyclic process.

The Three Laws of Thermodynamics

The laws of thermodynamics are foundational principles governing energy transformations. Understanding them is crucial for solving most problems in this chapter.

Zeroth Law of Thermodynamics: This law provides the basis for the concept of temperature. It states that if body A is in thermal equilibrium with body C, and body B is also in thermal equilibrium with body C, then bodies A and B are in thermal equilibrium with each other. This means they are at the same temperature. It might seem obvious, but it's the fundamental principle that allows us to use a thermometer.

First Law of Thermodynamics: This is essentially the law of conservation of energy applied to thermodynamic systems. It is mathematically stated as ΔQ = ΔU + ΔW. Here, ΔQ is the heat supplied to the system, ΔU is the change in the internal energy of the system, and ΔW is the work done by the system on its surroundings. It's vital to get the sign conventions right: heat supplied to the system is positive, work done by the system is positive, and an increase in internal energy is positive. For a cyclic process, where the system returns to its initial state, ΔU = 0, so ΔQ = ΔW.

Second Law of Thermodynamics: While the first law states that energy is conserved, it doesn't say anything about the direction of energy flow. The second law fills this gap. It has two famous statements:

1. Kelvin-Planck Statement: It is impossible to construct a heat engine that, operating in a cycle, produces no other effect than to extract heat from a single reservoir and perform an equivalent amount of work. This implies no heat engine can have 100% efficiency.
2. Clausius Statement: It is impossible to construct a device that, operating in a cycle, produces no other effect than the transfer of heat from a colder body to a hotter body. This explains why heat spontaneously flows from hot to cold and why refrigerators require external work to operate.

Must Remember Formulae and Concepts

• Sign Convention (Physics): Heat absorbed by system: ΔQ > 0. Work done by system: ΔW > 0. Increase in internal energy: ΔU > 0. (Note: Chemistry uses a different sign convention for work).
• First Law of Thermodynamics: ΔQ = ΔU + ΔW
• Work Done during volume change: W = ∫ P dV. For constant pressure, W = P(V₂ - V₁).
• Internal Energy of an Ideal Gas: Depends only on temperature. ΔU = nCvΔT. For an isothermal process, ΔU = 0.
• Mayer's Formula: Cₚ - Cᵥ = R, where Cₚ and Cᵥ are molar specific heats at constant pressure and volume, and R is the universal gas constant.
• Ratio of Specific Heats (γ): γ = Cₚ / Cᵥ. For a monatomic gas, γ = 5/3. For a diatomic gas, γ = 7/5.
• Adiabatic Process Equation: PV^γ = constant.
• Efficiency of a Heat Engine (η): η = Work Done / Heat Input = W / Q₁ = 1 - (Q₂ / Q₁), where Q₁ is heat absorbed from the hot reservoir and Q₂ is heat rejected to the cold reservoir.
• Efficiency of a Carnot Engine (Ideal): η = 1 - (T₂ / T₁), where T₁ and T₂ are the absolute temperatures of the source and sink respectively.
• Coefficient of Performance (COP) of a Refrigerator: COP = Q₂ / W = Q₂ / (Q₁ - Q₂)

Comparison of Thermodynamic Processes

Worked Example: First Law of Thermodynamics

• Calculating Change in Internal Energy A system absorbs 2000 J of heat and does 500 J of work on its surroundings. What is the change in its internal energy? Solution: - Given: Heat absorbed, ΔQ = +2000 J - Work done by the system, ΔW = +500 J - Using the First Law of Thermodynamics: ΔQ = ΔU + ΔW - Rearranging for ΔU: ΔU = ΔQ - ΔW - Substituting the values: ΔU = 2000 J - 500 J = 1500 J Answer: The internal energy of the system increases by 1500 J.

Exam Traps and Scoring Tips

Sign Conventions are CRITICAL: This is the most common source of errors in thermodynamics numericals. Remember the convention for Physics: Work done BY the system is positive (+ΔW), while work done ON the system is negative (-ΔW). Heat GIVEN TO the system is positive (+ΔQ), heat REMOVED FROM the system is negative (-ΔQ). Always write down your sign convention at the beginning of a problem to avoid confusion.

P-V Diagrams: Be very comfortable with interpreting P-V diagrams. The area under the curve represents the work done. For a cyclic process, the area enclosed by the loop is the net work done. Also, remember that the adiabatic curve is always steeper than the isothermal curve. Questions often ask to identify processes on a given diagram.

Practice Questions with Solutions

• What is the change in internal energy during a cyclic process? Zero. The system returns to its initial state, so the change in any state function (like internal energy) is zero.
• Why is the specific heat at constant pressure (Cₚ) always greater than the specific heat at constant volume (Cᵥ)? At constant pressure, the heat supplied is used for both increasing the internal energy and doing work (expansion). At constant volume, all the heat supplied is used only to increase the internal energy. Thus, more heat is required at constant pressure for the same temperature rise.
• Can the efficiency of a real heat engine be 100%? Why or why not? No. According to the Second Law of Thermodynamics (Kelvin-Planck statement), some heat must always be rejected to a colder reservoir (sink). Therefore, Q₂ can never be zero, and the efficiency η = 1 - (Q₂/Q₁) can never be 1 (or 100%).
• What is the work done in an isochoric process? Zero. An isochoric process occurs at constant volume (ΔV = 0). Since work done is W = PΔV, the work done is zero.

Frequently Asked Questions

Frequently Asked Questions