Chapter 11 - Thermodynamics

Thermodynamics studies heat, work and energy at the level of complete systems. It does not attempt to follow the motion of every molecule. Instead, it uses measurable quantities such as pressure, volume and temperature to describe the state of matter.

Thermodynamics Class 11 establishes the laws that govern energy transfer and the natural direction of thermal processes.

System, surroundings and state variables

A thermodynamic system is the part of the universe selected for study. Everything outside it is called the surroundings.

The state of a system can be described using variables such as:

• Pressure

• Volume

• Temperature

• Internal energy

An equation of state relates the state variables of a substance.

Thermal equilibrium and the zeroth law

Two systems are in thermal equilibrium when no net heat flows between them.

The zeroth law states that if two systems are separately in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

This law provides the basis for comparing temperatures and using thermometers.

Heat, work and internal energy

Heat and work are two methods of transferring energy across the boundary of a system.

Internal energy is a property of the system’s state. Heat and work are not quantities permanently stored inside the system; they describe energy transfer during a process.

First law of thermodynamics

The first law applies conservation of energy to thermodynamic systems.

Heat supplied to a system may:

• Increase its internal energy

• Be used by the system to perform work

• Do both

Students must follow one sign convention consistently. Many incorrect answers result from switching conventions in the middle of a calculation.

Thermodynamic processes

The chapter covers:

• Isothermal processes

• Adiabatic processes

• Reversible processes

• Irreversible processes

• Cyclic processes

In an isothermal process, temperature remains constant. In an adiabatic process, no heat is exchanged with the surroundings.

In a cyclic process, the system returns to its initial state, so the net change in internal energy is zero.

Pressure-volume diagrams

A pressure-volume diagram represents the path followed by a system.

The area under a pressure-volume curve gives the work done by the gas during the process.

The change in internal energy depends only on the initial and final states, but heat and work may depend on the path.

Second law of thermodynamics

The first law tells us that energy is conserved, but it does not tell us which processes occur naturally.

The second law explains the direction of thermal processes. For example, heat flows naturally from a hotter body to a colder body, not the reverse without an external effect.

Common difficulties

Students frequently:

• Treat heat as a state variable

• Confuse internal energy with temperature

• Use inconsistent signs for heat and work

• Assume isothermal means no heat transfer

• Assume adiabatic means constant temperature

• Forget that internal-energy change is zero in a complete cycle

• Misread the area under a pressure-volume graph

How Teachoo helps

Teachoo provides process diagrams, sign-convention explanations, derivations, NCERT solutions, numerical questions and conceptual MCQs.

Solutions begin by identifying the system, process, known quantities and sign convention.

Frequently Asked Questions

What does thermodynamics study?

It studies heat, work, internal energy and the macroscopic variables that describe the state of a system.

Is heat contained inside a body?

A body has internal energy. Heat refers to energy transferred because of a temperature difference.

What is the difference between an isothermal and an adiabatic process?

Temperature remains constant in an isothermal process. No heat is exchanged in an adiabatic process.

Is internal energy a path function?

No. Internal energy is a state function. Its change depends only on the initial and final states.

Why is the second law needed if energy is already conserved?

The first law gives energy conservation. The second law determines the natural direction and limitations of thermodynamic processes.