Schooling · Class 11 · Physics · Thermodynamics · Chapter on a page
CBSE Class 11 · Physics · NCERT Physics Part-II
Thermodynamics: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- A thermodynamic system is any part…
- The state of a gas is described by…
- Heat Q is energy transferred because…
- The first law of thermodynamics
- For an ideal gas undergoing…
- The second law states that heat…
- A heat engine absorbs heat Q₁ from a…
- Formulas and facts
- ΔU = Q − W
- W = P ΔV
- W = nRT ln(V₂ / V₁)
- For an adiabatic process with an…
- Cₚ − Cᵥ = R
- η = 1 − T_cold / T_hot
- Coefficient of performance of a…
- Worked examples
- First-law calculation
- Work in isothermal expansion
- Carnot engine efficiency
- Watch out for
- Confusing sign conventions
- Treating internal energy as…
- Using Celsius in Carnot efficiency
- Believing a process that is quick is…
- Thinking the Carnot engine is a real…
What this chapter is about
Thermodynamics studies how energy moves between a system and its surroundings when heat flows or work is done. In earlier chapters you met kinetic and potential energy of single objects; now you deal with the internal energy of a large collection of molecules and the rules that govern how this energy can change form. The subject grew from practical questions about steam engines but now underlies chemistry, biology and every branch of engineering.
Key ideas
- A thermodynamic system is any part of the universe you choose to study; everything else is the surroundings. Systems can be isolated (no exchange of energy or matter), closed (energy can cross, matter cannot) or open (both can cross).
- The state of a gas is described by state variables such as pressure P, volume V, temperature T and internal energy U. A change between two states can follow many different paths, but U depends only on the state, not on the path.
- Heat Q is energy transferred because of a temperature difference; work W is energy transferred when a force moves through a distance (for a gas, work = force on piston × displacement).
- The first law of thermodynamics: ΔU = Q − W. Energy is conserved; the change in internal energy equals heat added minus work done by the system.
- For an ideal gas undergoing different processes, work and heat exchange take special forms: isothermal (constant T), isobaric (constant P), isochoric (constant V), and adiabatic (Q = 0).
- The second law states that heat cannot flow from a colder body to a hotter body without external work; equivalently, no heat engine can convert all absorbed heat into work. This law fixes the direction of natural processes.
- A heat engine absorbs heat Q₁ from a hot reservoir, does work W, and rejects heat Q₂ to a cold reservoir. Efficiency η = W / Q₁ = 1 − Q₂ / Q₁. The Carnot engine, an ideal reversible engine, has the maximum possible efficiency η = 1 − T₂ / T₁ (temperatures in kelvin).
- A refrigerator removes heat from a cold body and dumps it into a warmer one by using external work. Its coefficient of performance is COP = Q₂ / W.
Formulas and facts to remember
- First law: ΔU = Q − W (sign convention: Q positive when heat enters, W positive when system does work on surroundings).
- Work done by an ideal gas at constant pressure: W = P ΔV.
- Work in an isothermal expansion from V₁ to V₂: W = nRT ln(V₂ / V₁).
- For an adiabatic process with an ideal gas: PV^γ = constant and TV^(γ−1) = constant, where γ = Cₚ / Cᵥ.
- Relation between molar heat capacities: Cₚ − Cᵥ = R (for one mole of an ideal gas).
- Carnot efficiency: η = 1 − T_cold / T_hot (temperatures in kelvin).
- Coefficient of performance of a refrigerator: COP = Q_cold / W = T_cold / (T_hot − T_cold) for an ideal Carnot refrigerator.
- Second law (Kelvin–Planck form): It is impossible to construct a heat engine that, operating in a cycle, converts all absorbed heat into work.
Common mistakes
- Confusing sign conventions: forgetting that W is positive when the system does work on the surroundings and Q is positive when heat enters the system → always state your convention and apply it consistently.
- Treating internal energy as path-dependent: trying to calculate ΔU from the process instead of from the initial and final states → remember U is a state function; ΔU depends only on the endpoints.
- Using Celsius in Carnot efficiency: plugging temperatures in °C into η = 1 − T₂ / T₁ → convert temperatures to kelvin first.
- Believing a process that is quick is adiabatic: assuming any fast expansion has Q = 0 → a process is adiabatic only when heat exchange is negligible, either because of good thermal insulation or because the process is too fast for appreciable heat flow (as in sound waves).
- Thinking the Carnot engine is a real device: assuming real engines can reach Carnot efficiency → Carnot efficiency is the theoretical upper limit; real engines have friction and irreversibility.
Quick revision
- First law: ΔU = Q − W; energy is conserved in every thermodynamic process.
- Work done by a gas at constant pressure: W = P ΔV; in isothermal expansion: W = nRT ln(V₂ / V₁).
- Adiabatic means Q = 0; the relation PV^γ = constant holds.
- Carnot efficiency η = 1 − T_cold / T_hot sets the maximum efficiency for any heat engine.
- The second law forbids 100 % conversion of heat to work and explains why heat flows from hot to cold on its own.