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CBSE Class 11 · Physics

Thermodynamics

Official NCERT chapter from Physics Part I–II (book code keph1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Thermodynamics” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 10 concepts

1Thermal equilibrium

Thermal equilibrium: two bodies in contact that stop exchanging heat have the same temperature. A 40 °C block and a 40 °C block share T; a 40 and a 20 still exchange. Equilibrium is a same-T look, not “both still in a photo”.

A frozen-looking film of a flame is not this equilibrium.

Figure. Bodies A and B share a wall. Thermal equilibrium means they have the same temperature, so heat has no preferred direction.

How it works

  1. Name the two bodies and the contactThe pair.
  2. Ask whether heat still movesThe test.
  3. Same T → thermal equilibriumThe heading.

2Zeroth law of thermodynamics

Zeroth law: if A is in thermal equilibrium with B, and B with C, then A with C — temperature is a comparable label. The law is why a thermometer (B) can speak for A and C. It is not the first law.

A pretty “zero” badge without the A-B-C sentence is a heading.

Figure. If A and B are each in equilibrium with the same body C, then A is in equilibrium with B. That is the zeroth law, and it is why a thermometer reading is a shared temperature.

How it works

  1. Name A, B, CThree bodies.
  2. See A~B and B~CThe two contacts.
  3. Conclude A~C — same T-labelZeroth.

3Heat, internal energy and work

Heat Q is energy transferred by a T-gap; work W is energy transferred by a move (PdV as taught); internal energy U is the store inside. Three words, three jobs. A spinning paddle can raise U without a hotter contact (work).

Calling every U-change “heat” is a miss.

Figure. Heat Q enters the gas. Some of it leaves as work W when the piston moves out; the rest stays as a change in internal energy.

How it works

  1. Tag the transfer as Q or WHow it entered.
  2. Tag the store as UWhat stayed inside.
  3. Keep the three apartHeat, work, U.

4First law of thermodynamics

First law (school sign as taught): ΔU = Q + W or ΔU = Q − W — use the book’s sign, but the idea is energy-account. A 10 J heat-in and 4 J work-out raises U by 6 J if that is the sign-story. You cannot print energy.

A perpetual “Q=0, W>0 forever” fails the account.

Figure. NCERT writes the first law as Q = \Delta U + W: the heat you supply is the internal-energy change plus the work the system does. Here 800 J splits as 500 J stored and 300 J of work.

How it works

  1. Write the taught sign-formThe first law.
  2. Plug Q and WThe account.
  3. Read ΔUThe store-change.

Q_in=10 J, W_out=4 J

If ΔU = Q − W_by the system, find ΔU.

  • Q10 J
  • W4 J
  • ΔU6 J

Pro tip. Account; name which sign-story you used.

5Specific heat capacity

Specific heat of a gas depends on the path: c_p and c_v as taught, c_p − c_v = R (molar, as framed). A larger c_p because expansion-work is allowed at constant P. Using one c for every gas-process is a miss.

Stealing the solids’ single c for an ideal-gas process-pair is a heading-mix.

Figure. For one mole of a diatomic ideal gas, C_V = 20.8 and C_P = 29.1 joule per mole kelvin. The gap is R: heat at constant pressure also pays for expansion work.

How it works

  1. Name constant-P or constant-VThe path.
  2. Hang c_p or c_vThe specific.
  3. Keep the taught differenceTwo heats.

6Thermodynamic state variables and equation of state

State variables (P, V, T, U as taught) have values at a state; the equation of state (ideal PV=nRT) links them. A path-variable (Q, W) needs the path. You cannot say “the Q of this gas” without a process.

Treating Q as a state like U is the miss.

Figure. A thermodynamic state is fixed by the state variables. For a given amount of gas, any two of P, V, and T determine the third through PV=nRT.

How it works

  1. Name P, V, T (and U) as stateThe point.
  2. Name Q, W as pathThe trip.
  3. Use PV=nRT to move between pointsThe equation of state.

7Thermodynamic processes

Processes: isothermal (T stay), adiabatic (Q=0 as taught), isobaric (P stay), isochoric (V stay). Each has a taught Q, W, ΔU pattern. A PV-sketch is the map. One named process is enough.

Calling every slow process isothermal without a T-check is a rush.

Figure. On P–V axes the three school processes have fingerprints: an isobar is flat, an isochore is upright, and an isotherm falls as PV stays 0.196 on these axes.

How it works

  1. Name what stays or what Q isThe process.
  2. Write the taught Q, W, ΔUThe pattern.
  3. Keep the PV-curve as a lookThe map.

8Second law of thermodynamics

Second law (school): heat does not by itself flow cold to hot; a taught engine cannot turn all heat into work in a cycle. The law is a direction-and-limit, not a new first-law account. A fridge needs work to pump heat.

A slogan “entropy” with no school sentence is a later-class dump if untaught.

Figure. A heat engine sits between a hot bath and a cold bath. Heat Q_H arrives, work W leaves to the side, and leftover heat Q_C must be dumped. The second law says Q_C cannot be zero.

How it works

  1. Name the forbidden lone cold-to-hotThe direction.
  2. Name the engine-limit as taughtThe second.
  3. Keep the first law as the account, this as the wayTwo laws.

9Reversible and irreversible processes

Reversible versus irreversible: a reversible school-process is a slow, balanced ideal; real friction and a finite T-gap are irreversible. A Carnot-story needs reversible pieces. Irreversible is the everyday, not “wrong science”.

Calling a real piston “reversible” because the sketch was neat is a miss.

Figure. The same start A and end B can be joined by more than one path. The smooth isotherm is a reversible expansion; the knee (drop P at fixed V, then expand at the lower P) is the usual irreversible sketch. Work depends on the path.

How it works

  1. Ask slow-and-balanced or finite-gap/frictionThe sort.
  2. Hang reversible / irreversibleThe heading.
  3. Keep reversible as the ideal limitThis class.

10Carnot engine

A Carnot engine is the taught ideal cycle between two T’s: efficiency 1 − T_C/T_H (kelvin) as framed. 600 K and 300 K → 1/2. No real engine in this story beats that. Carnot is a limit, not a car-brand.

Using 27 °C as 27 in T_C/T_H is a scale-miss.

Figure. A Carnot cycle on P–V axes: AB is the hot isotherm, BC an adiabat, CD the cold isotherm, DA an adiabat. Heat enters only on AB and leaves only on CD, so \eta = 1 - T_C/T_H.

How it works

  1. Write T_H and T_C in kelvinThe two baths.
  2. η = 1 − T_C/T_HCarnot.
  3. Read it as a ceilingThe engine-heading.
Carnot η between 600 K and 300 K is
  1. 1/2
  2. 2
  3. 27% from the Celsius numbers

1−300/600; kelvin.

Notes

  • Mapped to the official NCERT chapter “Thermodynamics”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • ΔU = Q − W (or the taught sign)
  • Carnot η = 1 − T_C/T_H (K)

Recap

Hold these pegs from the official chapter “Thermodynamics”. The wording is ExamMaster’s teaching, not a textbook recap.

Thermal equilibrium
Thermal equilibrium: two bodies in contact that stop exchanging heat have the same temperature.
Zeroth law of thermodynamics
Zeroth law: if A is in thermal equilibrium with B, and B with C, then A with C — temperature is a comparable label.
Heat, internal energy and work
Heat Q is energy transferred by a T-gap; work W is energy transferred by a move (PdV as taught); internal energy U is the store inside.
First law of thermodynamics
First law (school sign as taught): ΔU = Q + W or ΔU = Q − W — use the book’s sign, but the idea is energy-account.
Specific heat capacity
Specific heat of a gas depends on the path: c_p and c_v as taught, c_p − c_v = R (molar, as framed).
Thermodynamic state variables and equation of state
State variables (P, V, T, U as taught) have values at a state; the equation of state (ideal PV=nRT) links them.

Practise Thermodynamics

Reading is free and needs no account. Practice, mocks and progress live in the app.

  • A 4-question practice set that ends the chapter
  • 1 quick check with worked explanations
  • Timed mocks scored with the real marking scheme
  • Readiness tracked per topic, kept on your device
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