E ExamMaster

CBSE Class 11 · Physics

Kinetic Theory

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 “Kinetic Theory” 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
  • 8 concepts

1Molecular nature of matter

Matter as molecules: many tiny bits in motion, with empty space between in a gas-story. A smell-across-a-room is a setting; the science is bits that move and hit. A continuous-jelly model is the contrast this heading drops.

A smooth-looking gas in a jar is still bits, not jelly.

Figure. Three states, same particles. Solid: a tight lattice. Liquid: still touching, but the packing has slipped. Gas: wide gaps and motion arrows. Squares stand in for molecules — the vocabulary has no circle.

How it works

  1. Name tiny bits in motionThe molecular nature.
  2. Name hits on a wall as a pressure-look laterThe use.
  3. Refuse a no-bit jelly as this headingBits.

2Behaviour of gases

Gases: easy to squeeze, fill a volume, mix — as the school looks. Pressure and temperature rise with more hits and faster bits, as the next headings make precise. A liquid’s almost-fixed volume is the contrast.

Calling a liquid “a slow gas” without the lesson’s words is a rush.

Figure. At fixed temperature, Boyle's law is PV = constant. Against 1/V the graph is a straight line through the origin: double 1/V and P doubles. The origin point is on the series.

How it works

  1. Name fill-and-squeeze as gas-looksThe behaviour.
  2. Link later to hits and speedThe bridge.
  3. Keep liquid/solid as other drawersThis heading is gas-looks.

3Kinetic theory of an ideal gas

Kinetic theory of an ideal gas: P V = (1/3) N m v_rms² as taught, so temperature tracks the average kinetic energy. A hotter gas’s bits move faster. Ideal means the school neglects (point bits, no leftover forces) as named.

Using a single bit’s speed as if it were the rms of the crowd is a miss.

Figure. Pressure is not a push from outside. One molecule hits the wall, reverses its v_x, and hands the wall a momentum change Δp. The gas pressure is those hits, added up.

How it works

  1. Write the taught P–v_rms linkThe theory.
  2. Read T as a crowd-K_avgThe temperature-story.
  3. Keep “ideal” as the neglect-listThis class.

Double T, ideal

If T doubles (ideal, same gas), what happens to the average translational K per molecule as taught?

  • K_avgtracks T
  • Soit doubles
  • Notv doubles — v_rms tracks √T

Pro tip. Energy tracks T; speed tracks square-root.

4Law of equipartition of energy

Equipartition: each taught quadratic “degree” gets (1/2) kT per molecule (as framed). A monatomic ideal has 3 translations; diatomic adds taught rotations at school T. Equipartition is a share-rule, not a new force.

Giving a monatomic 5/2 kT without the lesson’s extra degrees is a steal.

Figure. Equipartition puts ½RT in each active degree of freedom. A mole of monatomic gas has f=3, so U=1.5 RT. Room-temperature diatomic gas has f=5, so U=2.5 RT. A non-linear polyatomic with f=6 has U=3 RT. Bar lengths are those multiples of RT.

How it works

  1. Count the taught degreesThe share-list.
  2. Write (1/2) kT eachThe law.
  3. Keep monatomic versus diatomic as the lesson split themHonest.

5Specific heat capacity

Specific heat from kinetic theory: c_v from the U-per-mole and ΔT, c_p = c_v + R as already met. A monatomic molar U = (3/2) RT → c_v = 3/2 R as taught. This heading ties heat-capacity to the bit-story.

Using the solids’ c for an ideal monatomic gas is a mix.

Figure. For an ideal gas, Cp is always Cv plus R. The lower segment is Cv = (f/2)R: 1.5 R, 2.5 R, 3 R. The top segment on every bar is the same R. Heights are 2.5 R, 3.5 R and 4 R.

How it works

  1. Write U from equipartitionThe store.
  2. Differentiate with T as taught for c_vThe specific.
  3. Add R for c_p if askedThe pair.

6Mean free path

Mean free path is the average coast between hits. Thinner gas, longer path; fatter bits, shorter path — as the taught write. A vacuum-ish jar has a long path. Mean free path is a length, not a time.

Calling it “the speed” is a heading-steal.

Figure. Mean free path λ is the average distance a molecule travels between collisions. The first run is drawn horizontal so the bracket is an honest length. Other molecules are squares — the vocabulary has no collision circle.

How it works

  1. Name density and size as the lesson didThe given.
  2. Write the taught λThe path.
  3. Read thin → long λThe use.

7A definition is a test you can run

Molecule / ideal-gas / λ is a test: bits, a P–v_rms write, or a coast-length. If you only say “kinetic”, you have a heading.

A steam-photo is not the test.

Figure. A definition is a test: name the asked quantity, write the matching formula, finish in the SI unit. Here the ask is kinetic energy. For m=4 kg and v=4 m/s, ½mv² = 32 J, not a watt and not a work angle.

How it works

  1. Name bits, P V versus v_rms, or λThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.

8Name the given before the unknown

The given is T or P, V, N. The unknown is v_rms or λ. Copy kelvin before you double T.

Using T=27 because the room was 27 °C inside v_rms² ∝ T without converting is a silent given.

Figure. Write the given mass and speed before you reach for ½mv². The unknown is KE. With m=4 kg and v=8 m/s the later arithmetic is 128 J, but that sum is not the first line.

How it works

  1. Copy T in K, or P, V, NThe given.
  2. Name v_rms or λThe unknown.
  3. Then the taught writeGiven first.
If T doubles (ideal), v_rms
  1. Grows by √2 — not a double
  2. Doubles
  3. Halves

K_avg doubles; speed tracks square-root.

Notes

  • Mapped to the official NCERT chapter “Kinetic Theory”. 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

  • K_avg tracks T
  • v_rms tracks √T
  • c_v from U(T); c_p=c_v+R (ideal, as taught)

Recap

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

Molecular nature of matter
Matter as molecules: many tiny bits in motion, with empty space between in a gas-story.
Behaviour of gases
Gases: easy to squeeze, fill a volume, mix — as the school looks.
Kinetic theory of an ideal gas
Kinetic theory of an ideal gas: P V = (1/3) N m v_rms² as taught, so temperature tracks the average kinetic energy.
Law of equipartition of energy
Equipartition: each taught quadratic “degree” gets (1/2) kT per molecule (as framed).
Specific heat capacity
Specific heat from kinetic theory: c_v from the U-per-mole and ΔT, c_p = c_v + R as already met.
Mean free path
Mean free path is the average coast between hits.

Practise Kinetic Theory

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
Continue with Google — freeNo card, no trial. Works offline once installed.