Class 11 Physics · NCERT notes for NEET

Thermal Properties of Matter: NCERT Notes, Formulas and Tricks for NEET

Temperature scales, thermal expansion, specific heat, latent heat and the three modes of heat transfer. NEET usually asks one or two questions from here, mostly on calorimetry, expansion and conduction.

1. Temperature, scales and thermal expansion
2. Specific heat, calorimetry and change of state 🔒
3. Heat transfer: conduction, convection, radiation 🔒

Topic 1 is open for everyone. The full chapter, quick revision and practice open with a plan, or start free with 5 full chapters.

Open the interactive version (simulations and tap-to-answer quick checks).

Topic 1

Temperature, scales and thermal expansion

Railway tracks have small gaps and power lines sag more in May than in January: both are just $Δ l = α l Δ T$ at work.

Temperature tells how hot or cold a body is. Heat is energy that flows only because of a temperature difference. Heat is measured in joule; temperature in kelvin (K) or $^{\circ} C$ .
Fixed points at 1 atm: ice point and steam point. Celsius 0 and 100, Fahrenheit 32 and 212, Kelvin 273.15 and 373.15.
Conversion: $\frac{t _{F} - 32}{180} = \frac{t _{C}}{100}$ and $T = t_{C} + 273.15$ . A kelvin and a Celsius degree are the same size; $1^{\circ} C$ difference $= 1. 8^{\circ} F$ difference.
Ideal gas: $p V = μ R T$ , $R = 8.31 J mol^{- 1} K^{- 1}$ . A constant-volume gas thermometer extrapolates $p \to 0$ at $- 273.1 5^{\circ} C$ : absolute zero.
Linear, area and volume expansion: $Δ l / l = α_{l} Δ T$ , $Δ A / A = 2 α_{l} Δ T$ , $Δ V / V = 3 α_{l} Δ T$ for an isotropic solid.
Metals have large $α_{l}$ (copper, aluminium); glass and invar have small $α_{l}$ . Liquids and gases are described by $α_{V}$ only.
Ideal gas at constant pressure: $α_{V} = 1/ T$ . At $0^{\circ} C$ this is $3.7 \times 1 0^{- 3} K^{- 1}$ , far larger than for solids and liquids.
Anomalous expansion: water contracts from $0^{\circ} C$ to $4^{\circ} C$ , so it is densest at $4^{\circ} C$ . Lakes freeze from the top down and fish survive in the $4^{\circ} C$ water below.
A rod clamped at both ends and heated cannot expand: thermal stress $= Y α_{l} Δ T$ (compressive), thermal force $= Y A α_{l} Δ T$ .
A hole or cavity expands on heating exactly as if it were filled with the same material.

Ice and steam points line up across all three scales. 100 Celsius degrees span 180 Fahrenheit degrees, and the scales meet at −40.

Between 0°C and 4°C water shrinks as it warms. Minimum volume and maximum density are both at 4°C, not at 0°C.

Left: a clamped heated rod pushes on the walls, and the stress YalphaDelta T does not depend on length or area. Right: the hole in a heated plate gets bigger, not smaller. — Left: a clamped heated rod pushes on the walls, and the stress $Y α Δ T$ does not depend on length or area. Right: the hole in a heated plate gets bigger, not smaller.

Formulas

$\frac{t _{F} - 32}{180} = \frac{t _{C}}{100}$
Fahrenheit-Celsius conversion
$T = t_{C} + 273.15$
Kelvin from Celsius
$Δ l = α_{l} l Δ T$
Linear expansion; $α_{l}$ in $K^{- 1}$
$Δ A = 2 α_{l} A Δ T, Δ V = 3 α_{l} V Δ T$
Area and volume expansion of an isotropic solid
$α_{V} = \frac{1}{T}$
Ideal gas at constant pressure ( $T$ in kelvin)
$σ_{t h} = Y α_{l} Δ T, F = Y A α_{l} Δ T$
Clamped rod: thermal stress and thermal force

Tips, tricks and hacks

1 : 2 : 3 rule

α_{l} : α_{A} : α_{V} = 1 : 2 : 3

. Percentages scale the same way: if length grows by 0.1%, area grows by 0.2% and volume by 0.3%.

Scale conversion shortcutUse

\frac{C}{5} = \frac{F - 32}{9}

. Check with

- 40

- 8 = - 8

. For a temperature DIFFERENCE drop the 32:

Δ F = 1.8 Δ C

, so a rise of 10°C is a rise of 18°F.

Density falls on heatingMass is fixed, so

ρ \propto 1/ V

and

Δ ρ / ρ \approx - α_{V} Δ T

. A liquid with

α_{V} = 1 0^{- 3} K^{- 1}

heated by 20 K loses about 2% density.

Pendulum clock in summer

T_{p} \propto l

, so

Δ T_{p} / T_{p} = \frac{1}{2} α Δ T

. A clock with

α = 2 \times 1 0^{- 5}

warmed by 10 K loses

\frac{1}{2} \times 2 \times 1 0^{- 5} \times 10 \times 86400 \approx 8.6

s a day.

Common mistakes

✗ A hole in a metal plate shrinks on heating because the metal expands into it.

✓ Every length in the plate, including the hole's diameter, scales by $(1 + α Δ T)$ , so the hole grows.

✗ Thermal stress in a clamped rod is larger for a longer rod.

✓ Stress $= Y α Δ T$ has no $l$ or $A$ ; only the force $Y A α Δ T$ depends on area.

✗ Using $Δ F = 1.8 Δ C + 32$ for a temperature change.

✓ The 32 shifts readings, not differences: $Δ F = 1.8 Δ C$ .

✗ Water is densest at $0^{\circ} C$ (ice point).

✓ Water is densest at $4^{\circ} C$ ; ice is even less dense than water, which is why it floats.

NEET traps

"Water expands on heating at all temperatures": FALSE; it contracts from $0^{\circ} C$ to $4^{\circ} C$ .

$α_{V} = 1/ T$ needs $T$ in kelvin: at $2 7^{\circ} C$ it is $1/300 K^{- 1}$ , not $1/27$ .

A question gives $α$ and asks the change in volume of a hollow sphere: the cavity expands like solid metal, so use $3 α$ on the outer volume.

Quick check: A metal plate has

α_{l} = 2 \times 1 0^{- 5} K^{- 1}

. If it is heated through 50 K, the percentage increase in its area is

0.1%
0.2%
0.3%
0.02%

Answer: B. $Δ A / A = 2 α Δ T = 2 \times 2 \times 1 0^{- 5} \times 50 = 2 \times 1 0^{- 3} = 0.2%$ .

Topic 2 · 🔒

Specific heat, calorimetry and change of state

Key points, graphs, every formula, tricks, common mistakes and NEET traps for this topic are inside Dhruv.

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Topic 3 · 🔒

Heat transfer: conduction, convection, radiation

Key points, graphs, every formula, tricks, common mistakes and NEET traps for this topic are inside Dhruv.

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Practise Thermal Properties of Matter questions

Topic-wise practice from Level 1 to Level 3, NCERT-based questions and NEET PYQs, with a full solution after every answer.

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