Thermal Properties of Matter — NEET Physics Notes

This chapter covers heat, temperature, thermal expansion, calorimetry, heat transfer mechanisms, and change of state — a fundamental topic that connects heat concepts to practical everyday phenomena tested in NEET Physics.

Quick Revision

• Temperature: Measure of average kinetic energy of molecules
• Heat: Energy transferred due to temperature difference
• Specific Heat Capacity: Heat required to raise 1 kg by 1°C (c = Q/(mΔT))
• Latent Heat: Heat absorbed during change of state without temperature change
• L = Q/m (latent heat of fusion/vaporisation)
• Thermal Expansion: ΔL = αL₀ΔT (linear), ΔA = 2αA₀ΔT (area), ΔV = 3αV₀ΔT (volume)
• Heat Transfer: Conduction, Convection, Radiation

Standard Study

Temperature Scales

Scale	Freezing Point	Boiling Point	Absolute Zero
Celsius (°C)	0°C	100°C	−273.15°C
Fahrenheit (°F)	32°F	212°F	−459.67°F
Kelvin (K)	273.15 K	373.15 K	0 K

• Conversion: C = (F − 32) × 5/9; K = C + 273.15

Heat and Calorimetry

• Heat gained/lost: Q = mcΔT
• Water equivalent: w = mc (mass of water having same heat capacity)
• Principle of calorimetry: Heat lost = Heat gained (in isolated system)
• Joule’s Mechanical Equivalent: 1 cal = 4.184 J (specific heat of water = 1 cal/g°C)

Change of State

• Fusion (solid → liquid): Heat required = mL_f
• Vaporisation (liquid → gas): Heat required = mL_v
• Sublimation (solid → gas): Direct transition (e.g., dry ice, iodine)
• Latent Heat of Fusion (water): 80 cal/g = 334 J/g
• Latent Heat of Vaporisation (water): 540 cal/g = 2260 J/g

Thermal Expansion

Linear Expansion:

• ΔL = αL₀ΔT
• α = coefficient of linear expansion (per °C or per K)
• Final length: L = L₀(1 + αΔT)

Area Expansion:

• ΔA = 2αA₀ΔT
• β = 2α (coefficient of superficial expansion)

Volume Expansion:

• ΔV = 3αV₀ΔT
• γ = 3α (coefficient of volume expansion)
• For liquids, only volume expansion is significant

Anomalous Expansion of Water:

• Water expands on cooling from 4°C to 0°C
• Maximum density at 4°C
• Ice is less dense than water — floats

Heat Transfer Mechanisms

Conduction:

• Heat flows through solid without flow of matter
• Q/t = (kAΔT)/l (Fourier’s law)
• k = thermal conductivity (W/m·K)
• Good conductors: metals (Cu, Al); Bad conductors: wood, glass, air

Convection:

• Heat transfer by movement of fluid (liquid or gas)
• Natural convection: density differences drive flow
• Forced convection: fan/pump drives fluid movement

Radiation:

• Heat transfer via electromagnetic waves (infrared)
• No medium required
• Stefan-Boltzmann law: P = σAT⁴
• Newton’s law of cooling: dT/dt = −k(T − T_surroundings)

Deep Study

Stefan-Boltzmann Law Details

• Emissivity (e): 0 ≤ e ≤ 1; black body has e = 1
• P = eσAT⁴ (energy radiated per second)
• Net rate of heat transfer: P_net = eσA(T⁴ − T_s⁴)
• Wien’s displacement law: λ_max × T = b (b = 2.898 × 10⁻³ m·K)
• As temperature increases, λ_max shifts to shorter wavelengths

Newton’s Law of Cooling

• dT/dt = −k(T − T_s)
• T = T_s + (T₀ − T_s)e^(−kt)
• Useful for cooling curves and determining specific heat by cooling method

Heat Conduction Through Composite Wall

• Series: R_total = R₁ + R₂ + …; Q/t = ΔT/R_total
• Parallel: 1/R_total = 1/R₁ + 1/R₂ + …
• Thermal resistance R = l/(kA)

Change of State Details

• Sensible Heat: Heat that changes temperature without changing state
• Latent Heat: Heat that changes state without changing temperature
• Supercooling: Liquid cooled below freezing point without solidifying
• Superheating: Liquid heated above boiling point without vaporising

Exam Tips

1. Specific heat of water (1 cal/g°C = 4.184 J/g°C) is highest of all common substances
2. Anomalous expansion of water — ice floats, water has maximum density at 4°C
3. Linear expansion: ΔL/L = αΔT; Volume expansion: ΔV/V = γΔT = 3αΔT
4. Conduction: Q/t = kA(ΔT/l) — metals have high k, insulators have low k
5. Stefan-Boltzmann: P ∝ T⁴ — small temperature changes cause large changes in radiated power
6. Newton’s law of cooling is exponential — useful in calorimetry experiments
7. Heat flow in series: same current (heat flow rate); in parallel: same potential (temperature drop)

Common Pitfalls

• Confusing heat with temperature — heat is energy transfer, temperature is state variable
• Mixing up coefficient of linear expansion with area or volume expansion coefficients
• Forgetting to use Kelvin (not Celsius) in gas law and radiation calculations
• Not converting mass to kg or length to m — unit consistency is critical
• Confusing emissivity with absorptivity (they are equal for a body in thermal equilibrium)

Suggested Study Order

1. Temperature scales and conversion
2. Heat, specific heat, and calorimetry
3. Phase changes and latent heat
4. Thermal expansion — linear, area, volume
5. Anomalous expansion of water
6. Conduction and thermal conductivity
7. Convection and radiation
8. Stefan-Boltzmann and Newton’s law of cooling