Class 11 Physics: Thermal Properties of Matter Notes

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1. Introduction

• Thermal properties describe how matter responds to changes in temperature.
• Includes heat capacity, specific heat, thermal expansion, and latent heat.

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2. Heat Capacity

• Definition: Amount of heat required to raise the temperature of a body by 1 K.

$$C = \frac{Q}{\Delta T}$$C=ΔTQ​

• Units: J/K

Specific Heat (c): Heat required to raise 1 kg of substance by 1 K:$$Q = m c \Delta T$$Q=mcΔT

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3. Thermal Expansion

• Solids, liquids, and gases expand on heating.

1D (linear) Expansion:$$\Delta L = \alpha L \Delta T$$ΔL=αLΔT

2D (area) Expansion:$$\Delta A = 2 \alpha A \Delta T$$ΔA=2αAΔT

3D (volume) Expansion:$$\Delta V = 3 \alpha V \Delta T$$ΔV=3αVΔT

Where $\alpha$α = coefficient of linear expansion

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4. Relation Between Coefficients

$$\beta = 3 \alpha$$β=3α

• $\beta$β = volume expansion coefficient
• $\alpha$α = linear expansion coefficient

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5. Anomalous Expansion of Water

• Water contracts on cooling from 4°C to 0°C instead of expanding.
• Maximum density occurs at 4°C.

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6. Calorimetry

• Study of heat exchange between bodies.
• Heat lost = Heat gained (ignoring losses to surroundings):

$$m_1 c_1 (T_f – T_1) = m_2 c_2 (T_2 – T_f)$$m1​c1​(Tf​−T1​)=m2​c2​(T2​−Tf​)

Where $T_f$Tf​ = final equilibrium temperature

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7. Latent Heat

• Definition: Heat absorbed or released without change in temperature during a phase change.

$$Q = m L$$Q=mL

Where:

• $L$L = latent heat (fusion or vaporization)
• $m$m = mass

Common latent heats:

• Fusion (solid ↔ liquid)
• Vaporization (liquid ↔ gas)

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8. Thermal Stress

• Definition: Stress developed when expansion or contraction is restricted.

$$\sigma = Y \alpha \Delta T$$σ=YαΔT

Where:

• $Y$Y = Young’s modulus
• $\alpha$α = coefficient of linear expansion

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9. Key Points

• Heat causes temperature rise and/or phase change.
• Linear, area, and volume expansion depend on material properties.
• Calorimetry helps find specific heat or latent heat.
• Thermal stress arises if expansion is restricted.