JEE Physics Notes: Thermal Properties of Matter

Introduction

Thermal Properties of Matter deals with how substances behave under the influence of heat, including temperature changes, heat transfer, thermal expansion, and specific heat capacity. These concepts are essential for understanding thermodynamics and various real-world applications such as engines, refrigerators, and climate control.

1. Temperature and Heat

• Temperature: A measure of the average kinetic energy of particles in a substance.
• Heat: The energy transferred from one body to another due to a temperature difference.
• Units: Measured in Joules (J) or calories (1 cal = 4.18 J).
• Measurement of Temperature: Celsius (°C), Kelvin (K), and Fahrenheit (°F) scales.

2. Thermal Expansion

• When a substance is heated, its particles move faster and occupy more space, leading to expansion.
• Types of Expansion:
  • Linear Expansion: Change in length of an object due to heat.
  • Area Expansion: Change in surface area due to heat.
  • Volume Expansion: Change in volume due to heat.
• Formulas:
  • Linear Expansion: ΔL = α L₀ ΔT
  • Area Expansion: ΔA = β A₀ ΔT
  • Volume Expansion: ΔV = γ V₀ ΔT

3. Specific Heat Capacity and Heat Transfer

• Specific Heat Capacity (C): The amount of heat required to raise the temperature of 1 kg of a substance by 1°C.
• Formula: Q = mcΔT, where:
  • Q = Heat energy
  • m = Mass of the substance
  • C = Specific heat capacity
  • ΔT = Change in temperature
• Heat Transfer Methods:
  • Conduction: Heat transfer through direct contact (e.g., metal rod).
  • Convection: Heat transfer through fluid movement (e.g., sea breeze).
  • Radiation: Heat transfer via electromagnetic waves (e.g., sunlight).

4. Calorimetry and Latent Heat

• Calorimetry: Measurement of heat transfer during physical and chemical changes.
• Principle: Heat lost by a hot body = Heat gained by a cold body.
• Latent Heat: Heat required to change the phase of a substance without changing temperature.
  • Latent Heat of Fusion: Heat required to convert solid to liquid.
  • Latent Heat of Vaporization: Heat required to convert liquid to gas.

5. Thermal Conductivity

• Definition: A material's ability to conduct heat.
• Formula: Q = kA (ΔT / d), where:
  • k = Thermal conductivity
  • A = Cross-sectional area
  • ΔT = Temperature difference
  • d = Thickness of the material
• Good Conductors: Metals like copper and aluminum.
• Poor Conductors: Wood, rubber, and air.

6. Newton’s Law of Cooling

• Statement: The rate of heat loss of a body is proportional to the difference in temperature between the body and its surroundings.
• Formula: dT/dt = -k (T - Tenv).
• Used in designing cooling systems and heat management applications.

Conclusion

Understanding thermal properties is crucial for applications in engineering, meteorology, and material sciences. Mastering concepts like specific heat, thermal expansion, and heat transfer will help in solving JEE-level problems efficiently.