JEE Physics Notes: Transmission of Heat

Introduction

Heat transmission is the process by which thermal energy is transferred from one place to another due to a temperature difference. It plays a crucial role in understanding natural and artificial heating systems, thermal insulation, and engineering applications.

1. Modes of Heat Transmission

• Heat can be transferred through three main mechanisms:
• Conduction: Transfer of heat through direct contact.
• Convection: Transfer of heat via fluid movement.
• Radiation: Transfer of heat in the form of electromagnetic waves.

2. Conduction

• Definition: The process where heat flows through a solid without movement of particles.
• Formula: Q = kA (ΔT / d), where:
  • Q = Heat transferred per second
  • k = Thermal conductivity of the material
  • A = Cross-sectional area
  • ΔT = Temperature difference
  • d = Thickness of the material
• Examples: Metal rods heating up, cooking utensils.
• Good Conductors: Metals like copper and aluminum.
• Poor Conductors (Insulators): Wood, rubber, glass.

3. Convection

• Definition: Transfer of heat due to the movement of a fluid (liquid or gas).
• Types:
  • Natural Convection: Heat transfer occurs due to density differences (e.g., sea breeze, hot air rising).
  • Forced Convection: Heat transfer occurs with external aid like fans and pumps (e.g., air conditioning, radiators).
• Examples: Boiling water, wind currents.

4. Radiation

• Definition: Heat transfer through electromagnetic waves, without the need for a medium.
• Stefan-Boltzmann Law: P = σ A T⁴, where:
  • P = Radiant power
  • σ = Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²K⁴)
  • A = Surface area
  • T = Absolute temperature in Kelvin
• Kirchhoff’s Law: A good absorber is also a good emitter of radiation.
• Examples: Sunlight, infrared heaters.

5. Thermal Conductivity

• Definition: The property of a material to conduct heat.
• Formula: k = (Q d) / (A ΔT).
• Units: W/mK (Watts per meter per Kelvin).

6. Blackbody Radiation

• Definition: An ideal body that absorbs all radiation incident upon it.
• Planck’s Law: Describes the spectrum of radiation emitted by a black body.
• Wein’s Displacement Law: λmax T = constant, where λmax is the wavelength corresponding to peak emission.

7. Newton’s Law of Cooling

• Statement: The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings.
• Formula: dT/dt = -k (T - Tenv).
• Applications: Cooling of hot objects, temperature control.

Conclusion

The study of heat transmission is essential for understanding how energy moves in different environments. Mastering conduction, convection, and radiation concepts is crucial for solving thermodynamics problems in JEE Physics.