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Heat Transfer Overview

• Heat Transfer Mechanisms: Conduction, Convection, and Radiation.

• Thermal Equilibrium: Heat transfer continues until thermal equilibrium is reached.

Types of Heat Transfer

1. Conduction

• Definition: Heat transfer in a medium without the movement of the medium itself.

• Mechanism:

  • Through vibrations of particles (atoms or molecules).

  • Motion of free electrons, particularly in metals.

• Good Conductors: Metalls (e.g., copper, aluminum).

• Poor Conductors: Non-metals (e.g., wood, plastic).

• Cannot occur in a vacuum because there are no particles to vibrate.

• Example: A metal spoon feels hot when placed in hot water.

2. Convection

• Definition: Heat transfer in a medium by the movement of particles in it.

• Mechanism: Hot fluid rises and cold fluid sinks, creating convection currents.

• Only occurs in liquids and gases; solids cannot participate due to fixed particle positions.

• Example: Refrigerators utilize convection currents to distribute cold air.

3. Radiation

• Definition: Transfer of heat energy by infrared waves; does not require a medium.

• Example: Heat from the sun reaches the Earth.

• Ray Factors: Affected by surface temperature, area, colour, and texture.

  • Black surfaces are good absorbers and emitters; white surfaces are poor.

Practical Applications of Heat Transfer

• Conduction: Used in cooking utensils like pans and car radiators.

• Convection: Observed in air conditioning systems and natural phenomena such as sea breezes and land breezes.

• Radiation: Heat is radiated from hot objects, such as a radiator emitting heat into a room.

Thermal Properties of Materials

• Good Absorbers/Emitters: Black, dark, rough surfaces.

• Poor Absorbers/Emitters: White, shiny, smooth surfaces.

Vacuum Flasks

• Design: Used to keep liquids warm; reduce heat loss via conduction, convection, and radiation.

• Features:

  • Insulating cap prevents heat loss through conduction and convection.

  • Silvered walls reflect heat back into the flask, minimizing radiation loss.

  • Vacuum layer minimizes heat transfer by conduction and convection.