pHySiCs :) 物理学

Chapter 8: Transfer of Thermal Energy

- Wednesday, September 3, 2008 -

Chapter 8: Transfer of Thermal Energy

- Thermal energy always flow from a region of higher temperature to a region of lower temperature.
- Thermal energy is transferred by either one of the three processes:
1. Conduction
2. Convection
3. Radiation

Conduction
- A process of thermal energy transfer without any flow of the material medium.
- Applications of conduction
Uses of good conductors of heat:

Uses of bad conductors of heat (insulators):

Solid:
- When thermal energy is supplied to one end of a rod, the particles at the hot end will vibrate vigorously. Particles will collide with the others, making them vibrate too. Therefore, kinetic is being transferred.
- When metal is being heated, the free electrons in it gain kinetic energy and therefore move faster.

Liquid and Gas:
- As the particles of liquids and gases are spaced further apart than those in solid, therefore collision between them are lesser, especially gases.
- The transfer of kinetic energy is slow.

Convection

- Transferring of thermal energy by means of currents in a fluid (liquids or gases)
- The expanded water that is caused by heated water at the bottom of the flask is less dense compared to the surrounding water, therefore it starts to rise.
- Cooler region of the water at the upper part of the flask sink due to the high density.
- Applications of convection

Radiation
- The continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
- Applications of radiation

Absorption of infrared radiation
- The absorption of radiant heat causes a temperature rise.

Emission of infrared radiation
- This emission causes the temperature of the objects themselves to fall.

Factors affecting rate of infrared radiation
1. Colour and texture of the surface: Dull/Black surfaces are better absorbers of infrared radiation.
2. Surface temperature: The higher the temperature of the surface of the object compared to the surrounding, the higher rate of infrared radiation.
3. Surface area: The larger the surface area, the higher the rate of emitting infrared radiation.

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