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This is bold, italic & underline. Thursday, September 4, 2008
Thermal Physics- Chapter 8

Chapter 8- Transfer of Thermal Energy

Transfer of Thermal Energy

Thermal energy always flows from a region of higher temperature to a region of lower temperature.

How is thermal energy transferred?

Thermal energy is transferred by any of these three processes: conduction, convection and radiation.

Conduction

Conduction is the process of thermal energy transfer without any flow of the material medium.
It occurs mainly in solids.

Conduction occurs when the vibration energy of the molecules is transferred from one molecule to the next. The free electrons in metals make metals a very good conductor of heat.

In conduction, heat is transferred from one atom to another by the vibration of atoms.
Heat is also transferred by the movement of free electrons from the hot end to the cool end, thus metals are good conductors of heat.

Most liquids are poor conductor of heat, so gases such as air are among the poorest conductors of heat.

The link below is a video on conduction. It shows a kind of conduction.
http://www.youtube.com/watch?v=V7QceNEIR-w

Convection

Convection is the transfer of heat energy involving movement of the material itself.
The heated portions of the gases or liquids becomes less dense and rise up, creating convection currents.
Similarly, convection currents are created when the colder portions of liquids or gases becomes denser and sink.

Convection is the transfer of thermal energy by means of currents in a fluid.

How does convection works?

Convection currents occurs only in fluids such as liquids and gases but not in solids.

The link below is a video on convection. It shows a kind of convection.
http://www.youtube.com/watch?v=5pG-tkbQgMo

Radiation

Radiation is the transfer of energy in the form of electromagnetic waves.
Radiation can pass through vacuum.

Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
Radiation does not require a medium for energy transfer.

Factors affecting rate of infrared radiation:
1. Colour and texture of the surface
2. Surface temperature
3. Surface area.

Applications of Thermal Energy Transfer

Common application of conduction:

Uses of good conductors of heat
1. Cooking utensils- usually made of aluminum or stainless steel
2. Soldering iron rods- made up of iron
3. Heat exchangers

Uses of bad conductors of heat
1. Handles of appliances and utensils- made up of woods or plastics
2. Table mats- usually made up of cork
3. Sawdust- used to cover ice blocks to reduce melting
4. Wooden ladles
5. Woolen clothes
6. Fibreglass, felt and expanded polystyrene foam

Common applications of convection
1. Electric kettles
2. Household hot water systems
3. Air conditioners
4. Refrigerators

Common applications of radiation
1. Teapots
2. The green house
3. Vacuum flasks

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