Transfer of Thermal Energy

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

2. Net flow of thermal energy occurs only when there is a difference in temperature.

3. Conduction

3.1. Transfer of thermal energy through a medium without any flow of the medium

3.2. Requires a medium

3.2.1. -> Vibration of particles about their positions and free electron diffusion in metals.

3.2.2. -> Vibration of particles in non-metals.

3.3. Applications of Conduction

3.3.1. 1. Cooking utensils

3.3.2. 2. Soldering iron rods

3.3.3. 3. Heat exchangers

4. Convection

4.1. Transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to a difference in density.

4.2. A convection current is the movement of fluid caused by a difference in the densities of various parts of the fluid.

4.2.1. Expanded fluid rises due to lesser density, colder fluid sinks due to higher density, gets heated, expand and cycle repeats.

4.3. Applications of convection

4.3.1. 1. Electric kettles

4.3.2. 2. Household hot water systems

4.3.3. 3. Air conditioners

4.4. Needs a medium

5. Radiation

5.1. Transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium.

5.2. Does not need a medium

5.3. Emission of radiation causes temperature of object to fall.

5.4. Absorption of radiation results in a temperature rise.

5.5. Factors affecting rate of emission and absorption of infrared radiation:

5.5.1. 1. Colour and texture of surface

5.5.2. 2. Surface temperature

5.6. Applications of radiation:

5.6.1. 1. Greenhouses

5.6.2. 2. Vacuum flasks