Transfer of Thermal Energy

Transfer of thermal energy.

1. Thermal energy is transferred ONLY when there is a difference in temperature.

-Transferred by conduction, convection and radiation

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

3. Thermal equilibrium is no gain or loss of thermal energy

CONDUCTION

Definition:

Conduction is the process of thermal energy transfer without any flow of the material medium.

Doing an experiment involving different metal rods coated with wax, a few observations and conclusions can be drawn.

1. Thermal energy flows through the material of each rod without any flow of the material itself.(similar to sound energy for e.g, the object is not transferred though the energy is) Such is called conduction.

2. Different materials conduct heat at different rates.

Conduction in liquids and gases

-process of conduction is inefficient

-particles in liquids and gases are spaced further apart than those in solids

-hence transfer of kinetic energy is slower

-hence air is a poor conductor of heat compared to water

-solids are still the best conductors

Fun fact:

Have you ever noticed that metals tend to feel cold? Believe it or not, they are not colder! They only feel colder because they conduct heat away from your hand. You perceive the heat that is leaving your hand as cold.

CONVECTION

Definition:

Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases)

How convection works:

When water is heated, it expands and becomes less dense than the surrounding water. It therefore starts to rise while the cooler regions in the upper part sink. This movement is due to a difference in its density and sets up a convection current.

Convection currents occur only in fluids but not in solids as convection involves the bulk movement of the fluids which carry thermal energy with them.

RADIATION

Definition:

Radiation is the continual emission of infrafred waves from the surface of all bodies, transmitted without the aid of a medium.

Absorption of infrared radiation

-Infrared radiation is absorbed by all objects and surfaces

Factors that affect rate of infrared radiation

1. Colour and texture of the surface

-Black surfaces are better absorbers of infrared radiation than shiny white surfaces.

2. Surface temperature

-The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.

3. Surface area

-The object with the larger surface area will emit infrared radiation at a higher rate.

Applications of thermal energy transfer:

CONDUCTION

1. Cooking utensils

2. Soldering iron rods

(these are the good conductors of heat)

3. Handles of utensils

4. Table mats

5. Woollen clothes

(these are the bad conductors of heat)

CONVECTION

1. Electric kettles

2. Air conditioners

3. Refrigerators

RADIATION

1. Teapots

2. The Greenhouse

END OF CHAPTER 8!!!