UNIT EIGHT: TRANSFER OF THERMAL ENERGY

EIGHT.ONE: Transfer Of Thermal Energy

What causes transfer of thermal energy?

3 small basins of water labelled A,B & C at 10degrees, 37degrees and 60degrees respectively
Your right hand feels cold when you immerse it in A.
Your left hand feels warm in C.
However when you immerse your hands in B, you will feel neither hot nor cold.
the water in B is the same temperature as your hands, therefore, there is thermal equilibrium.
There is no net gain or loss in temperature betweeen the water in B and your hands. Hence, you will feel neither hot nor cold.

• thermal energy is transferred only when there is a difference in temperature
• it always flows from a region of higher temperature to a region of lower temperature

How is thermal energy tramsferred?

Thermal energy is transferred through 3 processes: conduction, convection and radiation.

1)Conduction is the process of thermal energy transfer without any flow of material medium.
The two main mechanism for conduction are atomic or molecular (non-matals & metals) and free electron diffusion(for metals only).
Liquid and gases are poor conductors of heat compared to solid.
Here's a video about conduction.


2)Convection is the transfer of thermal energy by means of currents in fluid (liquids or gases)
It is the movement of fluid caused by the change in density in various parts of the fluid.

Here's video about convection.


3)Radiation is the continual emission of infared waves from the surface of all bodies, transmetted without the aid of a medium. It does not require a medium for thermal transfer.
Factors affection rate of infared radiation:
-colour and texture of surface
eg: dull, black shiny surfaces are better absorbers and emitters of infared radiation than shiny, white surfaces.
-surface temperature
-surface area

Here's a video on radiation.


EIGHT.TWO: Application of thermal energy transfer

Uses of good conductor of heat:
-cooking utensils like kettles and boilers
-soldering iron rods
-heat exchangers

Uses of bad conductor of heat:
-handles of appliaces and untensils like kettles and boilers
-table mats
-sawdust
-wooden ladles
-woollen clothes

Common applications of convection:
-electric kettles
-household hot water system
-air conditioners
-refrigerator

Common applications of radiation:
-teapots
-the greenhouse
-vacuum flasks