Chapter 8 – transfer of thermal energy

Firstly, thermal energy always flow from a region of higher temperature to a region of lower temperature.
Secondly, thermal energy is only transferred when there is a difference in temperature.
Lastly, It is transferred by three processes, conduction, convection and radiation.

Starting from conduction

Conduction is The transfer of thermal energy through a medium, without medium moving.

(looks yummy) heeee

(We take it as the spoon is made of silver)
when u place the spoon into the bowl of boiling hot soup, u realized that the spoon eventually becomes warmer. This is due to the process of conduction. The cold end of the spoon becomes warmer as thermal energy is transferred from the boiling soup (hot end of the spoon) towards the colder end of the spoon.


But How Does It Work?

(linking back to the diagram again)
When the spoon in the soup is heated, the free electrons in the spoon gain kinetic energy and move faster. These fast-moving electrons then spread into the cooler parts of the spoon and then transfer the kinetic energy to them. As a result, good conductors of heat are more capable of transferring thermal energy at greater speed.

Conduction in liquids and gases.
The process of conduction in liquids and gases is inefficient.
Linking back to the previous post, I have shown that particles in liquids and gases are spaced further apart than those in solids, hence, collisions between particles are less frequent in liquid and even lesser in gases. As a result, the transfer of kinetic energy is slower.

Application of conduction

Uses of good conductor of heat.
- cooking utensils > kettles, saucepans and boilers
- soldering iron rods
- heat exchangers

Uses of insulators
- handles of appliances and utensils > kettles saucepans etc.
- table mats
- sawdust
- wooden ladles
- woolen clothes
- fiberglass, felt and expanded polystyrene foam


Convection



Convection is the transfer of thermal energy by means of currents in a fluid.
It occurs only in fluids such as liquids and gases but not in solids as convection involves the bulk movement of the fluids which carry thermal energy with them.

Convection in liquid



In this case, the water at the bottom expands and gain more energy when it gets heated up. As the heated portion of the water is less dense than the surrounding water, it rises. Meanwhile, the cooler regions of the water in the upper part of the flask, being denser, sink.
(use the purple “arrows” to represent the movement of the water)

common applications of convection
- electric kettles
- household hot water systems
- refrigerators
- air conditioners.

Radiation

Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium
- able to take place in a vacuum

Factors affecting rate of infrared radiation.

color and texture of surface
- dull, black surfaces are better absorbers and emitters of infrared radiation than shiny, white surface.

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

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

applications
- teapots
- the greenhouse
- vacuum flasks

Hence. In conclusion for Unit 8






End of Unit 8.