Monday, September 1, 2008

Chapter 8

Transfer Of Thermal Energy.

•Thermal energy always flows from a region of higher temperature to a region of lower temperature.
•The greater the difference in temperature, the greater the heat flow.

•3 different processes of thermal energy transfer:
1. Conduction
2. Convection
3. Radiation


Difference between thermal energy and temperature.

Temperature
-Temperature is not a form of energy, it is only a measurement.
-SI Unit: Kelvin (K)

Thermal Energy
-A flow of energy from a hot object to a cold object
-Unit: joule (J)

>Eureka! Episode 21 - Temperature vs. Heat, Video-YouTube

1. Conduction

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

•Conduction is a point-to-point (by contact) transfer of energy. Molecules that are energetic impart energy to those nearby that are less energetic, thus increasing their temperature.

Heat transferred by conduction in 2 ways:
I. Through the vibrations of particles (atoms and molecules)
II. Through the motion of free electrons

I. Vibrations of particles

- Particles in solids vibrate at their fixed positions.
- Strong attractive force between them.
- When heat is supplied to one region of particles, they vibrate stronger and cause their neighbouring particles to vibrate stronger as well.


II. Motion of free electrons

Metals
- Good conductors of heat
- Have free electrons.
- Carry heat with them when they move from the hotter region to the colder region.
- The heat transfer is speeded up in this way.


Non-metals
Eg. wood, plastics or glass
- Are insulators
- Do not have such free electrons
- Particles are further apart
- Collision between particles are less frequent

>Eureka! Episode 24 - Conduction, Video-YouTube



2. Convection

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

• Convection requires the movement of particles, so this process can only take place in liquids and gases.


Convection Current

- Is created due to change in density in different parts of the fluid.


>Eureka! Episode 27 - Convection, Video-YouTube



3. Radiation

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

- Does not require a medium for energy transfer.
- Take place in a vacuum.
- Heat from the sun reaches us by infrared radiation.

- Absorption of Infrared Radiation
•causes a temperature rise

- Emission of Infrared Radiation
•causes the temperature of the objects themselves to fall

-The rate of heat transfer by radiation is affected by the following factors:
I. Colour and texture of the surface
II. Surface temperature
III. Surface area


Factors Affecting rate of infrared Radiation

I. Colour and texture of the surface
-All objects absorb and radiate heat in different rate.
-Dull and black surfaces are better absorbers and radiator of heat. They are poor reflector of heat.

II. Surface temperature
- Hgher the temperature of the surface of an object compared to the surrounding temperature, the higher the rate of infrared radiation from the object to the surroundings.

III. Surface area
- Larger the surface area of an object, higher the rate of infrared radiation from the object to the surroundings.

>Eureka! Episode 29 - Radiation Waves, Video-YouTube



Applications of Thermal Energy Transfer


Applications of conduction

Common applications of conduction.
Good conductors of heat:

- Cooking Utensils
Eg. Kettles, Boilers

- Soldering Iron Rods
• tip is made of copper

- Heat Exchangers
• used in large laundry facility, help to save energy


Uses of bad conductors of heat (insulators):
• Minimise loss of thermal energy
• Prevent thermal energy from being transferred quickly

Bad conductors of heat:

- Handles of Applications and Utensils
• utensils can be picked up without scorching our hands

- Table Mats
• usually made of cork

- Wooden Ladles
• useful for stirring or scooping hot soup or rice that has just been cooked

- Woolen clothes
• keep people warm



Applications of convection

- In boiling water.

- A radiator in a room heats up the room by convection.




- Hot water system uses the concept of convection.

• With a convection circulation system set up, the hot water storage tank gradually becomes filled with hot water from the top downwards.
• When hot water is run off, an equal volume from the cold supply tank enters the hot storage tank at the bottom. The whole system is thus kept constantly full of water and no air can enter.



- Coastal breeze is an example of natural convection.


Applications of radiation

- Green house effect
• A warm environment.
• Short infrared radiation from sun passes through the glass panels of a green house, and is absorbed by the plants and soil inside.
• This radiation is reflected by the glass panels
• Temperature inside the green house increases until it reaches a thermal equilibrium suitable for plant growth.


- Vacuum flask
• Is a container that is used to keep liquids warm for a longer period of time.
• Its design has important features to reduce heat transfer from inside the flask to the surroundings through conduction, convection and radiation.



Plastic cap
• Made of material which is an insulator
- prevents heat loss by convection because hot air cannot escape

Trapped air
• air is a very poor conductor of heat
- reduces heat lose by conduction

Vacuum
• no particles in vacuum
- reduces heat loss by conduction and convection as both processes need particles to transfer heat.

Thin silvered glass walls
• silvered surface is a good reflector of heat
- it reflects heat back inside the flask, reduces heat loss by radiation



This picture somewhat sums up on chapter 8,



>Easy to Understand; web on
Conduction, Convection and Radiation.

Posted at 1:21 AM