1st post on Chapter 8
Friday, September 5, 2008
3:45 PM
CHAPTER 8 Oh WOW D:
Transfering of Thermal Energy
TRY THIS OUT (!!!)
Your hand will feel cold when immersed into the Blue basin.
While your left hand will feel warm in the Red basin.
However when your both hands are placed in the Pink basin,
(which is kept at body temperature)You will feel neither hot or cold
Obviously from that,we can feel if something is hot or cold (:
Therefore shows that Thermal Energy
- Only transferred when there is a difference in tempertaure
- Always flows from a region of higher temperature to a region of lower temperature
How is Thermal Energy transferred (?)
THREE PROCESSES!
Radiation
Conduction
Convection
Red Crazy Cow (Oh?)
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Radiation
What is radiation?
- It is the continual emission of infared waves from the surface of all bodies,transmitted without the aid of a medium.
Unlike Conduction & Convection
- Radiation DOES NOT require a medium for energy transfer
- Radiation can take place in a vacuum
Example(!)
Thermal Energy from the Sun reaches the earth by the process of Radiation!
Conduction or Convection is not possible because of the vacuum between the Sun and the Earth.
Thermal Energy from the infrared waves is call RADIENT WAVES.
Actually,all objects emit some radient heat.
The hotter the object,the greater the amount of radient heat emitted! Hah It's as simple as that(:
Absorption of infrared radiation
Infrared radiation
- Is absorbed by all objects & surfaces.
- Absorption of radient heat causes a temperature rise
Emission of infrared radiation
Infrared radiation
- Is emitted by all objects and surface
- This emission causes the temperature of the objects to fall
- A good emitter of radient heat is also a good absorber of radient heat.Conversely,a poor emitter of radient heat is also a poor absorber of radient heat.
Factors affecting rate of infrared radiation
Again,it depends on three factors.
1. Colour & texture of the surface
- Dull,black surface are better absorbers of infrared radiation.
- They are better than shiny & white surfaces
- Dull surfaces are better emitters of infrared radiation
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
- Compare two objects with same mass & material
- The object with larger surface area will emit infrared radiation at a higher rate than the object with smaller surface area.
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Conduction
Conduction is the process of thermal energy transfer without any flow of the material medium.
Here's an exampple:
- Place three small pieces of candle wax on the piece of thick metal wire
- Hold one end to the wire and the other end to the flame of a burner
- The wax will melt beginning with the piece nearest to the flame working down to the farthest piece.
- This demonstrates that the heat is traveling through the length of the wire.
Conduction in liquids and gases
In liquid & gas,
- Thermal energy can also be conducted
- Process of conduction is inefficient
- Particles are spaced further apart than those in solid
- Collisions between particles are less frequent
Example:
This experiment shows that the rate of thermal energy transfer by conduction from the water at the top to the tottom of the test tube is extremely slow.And therefore shows that water is a poor conductor of heat.
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Convection
What is Convection?
Convection is the transfer of thermal energy by means of currents in a fluid (Liquid&Gases)
How does convection work?
- Water at the bottom of the flask is heated,It will expand.
- The expanded water starts to rise as it is less dense than the surrounding water
- The cooler regions of the water in the upper part of the flask,being denser, It starts to sink.
Hot air rises,while cool air sinks.
Convection currents occur only in fluids such as liquids and gases but not in solids.
A convection current is the movement of fluid caused by the changes in density in various parts of the fluid.
In some situations,all three
Radiation
Conduction
Convection
Happens at the same time like this -
