Meng Ying's Physics Blog

Mr Lim,

Meng Ying's physics blog address is
mengying-physicsproject2009.blogspot.com

Radiation

Radiation is the method of heat transfer that does not require a material medium, it can take place in vacuum.

How Radiation Work
In radiation, the source of heat transmit energy in the form of waves. These waves constitute part of electromagnetic spectrum and are known as infrared radiation. These waves are transformed into heat again when they are absorbed by another body. The hotter an object is, the more energy it radiates.

Example: Sun and Earth
Between the Sun and Earth, there is a vacuum. Conduction and convection cannot take place, which means that radiation is the only one that is taking place. The Sun's energy is traveling to Earth as electromagnetic waves, which is why we have solar energy.

Good and Poor Emitters of Radiation
To emit radiation is to radiate heat.

Objects with black, dull surface radiates more heat.
Objects with shiny surface radiates less heat.

E.g The teapot is usually made of shiny materials so that the tea in it will stay hot for a longer period of time.

Good and Poor Absorbers of Radiation
When a body absorbs radiation, the body's temperature rises.

Objects with dull, black surface absorbs more heat.
Objects with shiny surface absorbs less heat.

*Good Emitter = Good Absorber
Bad Emitter = Bad Absorber

E.g. Solar heating panels are painted black in order to absorb as much radiation from the Sun as possible.

Vacuum Flask

















1. Screw-on Stopper
2. Outer plastic or stainless steel case
3. Outer layer of glass coated with reflective material
4. Vacuum.
5. Inner layer of glass (or stainless steel)
6. Supports and keep inner vacuum chamber in place
7. Additional insulation reduces heat losses and cushions flask against impacts.

The vacuum (7) prevents conduction. The tight stopper(1) prevents air from entering or leaving, which stops convection. The inner layer of reflective material (3) prevents radiation as it is of shiny material, which will reflect radiation instead.
http://www.explainthatstuff.com/vacuumflasks.html

Convection: Land Breeze, Sea Breeze

Land Breeze













In the night, the temperature of the ocean is warmer than the land. The warm air then rises to form clouds. The cool air above the land will sink and rushes towards to ocean the fill the space there, thus we get our land breeze.













In the day, it is the exact opposite of the night. Instead of the ocean being warmer than the land, the air above the land is now warmer than the air above the ocean as the sun is out and the ocean is a poorer conductor of heat. The warm air above the land rises up to form clouds while the cool air above the ocean sinks. The cool air then rush towards the land, thus we get our sea breeze.

Convection: Convection in Air

He places a joss sticks into the first hole. The smoke of the joss sticks then went towards the direction of the candle flame. As the smoke from the joss sticks are hot air, they are already expanded and rises up. As they are in the box, they rises up and reaches the top of the box. When hot air touches the top, they are being pushed to the side by the recently warmed air, which is the candle flame in this case.

Convection in Radiators
Radiators heats up the room by giving off heat by radiation and convection. The radiator warms the air that is around it and the warm air rises, which will pull the colder air towards the radiator, and the radiator will heat the air and the cycle restarts again.

The reason why radiators are usually placed on the floor is because hot air rises, cold air sinks.

Convection: In Liquid

The video shows how convection is like in liquid.

http://euiveoivoaviik.43.gs/mod_tech/node76.html















As water is being heated, the hot water rises while the cool water falls. This is because the cool water are denser than the hot water.
*The heat element is at the bottom.
















This shows the convection current in the liquid when the liquid is being heated.
The hot liquid is represented by the red arrows, which is going upwards.. The liquid that is colder is represented by purple arrows, which is going downwards.

Another Example of Convection: Lava Lamp


The lava lamp heats the wax, which will cause the wax expand and become less dense. As it is less dense, it rise upwards. As the wax cools and becomes denser, it will drop to the bottom again and gets heated again, and the whole cycle repeats.

Another Way of Convection: Ice Cube Convection


The green star-shaped ice cube was put into the warm water. The ice slowly melts and the water sinks down as it is colder and denser than the warm water. The warm water then rise up.