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.

Convection

Convection is the main mode of heat transfer in liquids and gases.
It is the process by which heat is transmitted from one place to another by the movement of heated particles of gas or liquid.

Convection only takes place in gases and liquids, but not in solids as the particles cannot move far away from their places in solids. It is the particles in hot liquid or gas that transmit the heat energy. It involves the movement of the hotter fluids form the hot region to the cool region.

How Convection Work
When liquid is being heated, the layer of water that is closer to the heat source will expand and become less dense as compared to the layer of water above. Expanded water will then rise while the layer of water that is not expanded will sink. This is then called the convection current.

Conduction: Conduction in Liquids and Gases

Liquids and Gases are poor conductors of heat.

In liquids and gases, the atoms are spaced further apart as compared to solids, which will cause the collisions between the atoms to be lesser.

Liquids conduct thermal energy slower than solid, but faster than gases, as the atoms are spaced further apart when compared to solid but closer to each other when compared to liquid.

In liquids, it is the impurities (e.g. salt, dirt) that are conducting heat, not the water itself.
Thus, distilled water DO NOT conduct heat.

Gases conduct thermal energy the slowest among the 3 states as the atoms in gases is spaced furthest apart.

Gases are usually insulators of heat (poor conductors of heat).
e.g. Wool used as winter clothings.
It can trap pockets of air in it, reducing heat transfer by conduction.

Conduction: Conduction in Metals.

Metals are good conductors of thermal energy as the atoms of metals are very closely packed together so the vibrations are passed on very quickly and they also have a lot of free electrons. When the metal is being heated, the free electrons that are close to the heat source is also heated. As they are heated, they will move around the metal faster than usual, colliding into atoms and other electrons, which will make the others vibrate faster as well. That is why metals are good conductors of heat.
http://www.apqj64.dsl.pipex.com/sfa/id81.htm

Video
The following video shows that different metals conduct heat at different rates.

In the video,there are 5 rods that are of the same length attached to the handle. At the end of each rod, there is a flag attached to the rod with wax.

Flag #1 - Steel
Flag #2 - Brass
Flag #3 - Aluminum
Flag #4 - Stainless Steel
Flag #5 - Copper

As they start to heat the metal, heat is conducted along each rods which will melt the wax when the heat reaches the end of the rod, which will cause the flag to drop off. As the rate of thermal conductivity of each metal is different, the time taken for each flag is different as well.

From Fastest to Slowest
#3 -> #5 -> #2 -> #1 -> #4

So, from the experiment, we can see that aluminum is the best conductor of heat out of the 5 metals but stainless steel is not as good as the others.

Why does different metals have different rate of thermal conductivity?
The rate of thermal conductivity is depends on many factors such as the amount of free electrons there is, how fast the free electrons move and how far they have to move before colliding into something else and change direction(e.g. electron)

How fast a metal conducts heat is based on the molecules the metal is made up of. Good conductor of heat have atoms that allows heat to flow right through without having to collide or bump into anything. Some metals do not conduct heat that quickly is because they can't really let the heat to flow right through, making the heat bounce around, thus it takes a longer time to heat up.

Conduction

Conduction is the main mode of heat transfer in solids.
It is the process by which heat is transmitted through a medium from one particle to another.

In conduction, heat is transferred from one atom to another by the vibration of atoms. Heat is also transferred by the movement of free electrons from the hot end to the cool end (which is why metals are good conductors of heat.) Poor conductors of heat are liquids and gases.

Conduction occurs in direct contact
e.g. solid to solid (shaking of hands), solid to liquid (diving into the water)

How Conduction Works
When one end of a rod is heated, the particles at this end of the rod gain energy and start vibrating faster, which will then cause the particles to collide with the other particles. Some of their energy is transferred to the other particles which will let the other particles gain kinetic energy. In this way, heat is passed along by the vibrating particles.

Overview of Chapter 10

Chapter 10: Transfer of Thermal Energy

Thermal Energy (Heat) is the form of energy that is transferred from a region of higher temperature to a region of lower temperature.

Transfer of thermal energy only occurs when there is a difference in temperature.

Transfer of thermal energy is based on 2 things
1. Temperature
2. Flow of Heat

1. Temperature
Temperature represents the amount of thermal energy available.

2. Flow of Heat
Flow of heat represents the movement of thermal energy from place to place.

The three main methods of heat transfer are:
1. Conduction
2. Convection
3. Radiation