proudly done by alyssa and me! (:






the match glows (because of radiation) but does not light up as air is a bad conductor of heat!




ignore the hands, thanks, (:




you can see the heat rising in the shadow! (:




The candle heats up the air above the candle, causing the air to have more kinetic energy and allowing it to have more space between the molecules. Therefore, it becomes less dense and rises, causing the spiral to turn.




Cold red dye and hot green dye are put into a transparent tub. The red dye sinks while the green dye rises. This is because the molecules in the hot dye has more kinetic energy and vibrates faster, causing it to have more space between the molecules. As such, it will become less dense and rises. As the molecules in the cool dye has less kinetic energy, it would in turn be denser than the room temperature water and sinks.




Results,

Labels: video

still updating.

Radiation (heat transfer by electromagnetic waves)
The continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.

Radiation does not require a medium for energy transfer, which means that radiation can take place in a vacuum.

• An example of radiation is the way thermal energy from the Sun reaches the Earth. (As there is a vacuum between the Sun and the Earth, conduction and convection are not possible ways of thermal energy reaching the Earth from the Sun.) The Sun emits electromagnetic waves and the infrared waves within the electromagnetic waves makes us feel warm.

Thermal energy from infrared waves is called radiant heat and all objects emit some radiant heat. The hotter the object, the greater the amount of radiant heat emitted.


ABSORPTION AND EMISSION OF INFRARED RADIATION


Experiment 6 (:

1. Fill two cans (a bright, shiny can and the other a dull black can) with an equal amount of water.
2. Place a thermometer in each can and place both cans under the sun.
3. Start recording the temperature.

• The temperature of the water in the dull, black can increased faster than that in the bright, shiny can as the dull, black can absorbed radiation at a faster rate compared to the bright, shiny can.
  
• Therefore we can conclude that dull, black surfaces absorb infrared radiation faster compared to shiny, white surfaces.

Experiment 7 (:

1. Fill two cans (a bright, shiny can and the other a dull black can) with an equal amount of boiling water at the same time.
2. Place a thermometer in each can and place both of them in the same area.
3. Start recording the temperature

• The temperature of water for the dull, black can falls at a faster rate than the bright, shiny can as the dull, black can emitted thermal energy at a faster rate compared to the shiny tin.
• Therefore we can conclude that dull, black surfaces are better emitters of infrared radiation than shiny, white surfaces.

FACTORS AFFECTING THE RATE OF INFRARED RADIATION

- Colour and texture
From Experiment 6 and 7, we can see that dull, black surfaces are better absorbers of infrared radiation than shiny, white surfaces and that dull, black surfaces are better emitters of infrared radiation than shiny, white surfaces respectively.


NOTE!
This is why you would be cooler if you wear light or white clothes during hot weather! (:

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

- Surface area
If we compare two objects of the same mass and material, but with different surface areas, the object with the larger surface area.

APPLICATIONS OF RADIATION

Teapots






Sterilisation of Medical Products
To sterilize is to make things free of microorganisms. Large doses of radiation will kill microorganisms and in recent years the medical industry has introduced radiation to sterilize medical products which do not lend themselves to sterilization by heat or steam.

Food Preservation
In the food irradiation process, a variety of fresh vegetables, seafood and meats are subjected to high energy electron beams to obtain various radiation treatments, depending on dose and complexity of the food. Irradiation can delay sprouting in root crops; kill insects in vegetables and stored grains; kill parasites in fresh meats and seafood; delay ripening and decay in fruits; and kill bacterial spores that cause trichinosis and botulism. All this can be accomplished without making the food radioactive.

Crystal Colour Enhancement
Radiation has been used to produce colour in topaz. Uncolored rough, preform or faceted topaz is subjected to the electron beam to form color centers in the material. The irradiation speeds up the natural process of coloration and results in gems with the much sought after sky blue or super blue color. Topaz that would normally be discarded becomes commercially and aesthetically desirable.

Smoke Alarms
Some smoke alarms use the radiation emitted by a radioactive sample to detect fire. Smoke interferes with the radiation emitted by the atoms. This change in radiation levels sets off the alarm.

Mapping Ocean Currents
Radioactive atoms can be detected easily because of the radiation they emit. This allows scientists to track the location of a small sample of radioactive atoms—for example doctors track how a substance spreads through the body and oceanographers track the movement of water currents. Only very small amounts of radioactive elements are needed for this.

Nuclear Power
The energy released by radioactive decay can boil water, and the steam is then used to generate electricity. Nuclear power involves the use of much larger quantities of radioactive material than other uses of radioactivity.

Chemotherapy
Larger amounts of radiation can be used to destroy cancerous cells as an alternative to chemotherapy.

Infrared Cameras
Hot objects give out energy through radiation in the form of electromagnetic waves. Warm objects like the human body mainly radiate infrared radiation. This is why infrared sensitive cameras can "see" people clearly even in the dark. Therefore, police use infrared cameras to track down criminals in the dark.

Flash animation: Radiation energy transfer from sun to Earth and global warming

Convection (heat transfer through a fluid)

The transfer of thermal energy by means of currents in a fluid (liquid or gas)
*It does not take place in solids as convection involves the bulk movement of the fluids which carry thermal energy with them whereas the particles in solids are not free to move around.


CONVECTION IN LIQUIDS.
Convection is the movement of gases or liquids from a cooler spot to a warmer spot.

• When the water at the bottom of the flask is heated, it expands. The expanded water is less dense than the surrounding water and therefore starts to rise.

• In doing so, the cooler regions of the water in the upper part of the flask, being denser, sink.
• This movement of the liquid due to a difference in its density sets up a convection current.
• From this, we can see that a convection current is the movement of fluid caused by the change in density in various parts of the fluid.
  

*You can add coloured dye or potassium permanganate to see the convection currents.




Experiment 4 (:

1. 
   

1. Fill the flask with water. Carefully place some potassium permanganate crystals at the bottom of the flask.
2. Place a Bunsen burner with a small flame under the flask and observe the crystals.
   

Convection currents are seen by adding heat to coloured dye and water.





Convection currents seen by using potassium permanganate.





Convection currents seen by having a large jar which contains ice water and a small jar that contains hot water coloured with red food coloring.

• When the flame is placed at a particular spot, ie. at the middle of the beaker, it will be easier to see the convection currents.
• However, if the heat that is heating the beaker is covering the entire base of the beaker, the convection currents would be very messy.

• In the above videos, we see the convection currents through adding heat. In the video below, we can see convection currents by adding a ice and warm water.

CONVECTION IN GASES.

Experiment 5 (:


i just found a picture depicting the picture above. HURRAY! (: (30/06/09)

1. Place the candle below one of the chimneys. Light the candle.
2. Introduce smoke into the other chimney by placing a piece of smouldering paper over it and observe the movement of the smoke.

• This is because the air above the candle gets heated and expands.
• As the air is now less dense than the surrounding air, it rises out of the chimney.
• In doing so, the cooler surrounding air, being denser, sinks through the other chimney to replace the less dense air.
• This movement of air in and out of the chimneys due to a difference in density sets up a convection current.

APPLICATIONS OF CONVECTION
Electric kettles
The heating coil of an electric kettle is always placed at the bottom of the kettle to aid transfer of thermal energy in water by convection. When the power is switched on, the water near the heating coil is heated up, expands and becomes less dense. The heated water therefore rises while the cooler regions in the upper part of the body of water descend to replace the heated water. A convection current is set up.



Household hot water systems
Water is heated in the boiler by gas burners. The hot water expands and becomes less dense. Hence, it rises and flows into the upper half of the cylinder.

This picture shows a typical household water system.

1. Water is heated in the boiler by gas burners. The hot water expands and becomes less dense. Hence, it rises and flows into the upper half of the cylinder.
2. To replace the hot water, cold water from the cistern falls into the lower half of the cylinder and then into the boiler due to the pressure difference.
3. The overflow pipe is attached to the cylinder just in case the temperature of the water becomes too high and causes a large expansion of the hot water.
4. The hot water tap which is led from the overflow pipe must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap.

Note: The diagram below was edited by me to allow you to see the parts of the water system listed in the above points. (:


Air conditioners
An air conditioner is always installed near to the ceiling of a room to facilitate the downward flow of cooled air to set up convection currents. The rotary fan inside an air conditioner releases cool dry air into the room. As cool air is denser, it sinks. The warm air below, being less dense, rises and is drawn into the air conditioner where it is cooled. In this way, the air is recirculated and the temperature of the air will eventually fall to the desired value.



Refrigerators
The freezing unit is usually placed at the top to cool the air and facilitate setting up of convection currents. The convection currents inside the refrigeration cabinet help cool the contents inside.



Heaters
Domestic heaters are usually placed near the ground. This facilitates the rising of warmed air and sets up a convection current that circulates around the whole room. Thus, the room heats up.


Formation of land and sea breezes
Water has a larger heat capacity than land. Therefore it holds heat better and it takes longer to change its temperature, either upward or downward.During the day, the land heats up and it warms the air close to the ground. The warmer, lighter air begins rising. As the air above the ocean is cooler than that over the land, the air over the ocean is heavier and more dense than the warm air over land. The cool air nudges its way inland to replace the rising air, and can create a strong wind across the ocean and on shore. The bigger the temperature contrast between the air temperature inland and the water temperature, the better chance of a sea breeze developing and the stronger it will be.During the night, water cools off more slowly than the land and the air above the ocean is slightly warmer than that over the land. Therefore, the warmer, lighter air above the ocean begins rising. As the air above the land is cooler than that over the ocean, the air over the land is heavier and more dense than the warm air over the ocean. The cool air moves towards the ocean, replacing the rising air. This creates a land breeze.


Shimmering images and Mirages

1. In this photo, the shimmering of telegraph poles is caused by convection. As the hot road heats up the air next to it, the air rises by convection and creates the same effect as the air above a hot radiator.
2. Also, it isn't water on the road but a mirage. It is also caused by convection too.
   

Formation of clouds
This cloud is known as a hammerhead or anvil and this is another result of convection. It is formed as when the hot ground heats up, the air above it rises. It continues to do so until at altitude, water vapour in the rising air column condenses out as tiny water droplets and becomes a cloud.

Hot air balloons and Gliders

• Hot-air balloons use convection in order to rise into the air. The air inside the balloon is heated. As the hot air rises, so does the balloon. For the balloon to descend, the air in the balloon is cooled or allowed to escape.

• Convection currents in the air allow gliders to fly. The glider gains height from rising, warm currents of air known as thermals. These currents are formed by air being heated by the ground; the heated air becomes less dense and rises.

Labels: post 4

Conduction (heat transfer through direct contact)
The process of thermal energy transfer without any flow of the material medium.

There are two types of conduction:

• molecular vibration
• free electron diffusion
  

MOLECULAR VIBRATION (occurs in ALL solids)

• Particles in matter are kept in constant random motion by the energy they possess.
• The higher the temperature, the higher the average kinetic energy.

• Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions.

When an object is heated, the particles on the hotter side of the substance gain energy and vibrate more rapidly than those on the colder side. When a particle that is vibrating vigourously collides with its less rapidly vibrating neighbour, part of its kinetic energy is transferred to the neighbouring particle and makes it vibrate more. Through the continuous collisions of these neighbouring particles, energy is transferred from the hotter side of an object to the colder side. When thermal equilibrium is reached within the whole substance, there is no net flow of thermal energy between the both sides.

• Conduction is most effective in solids.

1. Solids are the best conductors of heat followed by liquids and then gases.
2. The particles in a solid are very closely packed and their positions are more or less fixed relative to each other. The force of attraction between adjacent particles is strong and therefore making heat transfer by collision is very efficient.
3. The particles in liquid are able to slide about each other, which means that the force of attraction between the particles is not as strong as the particles in solids. Thus, liquids are usually poor conductors of heat.
4. In a gas, the particles are far apart, making energy transfer by collision very inefficient. Thus gases, like air, are very poor conductors of heat.
   

FREE ELECTRON DIFFUSION (occurs in METALS only)

• Different materials conduct heat at different rates.

-Substances that allow thermal energy to move easily through them are called conductors.
-Substances that do not allow thermal energy to move through them easily are called insulators

• Metals like copper and aluminium are good conductors of heat because they contain many free electrons which move randomly between the atoms or molecules. Free electrons in metals gain kinetic energy and move faster as a result. These fast-moving electrons then diffuse or spread into the cooler parts of the metal. In the process, they collide with the atoms in the cooler parts of the metal and transfer their kindetic energies to them, in addition to the molecular vibrations between the atoms or molecules. Therefore free electrons are effective in transferring heat by collision.
• Metals tend to feel cold as they conduct heat away from your hand. As such, we perceive the heat that is leaving our hand as cold.
• Non-metals like glass, wood and polystyrene are usually poor conductors of heat, or good insulators, because they do not have free electrons to help transfer heat. Therefore, non metals are poor conductors of heat as they can only rely on the collision of atoms or molecules to transfer heat.

Molecular vibrations is a slow process while free electron diffusion is a fast process.
As such, metals heat up faster as they have 2 mechanisms of conduction occuring at the same time- molecular vibrations and free electron diffusion. Also, in metals, free electron diffusion is the main mechanism, which is faster.



APPLICATIONS OF CONDUCTION


-Uses of GOOD conductors of heat
If thermal energy has to be transferred quickly through a substance, good conductorsof heat such as metals are used.

1. Cooking utensils like kettles, saucepans and boilers are usually made of aluminium or stainless steel where direct heating is involved.
   
   
   
2. Soldering iron rods are made of iron with the tip made of copper, as copper is a much better conductor of heat than iron.
   
   
   
3. Heat exchangers, such as those used in a large laundry facility help save energy.
   
   
   

-Uses of BAD conductors of heat (good insulators)
Insulators are very useful if we want to minimise loss of thermal energy, or prevent thermal energy from being transferrred quickly.

1. Handles of appliances and utensils like saucepans, kettles, teapots, irons and soldering iron rods are made of wood or plastics which are poor conductors of heat. In this way, the hot utensil or iron can be picked up without scalding our hands.
   
   
   
2. Table mats are usually made of cork so that hot kitchenware can be placed on them without damaging the table-top.
   
   
   
3. Sawdust is used to cover ice blocks because of its good insulating property.
   
   
   
4. Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked.
   
   
   
5. Woollen clothes are used to keep people warm on cold days by trapping air.
6. Fibreglass, felt and expanded polystyrene foam which trap large amounts of air are employed as insulators in the walls of houses, ice boxes and refrigerators. Double-glazed windows have air trapped between two panes of glass, which reduce thermal energy transfer through windows.
   click to enlarge! (:
   
   
   
   
   
   
   

   Experiment 2 (:
   
   1. Coat the parts of the rods that are on the outside of the tank evenly with melted wax.
   2. Pour boiling water into the bath, so that the ends of the rods are submerged.
   3. Record the length of wax that melts in a given interval of time for each of the four rods.
      
   • In this experiment, thermal energy is transferred from a region of higher temperature to a region of lower temperature.
   • The wax on the rods melts as thermal energy is transferred from the boiling water (hot end) towards the colder end of the rods.
   • But the length of melted wax on each of the four rods are different- shortest length of unmelted wax for the copper rod and longest length of unmelted wax for the wooden rod, as seen in the diagram.
   • Therefore we can conclude that thermal energy flows through the material of the rods without any flow of the material itself, which is conduction.
     
   • We can also conclude that different materials conduct heat at different rates. Since the length of unmelted wax for the copper rod is the shortest and the length of unmelted wax for the wooden rod is the longest, it can be concluded that copper is a good conductor of heat and wood is a poor conductor of heat (good insulator).
   The reason why metals melt the wax faster than the non-metals is because of the free electron diffusion stated above.
   
   
   
   Experiment 3 (:
   
   1. Place an ice cube at the bottom of a test-tube and use a gauze to trap the ice and prevent it from floating.
   2. Fill the test tube with water till its almost full.
   3. Heat the test-tube at the upper end, as shown in the diagram below.
   4. Observe the water being heated and the ice below it.
      
      
   • When the water is being heated, we will realise that the water at the upper end of the test-tube soon starts to boil, while the ice melts very slowly.
   • This shows that the rate of thermal energy transfer by conduction from the water at the top to the bottom of the test-tube is extremely slow, which means that water is a bad conductor of heat. As such, heat cannot travel efficiently to the ice.
   • Also, as the fire is at the top, the convection current will only be found at the top too. As such, the water that is heated will remain at the top. This is because when the water molecules expand, the density decreases and the water will rise while the cooler water, which has a higher density would sink to the bottom. Thus the ice melts very slowly.
     
   
   
   
   link: Conduction & Convection
   The applet on the left shows the conduction in a solid bar and the applet on the right shows convection in a gas. For the left applet, above the bar is a graph showing the temperature at each point on the bar. Note you can start, stop and pause. You can turn off the heat applied to the end of the bar. For the right applet, the bottom slider controls the position of the heat source. You can turn the heat off and on by clicking in the box. As the particles heat up, they change color from blue to red.
   
   

Labels: post 3

Thermal energy is transferred by three processes:
-conduction
-convection
-radiation

Labels: post 2

What Causes Transfer of Thermal Energy?
- Thermal energy is transferred only when there is a difference in temperature.
- Thermal energy always flows from a region of higher temperature to a region of lower temperature until the two objects reach the same final temperature.


Experiment 1 (:
Temperatures of the basins.
- Cold: 10° C
- Warm: 37°C
- Hot: 70°C


Step 1:
When you place your left hand in the hot water basin, it feels hot because it gains thermal energy from the hot water.
Step 2:
When you place your right hand in the cold water basin, it feels cold because it loses thermal energy to the cold water.
Step 3:
After removing your hands and drying them with a cloth, when the effects of hotness and coldness in your hands has subside, place your hand in the warm water basin. As your hands feel neither hot nor cold as there is thermal equilibrium. As such, there is no net gain or loss of thermal energy between the water in the warm water basin and your hands.

Alternatively, you can replace step 3 with this.
Step 3:
After putting your hands in the respective basins after a period of time, remove your hands and immediately place your hands into the warm water basin. Your left hand would feel that the water is cooler while your right hand would feel that the water is warmer. Why is this so?


-Your left hand, which was in the hot water basin, transferred heat between the water and your hand until there was thermal equilibrium. Therefore, the temperature of your hand would be higher than it was in the beginning. As such, when you place your hand in the warm water basin, which is about the normal temperature of human beings, the warm water would quickly conduct the heat away, which leaves us with a cooling effect.

-Your right hand, which was in the cold water basin, transferred heat between the water and your hand until there was thermal equilibrium. Therefore, the temperature of your hand would be lower than it was in the beginning. As such, when you place your hand in the warm water basin, which is about the normal temperature of human beings, heat from the warm water would be conducted towards the hand suddenly, which leaves us with a hot sensation.

Labels: post 1