Sunday, July 5, 2009


Go to this website for a simple mcq quiz on the whole chapter of heat transfer(:

3. Application of radiation in real life

Radiation can be found in...

Teapots have shiny surface and since shiny surfaces are bad emitters of radiation, shiny teapots are able to keep the liquid in it warm for a longer period of time as compared to black-coloured teapots.
And in addition, shiny surfaces are bad absorbers, thus shiny teapots or container can keep cold liquid cold for a longer period of time.

Greenhouses are used in cold climates to help grow plants by trapping heat.
Daytime, infared radiation from the sun pass through glass roof into the greenhouse.
This warm up the plants and help them grow. However, the plants too emits infared radiation but the infared radiation emitted is different and cannot pass through the glass roof thus it gets trapped inside.
Overtime, the temperature in the greenhouse will increase.

-Vacuum/Thermos Flasks
The main use of the vacuum flasks is to keep the liquid in it hot by minimising the loss of heat.
And it is done through 4 possible ways: conduction, convection, radiation and evaporation.

-The stopper of the vacuum/thermos flasks is usually made of plastic which is a poor conductor of heat.
-Conduction will not take place in the trapped air since air is a poor conductor of heat.
-Conduction and convection also cannot take place through the sides as there is a vacuum between the double-glass walls of the flask.
-And to prevent radiation, the walls of the glass are silvered to radiant heat back into the hot liquid to keep it hot.
-The only time conduction and convection can occur is when the plastic stopper is removed.
-However, heat loss through radiation is hard to stop as radiant heat can even pass through vacuum.

3. Radiation

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

-unlike conduction and convection, it does not require a medium for energy transfer.
-can take place in a vacuum (without any matter)

For radiation, heat is transferred by infra red waves.

-The heat from the Sun is tranferred by radiation to the Earth.
-This is something conduction and convection cannot do as there is no medium between the Sun and the Earth.
-Another example will be campfire.
-We could feel the heat from the campfire even though there is no medium in between.

Emission of infared radiation
All objects and surfaces emits infared radiation and this emission causes the temperature of the objects themselves to fall.

What are the factors affecting the rate of heat tranfer by radiation (infared radiation)? [both emission and absorbsion]

1. The colour and texture of the surface of the body

Black or dull suraface
- best radiator of heat, better absorer of infared radiation.
- better emitter of infared radiation

Shiny or white smooth surface
- worst radiator of heat, worse absorber of infared radiation.

2. The surface temperature of the body
The higher the temperature of the surface, the higher the rate of radiation.

3. The surface area of the body
The larger the surface area, the higher the rate of radiation.

Other facts on radiation...

-If the temperature of the object is higher than the surrounding, it
is a net emitter of radiation.
-If the temperature of the object is lower than the surrounding, it
is a net absorber of radiation.

-A rough and dull black surface is both a good emitter and a good absorber.
-A smooth and polished surface is both a poor emitter and a poor absorber.

Infared radiation cannot be seen with our naked eyes but the heat sensitive camera can shot the hot spots.

The hottest spot of the cat is its nose as seen by the camera.

A video by 'Eureka' on the topic, radiation waves.

Saturday, July 4, 2009

cont. Applications of convection in real life

Sea and Land Breeze

Another example of the works of convection in real life will be the forming of land and sea breeze.

1. Sea Breeze
-In the morning where the sun is up high and scorching hot, the land will heat up faster than the sea(water)
-The air above the hot/heated land will be heated and hot air rises.
-As a result, the hot air from the land rises while the cool air above the sea rushes towards the land, forming the sea breeze in the day.
-Sea breeze blows from the sea to the land.

2. Land Breeze
-As for the land breeze, it only forms during the night where the land is cooler than the sea.
-The land cools faster than the sea and thus the cool air above the land moves towards the sea.
-While the warm air above the sea rises.
-Land breeze is form and the breeze blows from the land to the sea.

As seen in the picture below, the formation of sea and land breeze is by convection.
Hot air rises and cold air sinks.

*Click on this link for animation on the formation of sea and land breeze!

2. Applications of Convection in real life

Convection can be found in many of the household appliances such as:

-electric kettle and radiator
The heating element of the electric kettle is found at the bottom. This is so that convection can be set up in the water to boil the water. The same reason goes for radiator.

The freezer(freezing compartment) containing the evaporator, which cools the air is located at the top of the refrigerator.
This is so because cold air is more dense than warmer air, so it will sinks and the warm air will rise up.
Soon, the entire space in the refrigerator will be cooled.

-Convection in with an air conditioner is similar to that of the refrigerator.
The air intake is located up high so that only warm air enters the unit.
After cooling, the cold air expelled from the outlet vent sinks to the floor while the warmer air rise towards the air intake.

And with these, convection is set up. The cycle will keep continuing to keep the surrounding air cold as long as the air-con is on.

-Household hot water system
The operation of a hot water system for a house is based on the principle of convection.
The system consists of a boiler, a water storage tank and a cold tank inter-connected by pipes arranged as shown in the figure.
Convection currents rise the hot water up from the boiler to the storage tank while cold water flows down to the boiler, where it in turn becomes heated.

-Car engines
Car engines are cooled by convection currents in the water pipes.
Water is a very good substance to carry the unwanted heat away from the engine to the radiator and the radiator is a heat exchanger where the hot water gives up its energy to the air.
When a car engine has been running for a long time, a lot of thermal energy/heat is produced.
It is necessary to cool the engine so that it does not over-heat.
The engine is surrounded by a water jacket.
When the water in the water jacket gets heated, it flows into copper tubes which include many cooling fins.
A fan causes air to flow past these tubes and cool the water in them.
The cooled water flows down, back into the engine through a hose at the bottom.
A water pump is normally employed to facilitate the flow of convection currents back into the jacket.

Friday, July 3, 2009

2. Convection

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

-is the main mode of heat transfer for both liquid and gas only.
-involves the movement of hotter fluids from the hot region to the cool region.
-cannot happen in solids as it needs particles to be free to move about.

How does convection work?

-As the water at the bottom of the flask get heated up, the molecules at the bottom move about with bigger vibration, expands and take up more space.
-As it expands, it's density will increase thus it will rise.
-But with this, the cooler water in the upper part of the flask becomes denser and will sink.
-As it sink, it gets heated up.
-This movement of less dense warm water rising and denser cool water sink form the convection current.
-And this this pattern will keep continuing.

This are some examples of convection experiments.

'Eureka' in convection.

1. Application of conduction in real life

One of the examples of conduction in real life is cooking, something essential in every household.

From the stove, heat is generated by the fire.
The heat from the stove will then be transmitted through the pot/pans to the oil and finally to the food. This process is conduction.

Pan-frying a fish fillet. The heat of the flame is transferred (on a molecule to molecule basis)first through the pan, then through the thin oil layer, and finally

However, speed of conduction varies from matetials. Metal conducts heat more quickly and on the other hand, wood and plastic, slower. Thus, metal is used to make cooking pots and pans while wood and plastic are used to make pot handles and cooking spoons so that our hand will not feel as much heat.

Another example will be barbeque.
The grate or grill upon which the meat rests, having a higher specific heat than the meat, conducts heat to the meat and that is why, when the grill is right, beautiful brown stripes will magically appear upon the surface of the steak.
(brown strips)
Then the exterior of the meat conducts heat to the interior, molecule by molecule, cooking it.

Another illustration is to put a metal spoon in a hot cup of coffee. At first the utensil's handle is cool. It then grows warm, and eventually hot. Conduction is the process that explain this illustration.

However, cooking not only involves conduction, it also involve the other two processes, convection and radiation.