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.

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