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What Is Thermal Convection?

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  • Written By: Ken Black
  • Edited By: Andrew Jones
  • Last Modified Date: 11 September 2014
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Thermal convection is a process by which heat is transferred via an object that is being heated. This is most commonly seen in liquids and gasses, and can be easily demonstrated using air as an example. It represents one of the major ways in which heat moves, with conduction and radiation also being common ways to transport heat.

Scientifically speaking, coldness in not quantifiable; there is simply more heat or less heat. Therefore, thermal convection is not simply about hot objects moving. It is about the way temperature as a whole seeks to moderate itself, either to its surroundings, or to the objects that cause it to become heated. By design, the movement of warmer and cooler gasses and liquids causes an overall moderation, though a perfect balance may never be achieved.

The principle of thermal convection can easily be seen by opening the door to a heated oven. When the door is opened, a rush of hot air immediately shoots upward from the oven. If one was to place a small flag at the base of the oven, the flag would wave in the direction of the oven as cool air fills in as the hot air rises. When that cooler air is heated, it will also rise.

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The air that is rising displaces the cooler air above it, often forcing it to the side, and eventually downward. That air will remain at that lower level until it is reheated, and begins to rise again. This heat flow process repeats itself until the heat source is neutralized, and the temperature throughout the area is consistent. If that does not happen, the process will continue indefinitely.

The process is also seen in water, and is not always helpful. For example, if the surface of a lake cools quickly, the water underneath, which is warmer, pushes up. With this upwelling may come dead matter, such as algae, that had been slowly decaying on the lake bottom. Being exposed to air and sunlight, the decaying matter increases its rate of decomposition, and could starve oxygen from the lake. Thus, in this example, thermal convection is responsible in an indirect of way of great harm to a living ecosystem.

Despite the preceding example, thermal convection is usually considered to be a helpful phenomenon. Most ovens and furnaces run on principles associated with thermal convection, thus making the lives of most people more comfortable. Also, temperature moderation in an ecosystem is often very helpful to the forms of life that live in the system. It is also one of the major driving forces in weather.

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orangey03
Post 4

It seems that the shallow parts of a lake are always the warmest. I guess this is because thermal convection does not have a whole lot of water to travel through, so the process is over rather quickly.

I have been wading in water only a few inches deep before in May, and it felt just as warm as the deeper water would on a late summer day. However, as soon as I waded out further, I would feel cold water hitting my skin.

Even in August, if I swam out deep enough, I could feel cold water down low, even though the surface was warm. I would imagine that some water is so deep that it might never receive thermal convection from radiation.

kylee07drg
Post 3

I have noticed thermal convection in action during the transitional seasons of spring in fall. The earth is neither hot nor cold during these times, and the level of heat is determined largely by the sun.

I have been outside on a warm spring day and felt the heat of the sun on the ground and in the air around me. However, as soon as the sun dips beneath the horizon, the air gets dramatically cooler.

The radiation that had been heating up the ground all day suddenly departed, so the leftover heat rose up quickly. The cooler air above rushed downward, and I got very chilly in a matter of minutes.

OeKc05
Post 2

@lighth0se33 – I've never thought about it that way before, but I suppose it's true. The hot water rises up, and the cooler water falls down to the bottom of the pan to be heated, until the whole thing is in a bubbling fit of heat.

My experience with thermal air convection mainly involves the oven. I can't seem to remember that whenever I open the door to a hot oven, an intense wave of heat will suddenly hit me in the face.

I guess I'm just hungry and impatient to get to my food, because I never wait long enough for the initial heat wave to get out of my way. I lunge in and feel a sudden burn on my skin. At least it doesn't cause actual burns or blisters.

lighth0se33
Post 1

I believe I've seen this thermal convection heat transfer in a pot of boiling water. I often have to boil pure water to make pastas and soups, and I've observed that the boiling process always starts at the bottom of the pan.

The bottom is the part that is in direct contact with the hot stove eye. So, it is only natural that this is where the convection would start.

When water begins to boil, little bubbles form on the bottom of the pan. As they form, they shoot upward toward the surface of the water. They keep doing this until finally, the whole pot of water is moving about in a boil.

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