Learn it: Convection

We saw in the “Conduction” section that water is a poor conductor. This is true of fluids (i.e. liquids and gases) in general. However, energy can be transferred with greater effect through fluids by a different process known as convection. It involves the actual movement of hot matter, which carries the internal energy with it from an object or place at a higher temperature to one at a lower temperature.

Convection is the transfer of energy by the circulation of a fluid.

Convection heat transfer is the movement of warm fluid (liquid or gas) particles from one location to another.

Convection may be explained as follows. When water, for example, is heated in a flask or pot, water at the bottom becomes hot by conduction through the glass.

When water is heated, it expands and so becomes less dense. The hot water therefore rises towards the surface, its place being taken by colder water which is heated in turn.

Thus currents - called convection currents - are set up in the water. These currents may be made visible by adding a few crystals of potassium permanganate to the water.

Characteristics
The following are some of the distinguishing characteristics of convection.

1. Convection makes possible the transfer of heat over much larger distances than is the case with conduction. The atoms/molecules themselves move, carrying heat energy with them, whereas with conduction heat is transferred from particle to particle by collision, the particles remaining in their places.

Smoke from the fire rises due to convection.

2. Convection takes place only in fluids (liquids and gases). It cannot take place in solids, as the atoms/molecules are fixed in their positions relative to each other.

3. The effectiveness of convection can also depend on the viscosity of the fluid in question.

For example, thin soup like chicken broth can be left over a gentle heat to simmer, but a thicker food like porridge tends to burn if it is not stirred while it cooks. This is because more viscous fluids are less able to transfer heat energy by convection.

Domestic heating system

With a convection circulation system set up, the hot water storage tank gradually becomes filled with hot water from the top downwards.

When hot water is run off, an equal volume from the cold supply tank enters the hot storage tank at the bottom. The whole system is thus kept constantly full of water and no air can enter.

Convection currents in the atmosphere and in the oceans are responsible for most meteorological changes. Clouds are formed when convection currents over the earth's surface carry warm, moist air upwards, where it expands and cools. The Trade Winds are formed when hot air over the equator rises and colder air flows in to take its place.

Land and sea breezes

On a hot summer day, the air over the land is hotter than the air over the sea. As a result, the hot air over the land rises and the cold air flows in to take its place, thus producing cool sea breezes. At night, the land cools more quickly than the sea, so the situation is reversed, with warm land breezes being formed.

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