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Convection

Convection is the transfer of heat throughout a fluid by the movement of the particles that make up that fluid.

A fluid is anything that can flow - liquid or gas.

Take a look at BOTH of these vid clips... the first one is on density, but that is very important if you are to understand convection!  

Many people explain convection by saying that 'heat rises'. Heat doesn't always rise.... it travels from hot to cold areas. We think that it does because of our experience of convection. Heated fluids rise! .... we experience this in our everyday lives and therefore the generalisation that heat always rises gets repeated.

Liquid

When a liquid is heated it's molecules become exited and start moving more vigorously (never just say they 'start moving' - they were not stationery to start with), creating more space between them. They spread out. More space between the molecules means bigger volume, the mass stays the same so it has less density which means that it floats up in surrounding cooler denser fluid. The cooler liquid then replaces the warm rising fluid and a current is set up (see illustration above). So, where gravity is a factor, such as here on earth, the natural fluid motion moves the heated, less dense fluid up and the cooler, more compact fluid down. This is why heating elements in a fish tank are located near the base of the fish-tank, while hot water tank outlets are located near the top of the tank.

Gas

When part of a gas is heated it's molecules in that region become excited. That means that the gas molecules in the region of the heat source move more rapidly creating pressure on the surrounding cooler gas. This makes more average space between particles near to the heat source than further away from it. So the gas near to the heat source has less density than the surrounding gas which means that it floats up in surrounding cooler denser fluid. The cooler fluid then replaces the warm rising fluid and a current is set up (see illustration above). So, where gravity is a factor, such as here on earth, the natural fluid motion moves the heated, less dense fluid up and the cooler, more compact fluid down. This is why heating vents are located along the floor, while air-conditioning outlets are located near the ceiling.

Convection is NOT POSSIBLE in a solid because the particles in a solid cannot move relative to one another. It is the principal mode of heat transfer within fluids.

The 'Chimney' Demonstration illustrates convection currents in gases.

Potassium Permanganate KMnO4 are dark purple needle shaped crystals - they are toxic and can stain your skin - handle with care! can be used as a marker to show convection in fluids.

When dropped into water the crystals dissolve quickly and form a bright magenta pink solution. This is most concentrated near to where the crystal was and so bright pink plumes of KMnO4 can be seen in the water - these can be used to track the movement of the water as it is heated.

Remember to always mention how it is difference in density that makes warm fluid rise and that cool fluid moves in to replace the displaced fluid - and to mention the term 'convection current' when you explain what happens....