Heat transfer - CCEAConvection in gases

A convection current in air can be demonstrated using the following apparatus.

• A burning splint or piece of rope is blown out so that it is smoky.
• When the smoky rope is held in air the smoke rises.
• The rope is then held over both chimneys in turn as shown above.
• When the rope is held above the candle the smoke rises.
• When the rope is held above the other chimney the smoke is drawn down the chimney. It then passes across the horizontal section and up through the chimney above the candle.

What is happening?

• The candle flame heats the air around it.
• The hot air expands and increases in volume.
• The density of the air decreases, and it floats upwards, rising through the chimney.
• Cooler air is sucked in through the other chimney to replace the rising, warm air – a convection current has been set up.
• The smoke from the smouldering rope shows the path of the cool air.

Applications

Early coal mines were ventilated with fresh air using a similar method.

Two shafts were dug down from ground level to the mine and a fire was lit beneath one of them.

As hot air rose upwards, fresh, clean, cool air was sucked down the other shaft and across the coal mine.

Mind you, lighting a fire in a coal mine did have some safety issues.

A convection current from a radiator

• Air close to the radiator is heated.
• It expands, becomes less dense and rises.
• It is replaced by the cooler, denser air which surrounds it.
• This air is in turn heated, expands becomes less dense and rises.
• The process continues, a convection current is set up and heat is transferred through the air and hence through the room.
• A radiator heats mainly by convection – not by radiation.