Heat on the move

The idea

Convection is heat transfer by the actual movement of particles in liquids and gases: the heated part expands, becomes lighter and rises, while cooler, heavier liquid or gas moves in, is heated and rises in turn.

A streak of colour

In class, a streak of colour is dropped into water in a beaker. The water is heated at the bottom. (A teacher does this. You will watch an animation.)

What will the colour do?

• Go up in the middle and come down at the sides
• Stay where it is
• Sink to the bottom and stay there

Heat the bottom

Press Heat the bottom. Watch the red and blue dots.

A loop of moving water

The red dots are the hot water. It expanded, got lighter and rose. The blue dots are the cooler, heavier water. It came down at the sides, was heated and rose too. The water particles themselves moved.

Convection is heat transfer by the actual movement of particles in liquids and gases: the heated part expands, becomes lighter and rises, while cooler, heavier liquid or gas moves in, is heated and rises in turn.

• Method | How heat moves | Where
• Conduction | Particles stay in place and pass heat on | Mainly solids
• Convection | Particles move from place to place | Liquids and gases

Notes

Convection is heat transfer by the actual movement of particles in liquids and gases: the heated part expands, becomes lighter and rises, while cooler, heavier liquid or gas moves in, is heated and rises in turn.

Check yourself

In convection, how does heat move?

In the beaker, which water rises?

Cooler, heavier water at the sides of the beaker …

Convection takes place in …

• The particles stay in place and pass heat on. That is conduction. In convection the particles actually move.
• It moves through empty space. That is not convection. Convection needs liquid or gas particles that can move.
• The particles themselves move from one place to another — correct. Yes!
• The hot water at the bottom, which is lighter — correct. Yes!
• The cool water at the sides. The cooler water is heavier, so it comes down.
• Water that has not been heated. Water that has not been heated is cooler and heavier, so it moves down.
• rises first. It is the hot water that rises. Cooler water comes down.
• comes down to take the place of the rising water — correct. Yes!
• turns into smoke. Water in the beaker does not turn into smoke. Cooler water simply comes down.
• only solids. In solids, heat moves mainly by conduction.
• only metals. Metals are solids. Convection needs particles that can move, as in liquids and gases.
• liquids and gases — correct. Yes!