A coal fire is burning in the bottom of a furnace, with a chimney above it. Air moves into the furnace from an opening at the bottom and up the chimney. Describe how the process of convection causes this air movement.

The air is heated by the coal fire. This causes the air to expand as the molecules move further apart. As a result the air becomes less dense. The less dense air rises. The cooler air from outside the chimney displaces the hot air. Above the chimney the hot air cools. The cool air falls. This process of convection is repeated.