Passage VI

Two students explain why the smoke (a mixture of gases and carbon particles) from burning wood in a fireplace rises up the chimney from the fireplace. They also discuss how chimney efficiency (the volume of smoke flowing out the top of the chimney per second for a given temperature difference between inside and outside the chimney) is related to chimney height.

Smoke rises because the gases from burning wood are less dense than the air that surrounds the fireplace. Because the gases are hotter than the air, the gas molecules have a higher average speed than the air molecules. Consequently, the average distance between adjacent gas molecules is greater than the average distance between adjacent air molecules, and so the gas density is less than the air density. As a result, the upward buoyant force acting on the gases is stronger than the downward force of gravity acting on the gases, and the gases rise, carrying the carbon particles with them. The upward flow of smoke is maintained as new air enters the fireplace, causing more wood to burn.

As chimney height increases, efficiency increases. The taller the chimney, the greater the volume of hot gas, the stronger the buoyant force compared with the force of gravity, and the more rapidly smoke rises.Student 2

Smoke rises because wind blows across the top of the chimney. When no wind is blowing, the air pressure at the bottom of the chimney is slightly higher than the air pressure at the top of the chimney. However, when air at the top of the chimney moves at a higher speed than air at the bottom of the chimney, the pressure difference between the bottom, and the top of the chimney is so great that air is forced upward, carrying smoke with it. The departure of air from the bottom of the chimney, in turn, creates a pressure difference that forces new air into the fireplace, causing further burning and an upward flow of smoke.

As chimney height increases, efficiency increases. Generally, wind speed increases with altitude. The taller the chimney, the greater the difference in air speed, the greater the difference in air pressure, and the more rapidly smoke rises.