Demos: 3C-01 Double Chimney

A chamber has two glass chimneys. Smoldering cotton placed under one chimney produces a smoke column that exits that chimney. When a lighted candle is placed under the other chimney, the greater heat flow up that chimney causes the direction of convection to reversed, i.e. the smoke goes down the first chimney and ultimately goes up the second chimney.

Directions: The chamber beneath the chimneys has a glass front for visibility and a locking side door for placing the cotton inside. For convenience, the glass front slides in a track to make the second chimney easily accessible. Light the cotton pile and, using the tongs, place the tray in the chamber beneath one of the chimneys. In a few seconds, smoke will emerge from that chimney. Light the candle and place it under the other chimney. Close the door and in a few seconds the convection will reverse direction. When finished, take the cotton and dunk it in the beaker of water to extinguish the smoldering.

Suggestions for Presentation: If you have covered fluids, remind the students of Archimedes’ Principle as it relates to gases: Less dense air will rise in air of greater density. If not, student are pretty familiar with the phrase “hot air rises.” Ask them to think about a fireplace and how smoke is “drawn” up the chimney. After performing this part, ask what would happen if we could have a greater air flow up the second chimney.

Applications: Fireplaces

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