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Trying to Blow Out a Candle

This is a fun way to explore the behavior of moving air, and it makes a great thought challenge.

You will need:

  • a candle
  • a candle holder or some foil
  • a lighter or matches
  • a funnel
  • a mirror

WARNING! This experiment uses fire. Be very careful and safe.

Place the candle into a holder or make a holder by crumpling some aluminum foil around the base. Light the candle. Place the small end of the funnel to your mouth. You are going to blow through the funnel and try to blow out the candle. Easy, right? Try it and see what happens. Unless you have a clear funnel, do this in front of a mirror so you can see what is happening.

Not what you expected, was it? Instead of blowing the candle out, the candle flame is pulled towards the funnel! The harder you blow, the more the flame is pulled towards the funnel, but it does not blow out, unless you know the secret. This secret will also help explain what is happening.

Instead of pointing the center of the funnel towards the flame, point the lower edge of the funnel towards it. This time, you will quickly blow out the candle. Instead of flowing straight through the funnel, the air is flowing along the sides of the funnel.

The fast moving air flows through the tube of the funnel until it reaches the point where the funnel starts to spread. At that point, the moving air follows the surface of the funnel instead of traveling in a straight line. This is due to the Coanda Effect, where a fast moving fluid follows the curved surface of an object.