Scientific Principle Demonstration-Another Convection Wheel

Materials:

Metal can lid, metal shears, stiff wire, candle, any tool which will make a small, round dent in the metal can lid (a hammer and nail will suffice), pliers

• Using the pattern shown, make eight radial cuts in a metal can lid, being sure to leave a circle of radius approximately 1 cm uncut in the center.
  

• Using pliers, twist each segment of the can lid slightly so as to form a fan with eight tilted blades.
  

• Use a tool to make a small indentation at the center of the can lid. You may want to test first to see if the lid balances when supported at its center. If not, make the indentation at the point on which the lid is most balanced.
  

• Bend the stiff wire to form a base and a length of wire pointing upward. Balance the can lid on the wire by placing the indentation on the lid on the upward-pointing end of the wire.
  

• Place a lit candle under the fan, and watch the fan begin to rotate.

Questions for students:

(1) What causes the fan to rotate?

(2) If the fan is moved to a new position under the blades, will the direction of rotation change?

(3) How would the rotation change if three or four candles were under the fan?

(4) In the atmosphere, are there different temperatures at different heights?
     Will this cause the movement of pollutants by convection?

Discussion: The air warmed by the candle expands and becomes less dense than the surrounding air. As a result, the warm air floats upward. As it strikes the blades of the fan, it exerts a force on the blades causing the fan to turn.