Air is all around us, and moving
air is called wind. Wind is caused by cool air that moves in to replace
rising warmer air. Warmer air rises because the same number of molecules
takes up more space. Because it is less dense, the warm air floats
on colder air.
A convection oven uses the principle of moving
heated air throughout the oven, which cooks evenly from the top and bottom.
A conventional stove cooks from the bottom up.
Demonstrate the following to emphasize
moisture, wind, heat, and air.
- MOISTURE DEMONSTRATION:
Discuss the sources of water in air. Show condensation by putting a plastic
bag over a plant a day before you show students. Moisture will accumulate on
the plastic bag. Note that this is one reason why forests feel wetter than
deserts. Breathe on a mirror to show that you can get moisture from air.
- WIND AND HEAT CONVECTION
DEMONSTRATION: Place a candle underneath one hole in the convection
chamber (directions below). Place the chimneys (cardboard tubes from toilet
paper rolls) over the holes. Stick pins into incense cones so that you can
hang the cone in the top of the chimney opposite the candle. Light the candle.
Wait a minute or so, then light the incense.
Hot air rises over the candle and pulls in
cool air. The smoke from the incense will travel through the box because the
incense is at the cool air inlet. (As the incense burns down, it may get hot
enough for smoke to rise from it. If this happens, replace it with a new
cone.) Explain that this box shows how wind is formed. The candle is a hot
area on the earth, and cool surrounding air is being pulled in.
A CONVECTION CHAMBER:
MATERIALS: cardboard tubes, box (small wooden is ideal), glass cover
A convection chamber illustrates the
movement of cold and hot air. Hot air rises over the candle and pulls in cool
air. The smoke form the incense will travel through the box because the
incense is at the cool air inlet. As the incense burns down, it may get hot
enough for smoke to rise from it. If this happens, replace it with a new cone.
This box shows how wind is formed. The candle is a hot area on the earth, and
cool surrounding air is being pulled in.
- AIR DEMONSTRATION: Put a
balloon over the top of a flask. The result will be more dramatic if the flask
is cooled first. Place the flask in the hot water. The balloon will blow up.
Ask the students why the balloon blew up.
Most will say that the air came in through the rubber or glass.) Try to lead
the discussion so that they figure out that the air already inside the flask
expanded (got bigger) so it took up more room.