Two alligator clip leads (wires with clips on the ends)
3 or more C or D batteries
A pencil lead (.5 or .7 mm mechanical pencil refills work well)
Clear plastic or glass container with rectangular flat sides (such as a small aquarium)
Light source, such as a flashlight (a Mini Maglite with the reflector removed will produce sharp images)
Projection screen or white poster board
Food coloring (in a small dropper bottle or an eyedropper)
Attach one end of each alligator clip lead to either end of a single pencil lead.
Fill the container with water.
Place the pencil lead and attached clips in the water. Position the pencil lead so it is suspended more or less horizontally underwater, and tape the wires to the top edges of the container to keep the lead in place.
Shine the light source through the liquid, projecting the shadow of the pencil lead onto the screen or white poster board.
Line the batteries up in a series, as pictured. Secure the batteries together with tape if you wish. Hold the ends of the alligator leads against the battery terminals to conduct electricity through the pencil lead. Connecting and disconnecting the clips from the batteries will turn your device on and off.
To Do and Notice
Connect your leads to the batteries to let the heat start! Observe the convection currents rising from the pencil lead. Try connecting and disconnecting the batteries to better see the convection currents.
You can also vary the orientation of the alligator clips and wires to see if this has any significant effect on the convection pattern.
Add a few drops of food coloring to the liquid and observe the effects.
What’s Going On?
Like air, water expands as it gets warmer, so it becomes less dense. Because the water warmed by the current flowing through the carbon rod is less dense than the surrounding colder water, the warm water is pushed upward to the surface by the colder water, which is more dense and descends due to gravity, causing the food coloring to move along with it.
Because the cold and warm water have different densities, they have different indices of refraction. Light bends (refracts) as it passes from warmer to colder water or colder to warmer water. When light is bent towards an area of the screen, that area becomes brighter. When light is bent away from an area of the screen, that area becomes darker. The positions of warm and cold water are constantly changing, so the images projected on the screen shimmer and flow like heat waves in air.
A simple variation of this activity is to place a candle on a table and project its image onto a screen or wall with a flashlight. The point source of a Mini Maglite projects clear images of convection when used on a small-scale desktop experiment like this. Changing the distance from the point light source to the candle will change the magnification of the image of the convection currents projected.