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Here's a simple and visually appealing way to show convection currents in water. Warmer water rising through cooler water bends light, allowing you to project its motion onto a screen.
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.
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.
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Attribution: Exploratorium Teacher Institute