Three liquids of different viscosities can be independent variables in convection experiments. For instance, equal amounts of water, motor oil and automobile anti-freeze can be poured into separate small metal containers. An equal amount of heat applied to the bottom of each container will cause heat to transfer by convection through the liquid. The surface temperature of each container of liquid will indicate which liquid is best for heat transfer. The viscosity of the liquid is the independent variable, and the resulting surface temperature is the dependent variable. The container size and amount of applied heat are controlled variables.
Color can be used as an independent variable. Place three equal patches of different-colored cloth on a small sheet of ice then place the ice in a sunny area. After 15 minutes, remove the cloth patches and observe the melted area under each patch of cloth. Different colors will allow heat transfer by convection at different rates. In this experiment the different-colored cloth patches are the independent variables, the amount of melting is the dependent variable and the heat caused by the sun is the controlled variable.
Convection currents created by air movement can also be controlled by the scientist. Purchase two 5-pound blocks of ice and place one into each of two identical large coolers. Add 6 ounces of water into each of two identical 8-ounce plastic glasses and place each one into a cooler. Add an operating small battery-operated fan into one of the coolers. Seal both coolers and place them in a dark location. After one hour, open the coolers and measure the water temperature in each glass. Air movement created by the fan is an independent variable, and the resulting water temperature is the dependent variable. The identical coolers, glasses and ice blocks are the controlled variables.
Vibration convection differs from the transfer of heat and is used to explain how objects in the earth, such as rocks, fossils and bones, tend to move upward during earthquakes or other occurrences that cause ground vibrations. For this experiment place five stones of different sizes in the bottom of a two plastic dishpans. Cover the stones with 8 inches of a half and half mixture of potting soil and sand. Vibrate one of the pans for one minute at 10-minute intervals over the next hour while leaving the second pan undisturbed. Observe the location of the stones in each pan and note the location of the various-sized stones. The independent variable is the vibration, the stones' location is the dependent, and the dish pan and dirt are the controlled variables.