Convection: Movement of Heat through Fluids (page 2)
Try New Approaches
- How would reversing the two temperatures affect the results? Repeat the investigation, filling the jar with hot water and the eyedropper with cold water. How does this model an atmospheric condition called an inversion? For more information on convection currents, see Janice VanCleave's A+ Projects in Earth Science (New York: Wiley, 1999), pp. 175-179.
- The looping path that a convection current follows is called a convection cell. Use a diagram to show the motion of a fluid in a convection cell. For information, see Corinne Stockley, Illustrated Dictionary of Physics (London: U sborne, 2000), p. 28.
Design Your Own Experiment
- Convection currents are used to cool an area, such as in a refrigerator. Design a way to show how the location of the cooling coils in the top of a refrigerator creates a cooling convection current. One way is to use the motion of water at different temperatures. Do this by filling a paper cup with cold water and adding 2 drops of food coloring to the water. Stir. Cover the mouth of the cup with a small square of aluminum foil. Insert a %-by-lO-inch (O.3-by-25-cm) dowel through the center of the aluminum foil. The dowel should stand as vertically as possible. Place the cup in a freezer. When the water is frozen, remove the paper cup from the freezer, then remove the paper and the aluminum foil from the ice. The dowel should be firmly frozen in the ice. Fill a jar that has a mouth large enough for the ice to fit through about three-fourths full with hot tap water. Push the ice just below the surface of the water in the jar and secure it in this position by taping the dowel to the rim of the bottle. (See Figure 22.2.) Observe the contents of the bottle for 1 to 2 minutes. Notice the direction of any convection currents.
- If cooling coils were placed at the bottom of a refrigerator, what effect would this have on the creation of cooling convention currents?
- It is commonly said that "heat rises," but what is meant is that the lighter, less dense hot fluids rise through cooler, denser fluids. Design a way to show that warm air rises and cool air sinks. One way is to secure three thermometers to a wall. Lay three thermometers on a table for 3 or more minutes so they all register the same temperature, then put one thermometer near the floor, a second on the middle of the wall, and a third near the ceiling. If there is a ceiling fan, turn it off during the investigation. Record the initial temperature readings of the three thermometers in a Temperature Data table like Table 22.1. Then record the temperature readings on each thermometer every 15 minutes for 1 hour. Determine the absolute differences between the temperatures at the different heights. For example, if T1 is 75°F and T2 is 78°F, the absolute difference between T1 and T2 expressed as |T1 - T2| is the positive difference between these two values, which is 3°F (78°F -75°F). From your results, determine if there are convection currents in the room. If so, what is the direction of the currents?
Get The Facts
When Earth's surface is heated by solar energy, it warms the air above it, creating convection currents. The currents of rising, heated air are called updrafts or thermals. How do thermals affect air pollution? How do thermals affect the weather? For information, see Curt Suplee, Everyday Science Explained (Washington, D.C.: National Geographic Society, 1998), pp. 70-71.
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