Convection: Movement of Heat through Fluids

Heat is the movement of energy from an object with high thermal energy to one with lower thermal energy. Convection is the process by which heat is transferred by the bulk movement of a fluid (gas or liquid). Natural convection is the up-and-down movement of fluids due to differences in temperature. Forced convection is when an external device, such as a fan, causes the circulation of warm or cool air.

In this project, you will investigate how water's temperature affects its movement. You will determine how an area can be warmed or cooled by convection currents. You will also find the temperature at various heights in a room and use the results to determine if there are convection currents in that room.

Getting Started

Purpose: To determine how water's temperature affects its movement.

Materials

• 1-quart (1-liter) jar
• cold and hot tap water
• 4 or 5 ice cubes
• spoon
• coffee cup
• blue food coloring
• eyedropper

Procedure

1. Fill the jar about half full with cold water.
2. Add the ice cubes to the jar of water.
3. Use the spoon to stir the ice and water mixture to cool the water; then remove any undissolved ice.
4. Fill the cup about one-fourth full with hot tap water.
5. Add 10 or more drops of food coloring to the hot water. Stir.
6. Fill the eyedropper with the hot, colored water.
7. Place the tip of the eyedropper against the inside of the jar about 1 inch (2.5 cm) above the water. As you observe from the side of the jar, slowly squeeze the eyedropper so the water runs down the inside of the jar into the cold water. Make note of the motion of the hot, colored water after it enters the cold water.
8. Repeat step 7 four times.
9. Observe the contents of the jar periodically for 10 minutes or until no more changes are seen.

Results

Some of the blue colored water sinks a short distance in the cold water, while some immediately moves across its surface. Most of the colored water that sank very quickly rises and joins the other colored water on and near the surface of the cold water. After a time, the blue water sinks, and all of the water becomes slightly blue in color.

Why?

An object's thermal energy is the sum of all the kinetic and potential energy of all the particles making up the object. Heat is the movement of thermal energy from an object with high thermal energy to one with lower thermal energy. In other words, thermal energy is transferred from a warm object to a cooler one. As a fluid warms by the absorption of heat, its thermal energy increases, and thus the kinetic energy of its particles increases. This causes the particles to expand (mover farther apart thus occupying more volume), resulting in a decrease in density. So the hot, colored water is less dense than the cold water in the jar. When poured into the jar, the hot, colored water's momentum (value describing the amount of motion an object has) carries it below the surface of the cold water. But then the bulk of the hot, colored water rises above the cold water because the hot water is less dense. The greater the difference in the temperature of the water, the greater the difference in their densities.

As the hot, colored water cools, it descends, and eventually all of the water mixes. When all of the water is the same temperature, the colored water no longer moves up or down but rather spreads out by diffusion (the movement of fluids due to molecular motion). The rising and descending of a fluid due to differences between its density and the density of a surrounding fluid produces what is called a convection current. Convection is the process by which heat is transferred from place to place by the movement of a fluid. Natural convection is the movement of fluids due to difference in temperature. Differences in temperature cause the density at one place in a fluid to be different from that at another. Forced convection is the use of an external device, such as a fan, to cause the transfer of heat from place to place by the movement of a fluid.

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