University of Virginia
Physics Department

Convection Currents and Density

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

 

Motivation for Learning

Driving Question

What happens when you use ice cubes to cool a drink? After a few minutes, the liquid feels cooler than before you added the ice. What exactly happens to the particles that make up the ice cube when it melts? How does convection aid in the cooling of your drink?

 

Teacher Demonstration

Materials

Procedure

  1. Pour water into the shoebox until it is 2/3 full.
  2. Place the immersion heater at one end of the box.
  3. Place the bag of ice cubes at the other end against the outside wall.
  4. Using the slide projector cast the shadow of the box on a wall. Students will see the vertical convection currents as the water is heated.
  5. (Optional) Repeat the demonstration on an overhead projector. Students should notice that the primary convection currents are set up horizontally.

Background Information

Liquids and gases differ from solids because they can flow. Anything that can flow is referred to as a "fluid."

Both fluids and solids can gain or lose heat by one of three methods: Radiation, Conduction, or Convection. Radiation is the transfer of energy through electromagnetic waves; an example of this is the heat we feel from the sun, or the heat we can feel coming off a red hot stove. Conduction is the transfer of heat or energy between two objects that are in contact with each other. For instance, if you were to touch a red hot stove with your finger, the burn you would receive would be from conduction. Convection is the transfer of energy by the movement of matter.

The best way thermal energy (heat) is transferred in fluids is by convection. The particles move from one location to another, carrying energy with them. When heat is added to a fluid, its particles begin to move faster. Particles in a fluid have considerable freedom to move, and they tend to move further apart as they gain more energy. When a pot of water is heated, the stove burner heats the bottom and as it heats it expands and becomes less dense. Cooler, denser water at the top sinks and pushes the hot water upward. As the hot water rises, it cools, becomes denser and sinks, forcing the warm water to rise again. This movement causes convection currents. These currents transfer thermal energy from warmer to cooler parts of the fluid. Gradually all the water in the pot is heated.

 

Student Activity

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Materials (for each group)

 

Procedure

  1. Obtain an ice cube that has been strongly dyed with food coloring.
  2. Using tongs, gently place the ice cube in a beaker full of warm, fresh water. Keep the beaker and water as still as possible.
  3. Observe the ice - water mixture for several minutes. Write a detailed description of your observations.
  4. After you have made your observation in #3, measure the temperature at the surface, at the bottom of the mixture, and 3 measured levels in between. Record the temperatures in the data table. If you take these measurements too soon, you may tend to stir the water mixture. If you wait too long, temperature equilibrium may have already occurred. You may need to do the experiment twice (observing one and measuring temperature in the other).
  5. Empty the beaker and fill it to the same level as previously with warm water.
  6. Add salt to the water, stirring vigorously as you pour until no more salt will dissolve.
  7. Repeat steps 3 and 4 using salt water in place of fresh water. 

 

Data Sheet

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Observations of Fresh Water:

 

 

Fresh Water
Salt Water
Depth
Temperature
Depth
Temperature
surface   surface  
1/4 Down   1/4 Down  
1/2 Down   1/2 Down  
3/4 Down   3/4 Down  
Bottom   Bottom  

Observations of Salt Water:

 

 

 

Extensions

  1. Predict what would happen to the coloring and temperature of the liquid if you left the beaker containing fresh water and a colored ice cube untouched for 30 minutes.
  2. Test your prediction.
  3. Relate this activity to ocean currents. Why do fish go down to the bottom during cold weather?
  4. Think about ocean currents. Where do you think the water is the coldest?  

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

Answer the following questions:

  1. As the ice melts, what happens to the colored meltwater in the sample of fresh water?

  2. In the salt water?

  3. Do you observe convection currents in this activity?

  4. Of the 2 liquids used, which do you think is denser?

 Answers to Questions:

1. The meltwater will eventually move down into the container of water.

2. The meltwater will stay on top of the saltwater for a longer time than it did for the pure water.

3. Convection currents should be able to be seen.

4. Saltwater is denser.