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Solar Energy: Classroom Activities

Modeling the Absorption of Solar Energy


Goal

To understand how color affects the absorption of solar radiation.

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Background Information

Solar heating systems may be passive or active. In an active solar heating system, special equipment, in the form of a solar collector, is used to collect and distribute the solar energy. In a passive solar heating system, a home or building is designed to let in large amounts of sunlight. The heat produced from the light is trapped inside.

Both active and passive solar heating systems convert solar radiation into thermal energy, or heat. All substances have thermal energy. It is caused by the vibration and movement of atoms and molecules within them. The faster the molecules in a substance move or vibrate, the more thermal energy is in that substance. When solar radiation strikes a substance, it causes its atoms and molecules to vibrate and move faster, increasing the thermal energy of the substance. If a substance is in contact with a cooler substance, collisions between adjacent vibrating atoms and molecules in the two materials cause the energy of the vibrations to even out, cooling the hot substance and warming the colder one. Substances with darker surfaces absorb a greater percentage of solar radiation than substances with lighter surfaces.

In an active solar heating system, solar radiation strikes the bottom of the solar collector box and increases its thermal energy. The bottom of the box transfers this energy to any air, water, or other liquid that is passed through the box. Likewise, a building with a passive solar heating system is designed to absorb solar radiation and transfer the heat generated to other parts of the building. For example, an outside wall might be made of materials that heat easily in sunshine and then transfers its heat to the inside of the house.

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Activity Overview

Students determine how the temperature of water in three metal containers (one black, one white, one shiny) changes over time when heated.

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Materials and Equipment

Three cylindrical metal containers (all alike), black paint, white pain, aluminum foil, masking tape, graduated cylinder, water, stirrer, three thermometers, three heat lamps

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Activity

  1. Provide students the supplies. Have them make the outside of one of the containers black, another container white, and the final container shiny.
  2. Tell students to use a graduated cylinder and place exactly the same volume of water in each of the three containers.
  3. Instruct students to record the room temperature, before beginning. Then, students should measure the water temperature in each of the containers. Have them record the temperatures in the following data table:
    Time
    (min.)    		 Black Temperature
    (degrees C)    		 White Temperature
    (degrees C)    		 Shiny Temperature
    (degrees C)
    Before test      
    After 4 min.      
    After 8 min.      
    After 12 min.      
    After 16 min.      
    After 20 min.      
    After 24 min.      
  4. Instruct students to place each of the three containers under a heat lamp or in a sunny place. If they use a heat lamp, they should be sure to place each container exactly the same distance from the bulb.
  5. Every four minutes, tell them to stir the water vigorously and measure the temperature in each container. Have them repeat this procedure for 24 minutes and record their measurements on their data tables.
  6. Have students answer the following:
    1. Which container got the hottest?
      The dark container should have reached the highest temperature.
    2. Which container heated the quickest?
      The data should show that the dark container heated the fastest.
    3. What evidence do you have that shows a connection between color and the absorption of sunlight (or light from a heat lamp)?
      The results of the investigation indicate that dark surfaces absorb more radiant energy than do light surfaces.
    4. In your group, discuss your findings and what they show.
      Answers will vary.
    5. Can you think of any other examples where color of materials and heat are involved?
      Answers will vary. One example is that dark clothing makes the wearer warmer in the sunlight than does light clothing. Another example is that the skin of dark-colored persons heats faster in the Sun than the skin of light-colored persons.

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