Module 7

HOW HOT
IS THAT COLOR ?

Instructor Guide

Jennifer Heidema, Jim Little, Rick Ramirez,
Nancy Topoozian

This module is intended to focus on the absorption of radiant energy (sunlight) as a function of the color of an object. The key concepts are:

• The color of an object represents the radiant energy reflected by an object. All other colors of incident radiant energy are absorbed by the object.
• As an object absorbs energy, its temperature increases as a function of time.

Students design their own experiments to investigate the rate that the water temperature inside the container changes with time for containers of various colors exposed to direct sunlight. The experiment is in two parts:

• Containers of a single color (white, black or metallic).
• Regular soda pop cans of various colors.

During class discussion and introduction of concepts, have students differentiate between the concepts "temperature" and "heat." This is also a good time to introduce "kinetic energy". Temperature is a measure of the kinetic energy of the water molecules. The greater the kinetic energy (the faster the molecules' random motions inside the can), the greater the temperature. Energy can be transferred and transformed into various different forms (heat, radiant, motion or kinetic, potential, electrical, etc.). In this experiment a fraction of the sunlight (radiant energy) shining directly on the cans of different colors is absorbed by the surface (if painted), then by the metal can and transferred by conduction to the water inside. The energy absorbed is transformed into heat when the water molecules increase their random velocities (kinetic energies). The increase in the kinetic energies of the water molecules causes an increase in the temperature of the water. The students monitor this temperature increase as a function of time using the CBL temperature probes. In 15 minutes a reasonable temperature vs. time graph can be obtained if the experiment begins with cold water (ice water), and the sun shines directly on the can at reasonable geographic locations on Earth (continental U.S. or Europe).

From the data collected, students should be able to infer which colors absorb the most/least energy from sunlight and which colors reflect the most/least amount of energy. You can compare students' findings with the sequential order of the visible light spectrum (from longest to shortest wavelengths). The order of the colors in a rainbow also makes for another interesting comparison. Recall that blue light carries the most energy, yellow less, and red light carries the least amount of radiant energy.

If an object were a perfect absorber, it would absorb all the incoming radiant energy and reflect none, so the object would look completely black to our eyes. Since all of the radiant energy is absorbed by a black object, it heats up, and will emit the energy absorbed, but in the infrared (as heat). A perfect absorber (black object) is also a perfect radiator. If an object were a perfect reflector, it would absorb none of the incoming energy and reflect all radiation, so the object would appear completely white (if "white" light--light of all colors--were shining on the object). In general, objects containing a lot of white (light colors) reflect most incoming radiant energy, while those objects containing a lot of black tend to absorb a larger proportion of the incident radiant energy, and thus heat up faster than the lighter object.

Advanced preparation:

1. This module can be modified to instruct students at various grade levels.
2. Before starting this module, you may want to set aside class time for students to practice using the PHYSICS program with the three temperature probes attached. This will ensure that they will be more comfortable with its use during the lab portion, making the experiment run more smoothly for you as an instructor.
3. To limit the number of variables in this experiment, the three colored cans (in both parts of the experiment) should be the same size, shape, and made of the same material. Use black, white, and silver for the initial exploration, and regular soda cans of differenct colors for the second part of the experiment. Spray painting soda cans works well as long as the reflective properties (flat or shiny) is kept constant over the can surface.
4. Make copies of the student worksheet for each student. Be sure to copy the worksheet twice (front-to-back) to allow for the two parts to this experiment.
5. Since students need to decide how many temperature-time data points to collect in each experiment, they should be told that there is a limit of 200 T-t data pairs allowed by the PHYSICS program on a TI-83.
6. Be sure that students shade their calculators and CBL's from the sun during data collection, especially if you live in a hot climate with intense sunlight such as we have in the Phoenix area in the summer.
7. The plastic soda straws are used to guide the temperature probes down into the cold water, so the probe does not touch the can surface. The heating of the probe is different if it touches the can directly (on the sun side or the shadowed side of the can). This student guidance reduces the number of variables in the experiment (probe touching metal can is heated at different rate from probe immersed in water). If you wish to make this experiment more "exploratory", then eliminate the student instruction to insert the straw into the can and secure with a wad of clay. If the straw is used, the temperature probe should be inserted so its tip extends beyond the end of the straw, and is fully immersed in water in the center of each can.

1. You can build exploration into this lab, by not informing students about the temperature probe needing to be guided in the vertical direction by the plastic straw. Students will discover by repeated runs of the experiment that the probe touching the interior surface of the can is heated more rapidly than expected compared to a position in the water, near the center of the can. We suggest that the exploration of this variable (distance from can surface) not be pursued, simply because of time constraints. If done, however, it is most appropriate for the more advanced grade levels (Individual instructors are the best judges of their own students. We recommend that you experiement with and without the "guide straw", and decide what your own students should be doing.)
2. We suggest that a guide straw be suggested to the students, in which case the exploration component of this experiment then focuses on students making decisions about the length the experiment should be run, the sample time intervals and the number of samples to take (in response to the menu-driven PHYSICS program on the TI-83).
3. About 30 minutes into Part I, discuss with the class what is the best sample time and number of samples. Have students point out all extraneous, confounding variables that may have influenced their results. Suggest that they try to control these variables as much as possible in the next part of the experiment.
4. After the second part of the experiment, we suggest that each group give an oral presentation in the form of a skit, song, poem, cheer, or rap.

1. An application for this module would be to introduce solar energy and how used naturally, as well as the technological state of solar energy uses (e.g. the Mars Pathfinder rover vehicle "Sojourner" utilizes solar cells). Inviting a local solar heating expert works nicely to culminate this experiment.
2. For further investigation about energy absorption/radiation, experiments can be designed around any of the following variables: location of the probe in the container; surface texture, surface area, and/or density of the container; longterm effects of absorption and radiation; angle of the light source; and spectrum of the light source.

1. absorption
2. reflection
3. color
4. conduction
5. convection
6. radiation
7. temperature
8. emitters
9. heat
10. energy (kinetic, radiant)
11. slope
12. linear regression
13. linear and non-linear functions
14. linear decay and growth

1. observation
2. reasoning
3. variable identification
4. control of variables
5. interpreting graphs
6. analysis of data
7. verbal communication
8. written communication

1. Compare and contrast the concepts of "temperature" and "heat."
2. What causes the temperature of an object to increase?decrease?
3. Why can a perfect absorber also be a perfect radiator?
4. Design an experiment that would result in a linear decay graph.
5. Can you replicate our classroom experiment and get accurate results with soda cans using the following materials: a black, plastic container, a brown, glass bottle, and a styrofoam cup, a metal thermos bottle? Why or why not?
6. List and give an example of 3 common ways heat can transfer from one object to another.
7. Explain the function of the silver surfaces found on a thermos bottle.
8. Would cold substances stay cold inside a thermos bottle? Explain your reasoning.

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Last modified 12 Aug 1997
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