Lesson Using Heat from the Sun

Learning Objectives

After this lesson, students should be able to:

• Explain the three types of heat transfer: conduction, convection and radiation.
• Identify materials that are good insulators and conductors of heat.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Introduction/Motivation

Suggestions for ways to introduce the lesson:

• Bring in a completed solar oven and ask students how hot they think it can get. Let them discuss this and then challenge them to build ovens that reach a certain temperature (200 °F is a reasonable challenge). If the teacher were to bring in the oven as well as something that had been made in it, for instance cookies, the ability to harness the sun's energy to cook food may be even more intriguing to students.
• Lead a discussion of the different energy sources available and explain that solar energy is the cheapest way to harness heat energy. It is also one of the few renewable resources available.
• Show students a video or obtain statistics illustrating the use of solar ovens and solar energy throughout the world. Solar ovens are routinely used in many developing nations and remote regions. This lesson and activity are similar to a solar oven design project at Brigham Young University where solar overns were designed for developing countries. More information on similar projects may be found at the Solar Cooking Archive.

Anothr recommended approach to introduce this lesson:

• Introduce students to basic heat transfer concepts by asking them about situations in which they experience heat transfer in their daily lives. Why is the handle of a pot on a stove hot? Why does hot chocolate cool down when you blow on it? Why is asphalt pavement much hotter on a sunny day than a concrete sidewalk? Why do you wear a jacket in the winter?
• Introduce the idea of using these same concepts to heat an oven to about 200 °F to cook food. Students learn that using heat from the sun is an inexpensive way to create heat without impacting the environment significantly.
• Have students look at pictures of various solar ovens and brainstorm how the heat transfer concepts enable them to work. Have them evaluate the differences between some of the designs and speculate about which ones might collect and store the most heat. Then students can conduct the associated activity Cooking with the Sun - Creating a Solar Oven to design their own solar oven!

Lesson Background and Concepts for Teachers

The lesson covers the fundamental mechanisms of heat transfer: conduction, convection and radiation. The specifics of each type of energy transfer are provided in the Vocabulary/Definitions section. The teacher should understand the differences between conduction, convection and radiation. For instance, only via radiation can energy travel through a vacuum. The teacher should also understand the basic properties of a material that will affect its ability to collect energy from the sun, specifically a material's emissivity. A material with a high emissivity absorbs the majority of the sun's rays, for instance a black shirt, while a substance with a low emissivity reflects the majority of the sun's rays, such as a white shirt. The teacher should also understand the concepts of heat flow. Heat flows from an area of high temperature to an area of low temperature; thus, insulators are needed to store heat. Specific examples of each of these concepts are provided in the vocabulary definitions.

Conduction

Conduction is heat flow from one part of a solid object to another or between two objects in contact with one another. Perhaps the best way to understand this is through a discussion of molecular motion. As an object is heated, its temperature increases. By definition, temperature is the average amount of kinetic energy in a system. As the kinetic energy of the system increases, the random motion of the molecules comprising that system begin colliding more with each other as well as any objects that the system is in direct contact with. By molecules of the system colliding with an adjacent object, the system can transfer its heat and impart more kinetic energy onto the object, thus increasing its temperature as well. (Examples: Touching a hot or cold object, the handle of a soup spoon being hot because heat is conducted through it from the heated liquid.)

Convection

Convection is heat flow through fluid or gas movement or through a fluid or gas moving past an object. In conduction, heat flows through the material. In convection, heat flows as hot air or water flows away from a hot object. (Example: By blowing on soup, the air above the soup is being swept by at an increased rate. This increase in air flow causes a decrease in the temperature of the air directly above the soup thus allowing the warm liquid to dissipate some of its heat). Convection can also be thought of in terms of the transfer of energy due to density differences. If a gas or liquid expands and becomes less dense, it becomes lighter as well. Lighter materials rise while heavier materials fall. If a cooler material is above a warm material, the warmer material rises though the cooler material to the surface, dissipating its energy (heat) to the surrounding environment. Through this process, heat is transferred from an area of high heat to one of low heat, as would be expected.

Radiation

Radiative heat transfer is heat energy transferred through the movement of electromagnetic waves. Heat transfer via radiation is dependent on the temperature as well as the emissivity of the objects that are heating or cooling. Emissivity is a material property that is related to the color of the object. Radiation is the only type of energy transfer that does not require a medium to flow through. Radiation transfers energy via photons that impart their kinetic energy into a system when they strike its surface. This transfer of kinetic energy thus heats whatever substance the radiation is striking. Keep in mind that radiation can be either in the visible or invisible regions, with shorter wavelengths carrying more energy. All energy from the sun initially strikes the Earth as solar radiation.

Energy vs. Power

In an engineering and scientific sense, power and energy are not the same. Energy is the ability to do some sort of work and is measured in Joules (J) in the SI system of measurement. This work can be moving something or it can be heating something. Power is the rate at which energy is used to do work or in this case transferred in the form of heat. Power is measured in watts (W), which are defined as 1 Joule per second (J/s). The flow of heat is generally measured in units of power because it is the rate at which energy is transferred from one place to another. These concepts of energy and power can be difficult for students to grasp. For the purpose of this lesson, it is not crucial that students come away with an understanding of the difference between power and energy, but it is important to present these concepts accurately.

Associated Activities

• Cooking with the Sun - Creating a Solar Oven - Students build solar ovens using cardboard, a set of insulating materials (foam, newspaper, etc.), aluminum foil, and plexiglas.

  Watch this activity on YouTube

Lesson Closure

To wrap up this lesson, have students...

• Discuss other ways that solar energy and heat transfer concepts can be used in our daily lives to solve practical problems. One recommended way to prompt this is by talking about the concept of a solar house. North Carolina State University features a solar house and provides a great deal of information at http://www.ncsc.ncsu.edu/solar_house/NCSU_solar_house.cfm
• Evaluate the advantages and disadvantages of using renewable resources.

Vocabulary/Definitions

conduction: Heat flow due to the direct contact of two objects or within a solid object.

convection: Heat flow due to fluid movement, such as the movement of air or water.

emissivity: Property of the surface of an object that determines how much electromagnetic energy is reflected and how much is absorbed by an object in the form of heat. Emissivity is very dependent on color. (Examples: Aluminum foil has a low emissivity because it reflects a majority of heat and a black surface has a high emissivity because it absorbs a lot of heat; a black shirt gets hotter than a white one in the sun.)

insolation/solar radiation: The amount of power received on the Earth's surface per unit area (wtts per square meter in the SI unit system).

insulator/insulation: A material that does not conduct heat very well and has a low thermal conductivity. (Examples: A good jacket, fiberglass insulation, a sleeping bag, anything with air trapped in it.)

nonrenewable resource: A resource that is not replaceable after it has been used. (Examples: fossil fuels such as oil or natural gas, iron ore.)

radiation: Energy transferred through the movement of electromagnetic waves; heat transfer not requiring a medium.

renewable resource: A resource that is inexhaustible or replaceable by new growth; limitless supply (Example: Solar energy).

thermal conductivity: The property of a material that determines how well it conducts/transmits heat. (Examples: Metal generally has a high thermal conductivity, plastic generally has a low thermal conductivity.)

watt (W): The metric (SI) unit for power. 1 horsepower (hp) = 746 watts (W)

Assessment

Determine whether or not students have grasped the heat transfer concepts via the following methods:

• Class discussion to assess students' ability to accurately discuss the covered material. Ensure that students can adequately describe the three basic forms of heat transfer (conduction, convection and radiation).
• Use the attached crossword puzzles as quizzes to determine how well students understand the vocabulary.
• Have students make solar oven presentations to the class, describing how it works and why they made certain material/design choices, such as reflector and insulation types.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.