# LessonFrom Sunlight to Electric Current

### Quick Look

Time Required: 1 hour

Lesson Dependency: None

Subject Areas: Physical Science, Science and Technology

### Summary

Students explore the concept of current in electrical circuits, with current defined as the flow of electrons. Then photovoltaic (PV) cell properties are introduced. Generally constructed of silicon, PV cells contain a large number of electrons BUT they can be thought of as "frozen" in their natural state. A source of energy is required to "free" these electrons if we wish to create current. Sunlight provides this energy. This leads to the principle of "conservation of energy." Finally, with a basic understanding of the circuits through Ohm's law, students see how energy from the sun can be used to power everyday items, including vehicles. Engaging students in the engineering design activity of building model solar cars helps students learn these concepts.

### Engineering Connection

Acting as engineers, students build model solar cars, which helps them apply and understand Ohm's law, photovoltaic cells and conservation of energy.

### Learning Objectives

At lesson end, students should be able to:

• Define current.
• Explain why a solar photovoltaic panel is like a battery.

### 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.

###### Common Core State Standards - Math
• Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) More Details

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###### International Technology and Engineering Educators Association - Technology
• Energy is the capacity to do work. (Grades 6 - 8) More Details

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• Energy can be used to do work, using many processes. (Grades 6 - 8) More Details

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• Power is the rate at which energy is converted from one form to another or transferred from one place to another, or the rate at which work is done. (Grades 6 - 8) More Details

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###### North Carolina - Math
• Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) More Details

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###### North Carolina - Science
• Recognize that energy can be transferred from one system to another when two objects push or pull on each other over a distance (work) and electrical circuits require a complete loop through which an electrical current can pass. (Grade 7) More Details

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• Understand forms of energy, energy transfer and transformation and conservation in mechanical systems. (Grade 7) More Details

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• Analyze the nature of moving charges and electric circuits. (Grades 9 - 12) More Details

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### Introduction/Motivation

How might you take the same energy that the sun provides to cook food in our solar ovens and use it to power _______ (insert any one of a number of electric devices)?

Conduct this related demonstration to grab students' attention. The electrons in silicon (Si) generally do not contain enough energy to jump the "band gap" and so they are considered frozen. To demonstrate this, a bucket filled with ice cubes can represent silicon with electrons in the frozen state. Connect the bucket to another bucket by a clear plastic tube attached towards the bottom of each bucket. Represent the sun by a hair dryer. The hair dryer melts the ice representing the release of the electrons by the sun. Additionally, the sun raises the electric potential (voltage difference from one point to another) of each PV cell (creating an electric field at the PN junction—this does not need to be taught to the students). The melted ice (water) now flows in the direction of the other bucket, representing current.

### Lesson Background and Concepts for Teachers

How Photovoltaic Cells Work

Energy of sunlight is transferred to electrons, allowing them to jump to the next orbital and cross the band gap. These electrons are now mobile and current will flow.

Use the Photovoltaic Cell Handout for diagrams and an explanation of how photovoltaic cells work, including their design (layers, materials) and functionality. Following the handout, have students complete the hands-on Racing with the Sun - Creating a Solar Car activity.

For a more detailed understanding of photovoltaic cells, please see http://science.howstuffworks.com/solar-cell.htm.

### Lesson Closure

Distribute solar panels to each group of two students. With a digital multimeter (DMM), go around to each group and run a personal demonstration soliciting student input. For example, hook up the DMM leads to the solar panel when it is placed in the dark (in a desk for example). Expect students to understand that without any light energy, the panel will not create any current or voltage. Similarly, expose the panel to a bright flashlight and prompt students to explain why current and voltage now exist.

Note: For solar panels, use those that come with the Junior Solar Sprint solar car kit that will be used for the associated Racing with the Sun - Creating a Solar Car activity. Thus, this also serves as an excellent transition to the activity.

### Vocabulary/Definitions

conductor: A material in which electricity moves easily. That is, it is a material with low electrical resistance, one in which a fairly small voltage produces a fairly large current.

current: Flow of electrons.

photovoltaic cell: A semiconductor device that converts the energy of sunlight into electric energy.

voltage: Designates "electric pressure" that exists between two points and is capable of producing a flow of current when a closed circuit is connected between the two points (can also be understood with the analogy of elevation: just as a hill will have water flow down it, a voltage will have current flow in the direction from high to low).

### Assessment

Assessment is closely tied to the Lesson Closure use of solar panels. By doing this, you can directly ascertain whether the students "get it" based on their ability to discuss and describe the behavior of the solar panels. Further questioning can easily be worked in regarding the other concepts such as current, conservation of energy, light interaction, etc., as they all relate to the PV cell.

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### More Curriculum Like This

Upper Elementary Lesson
Let the Sun Shine!

Students learn how the sun can be used for energy. They learn about passive solar heating, lighting and cooking, and active solar engineering technologies (such as photovoltaic arrays and concentrating mirrors) that generate electricity.

Upper Elementary Lesson
Electrons on the Move

Students learn about current electricity and necessary conditions for the existence of an electric current. Students construct a simple electric circuit and a galvanic cell to help them understand voltage, current and resistance.

High School Lesson
Solar Angles and Tracking Systems

Students learn about the daily and annual cycles of solar angles used in power calculations to maximize photovoltaic power generation. They gain an overview of solar tracking systems that improve PV panel efficiency by following the sun through the sky.

### Contributors

Rahmin Sarabi; Roni Prucz

### Supporting Program

Techtronics Program, Pratt School of Engineering, Duke University

### 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.