The lesson will first explore the concept of current in electrical circuits. Current will be defined as the flow of electrons. Photovoltaic (PV) cell properties will then be introduced. Generally constructed of silicon, photovoltaic 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. Light from the sun provides this energy. This will lead to the principle of "Conservation of Energy." Finally, with a basic understanding of the circuits through Ohm's law, students will see how the energy from the sun can be used to power everyday items, including vehicles. This lesson utilizes the engineering design activity of building a solar car to help students learn these concepts.

The lesson utilizes the engineering design activity of building a solar car to help students learn about Ohm's law, photovoltaic cells, and conservation of energy. Solar Energy Current Voltage Photovoltaic cell Conservation of Energy Ohm's Law Be able to define current Be able to explain why a solar photovoltaic panel is like a battery. Students can be posed with the question of "how could you take the same energy that the sun provided to cook food in our solar ovens and use it to power _______(insert any one of a number of electric devices)?" A related demonstration can also grab their 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. The bucket should be connected to another bucket by a clear plastic tube attached towards the bottom of each bucket. The sun can then be represented 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) will now flow in the direction of the other bucket, representing current. 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. For a more detailed understanding, please see http://science.howstuffworks.com/solar-cell.htm Flow of electrons 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) A semiconductor device that converts the energy of sunlight into electric energy This is a material that allows electricity to move through it easily. That is, it is a material with low electrical resistance, one in which a fairly small voltage will produce a fairly large current. Racing with the Sun - Creating a Solar Car Solar panels should be distributed to each group of two students. With a digital multimeter (DMM), teachers can go around to each group and run a personal demonstration soliciting student input. For example, the leads on the DMM can be hooked up to the solar panel when it is placed in the dark (in a desk for example). Students should easily understand that without any light energy, the panel will not create any current or voltage. Similarly, the panel can be exposed to a bright flashlight and students can be prompted to explain why current and voltage now exist. These panels are part of the Junior Solar Sprint solar car kit that will be used for the upcoming "Racing with the Sun - Creating a Solar Car" activity and thereby serve as an excellent transition. Assessment is closely tied to the closure described above. The teacher 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. Photovoltaic Cell Handout