Today's task is for students to apply the conservation of energy and the definition of power to determine how much water is needed to operate the 300 watt projector at the front of the room for one hour. This lesson combines topics from a lesson in the energy unit, Work, Power and You as well as the previous lesson on electric generators, Generating Interest in Generators. Students first determine the total amount of electical energy the projector uses in 1 hour. Then, with a given efficiency of 50% for the generator, they know the total starting potential energy of the water. From this information, they can calculate the mass of the water and then the volume.
Central to this lesson are NGSS performance standards HS-PS3-3: Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy and HS-PS2-5: Plan and conduct an investigation to provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.
Science Practice 5: Using mathematics and computational thinking and Science Practice 7: Engaging in argument from evidence are applied as students make their calculations and explain how to improve the electric generation potential of the situation. CCSS applied are Math Practice 1: Make sense of problems and persevere in solving them, Math Practice 2: Reason abstractly and quantitatively and Math Practice 4: Model with mathematics.
At the start of class, students engage in a think-pair-share. Think-pair-share is a collaborative learning strategy were students work to solve a problem or answer a question. Students first think individually about a topic or answer to a question. Then they pair up with a classmate and share their ideas with each other.
The students have 1 minute to read and think about the following question, that I display on the board using the Think - Pair - Share power point.: "The projector displaying this text has a power rating of 300 watts. Explain what that means in terms of the types of energy." Then they have 1 minute pair and share their answers.
I call on a random pair to provide their answers. First I want someone to define power as the rate at which energy changes from one form to another. Second, students should identify the energy transformations happening, including the change from electrical energy into thermal, kinetic and light energies.
After students have a good understanding of the type of energy the 300 watt projector uses, I present the problem on slide 2 of the Conservation of Energy Revisited - Problem power point:
This projector operates at 300 watts. If we could hook up a small electric generator to produce that power and used water falling a distance of one meter to spin it, how many gallons of water would need to fall to keep it going for 1 hour? The generator has an efficiency of 50%.
This is a cooperative learning activity where students work in groups of two. Cooperative learning gives them the benefit of being able to talk through the problem as they work to understand it and persevere in solving it. The groups of two are to write their solution on a single piece of blank paper which I collect after 25 minutes.
This problem combines multiple topics that the students have learned over the past months: work and power, electric power, types of energy, energy conservation and transformation, electric generation and unit conversions. Since real-life science and engineering problems always combine concepts across many topics, it is important to do this in the classroom as well. Students have to dig through their notes to remember the definition of power and how to use it for this problem.
I let the students struggle for 5-10 minutes, at which point I present slide 3 of the power point. Some groups understand the energy transformations involved: potential energy of the water becomes kinetic energy of the water which then spins the generator that transforms into electrical energy. For the groups that struggle, this overview helps them to move forward. The last big hint is to talk about the 50% efficiency; this means that only half of the potential energy of the water becomes electrical energy.
When the time is up, I instruct all student groups to stop their work so we can review the solutions. I bring them through two paths to the solution with the Conservation of Energy Revisited - Solution power point. The solutions are:
These are really the same solution, just done in a different order. However, some groups solve this problem in one way and other groups it another. I want them to know that either solution is perfectly acceptable.
The answer is around 58,000 gallons. This is a tremendous amount of water! I point out that it is enough water to completely fill the classroom. All that water to power a single projector for an hour is too much! I give the students a few minutes to come up with a strategy to reduce the amount of water required to power the projector.
Before students leave, we review their answers which are primarily to increase the distance the water falls, though a few groups come up with the idea of "recycling the water". When I ask them how they do that, they say with a pump. I ask them how they power their pump and they understand their circular reasoning is not a good solution. I display the last few slides on the power point that show how dams are generally high and that this is intentional.
For homework, students are to watch a 5 minute YouTube video titled Energy 101: Electricity Generation. From this video, they create a one page visual that shows how electricity gets to their homes from the power plant. I collect today's student work as students exit.