Unit 2.4: Power

• Examine patterns of similarity between power produced by the human body and the power needed for basic commodities in everyday life in order to familiarize concepts of "power" and "energy."
• Continue to develop the concepts of direct and inverse proportionality as they relate to energy, power, and time.
• Use mathematical representations to support a claim regarding physical relationships among gravitational potential energy, mass, and height.
• Experience failure as a critical part of the scientific process.
• Engineer a method to transform kinetic energy into electricity (Hand-Crank Generator Extension Lab only).

This unit follows up from the previous unit, Energy, and compliments it well. Students will utilize the same quantitative skills (taking a mean, interpreting proportionality, plotting data, unit conversions, error) as in earlier units, but this time the data relate to new physics. In this way, your students are building quantitative skills in new and novel settings to deepen their understanding.

Plan for these materials to take about 100 minutes, plus an additional 90 minutes if the Hand-Crank Generator Extension Lab is also completed. These materials were developed for us in an in-person synchronous classroom, and you must have access to a staircase to complete the Power Lab (only one student per lab group needs to climb the staircase and there are other less exhausting tasks like using a stopwatch and measuring the height of the staircase). Most of the exercises rely on small group work and are best suited to smaller classes or a lab meet-up outside a traditional lecture room.

Teaching Materials:

Pre-Class reading materials and assignment - Instructor Copy:

U2.4 Pre-Class Instructor Copy.docx -- educator-only file

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All slides (instructor notes are in the notes of many slides - delete these if giving to students): Unit 2.4 All Slides v2 (PowerPoint 2007 (.pptx) 53.7MB Aug30 24)

Personal Power Rating Lab:

• Materials: Stairs to climb, meter stick, timer or camera, long string or rope that is not stretchy.
• Instructor File:
  Unit 2.4 Power Lab - Instructor Copy v2 -- educator-only file
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  Unit 2.4 Power Lab - Instructor Copy v2

  
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• Student "Reminder" Handout (students are mobile and might need some reminders of what they're working on): U2.4 Power Lab Reminder Handout.docx (Microsoft Word 2007 (.docx) 70kB Jul8 24)
• Student Handout (optional, use if not following Science Journal format): U2.4 Power Lab Worksheet.docx (Microsoft Word 2007 (.docx) 70kB Jul8 24)

DIY Hand-Crank Generator Extension Lab:

• Materials: Cardboard, Long nail, Magnets, Copper wire, Glue, Tiny LED light bulb (see the Teaching Notes & Tips section below for specifics on materials that worked well for us).
• Videos of working generator: U2.4 Generator Video 2.mov (Quicktime Video 13.4MB Jul8 24) and U2.4 Generator Video 1.mov (Quicktime Video 21.7MB Jul8 24)
• Student Handout (optional, use if not following Science Journal format): Unit 2.4 Generator Lab Handout v2 (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)

Reflection: Unit 2.4 Reflection Energy Power Volcano v2 (Microsoft Word 2007 (.docx) 69kB Aug30 24).

• Sample Student Reflections (this is Reflection 4): Reflection Examples Redacted.pdf (Acrobat (PDF) 1.8MB Jul8 24)

Check your Understanding: Energy v. Power:

U2.4 Power Lab - Instructor Copy.docx -- educator-only file

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The Lab(s) is/are assessed as a Science Journal, as always. Science/Lab Journals General Instructions/Rubric (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)

Other Materials: computer and projector, white board or chalk board with markers/chalk.

Pre-Class Assignment(s):

Read the document called Power vs. Energy Explanation

Watch this KQED video: What is the Difference Between Power and Energy?

• In the video, each child represents one unit of energy and children go down the slide for five seconds. Which slide transfers the most power, or are they all three the same? Explain why you think this.
• Challenge question: Now, let's consider the same slide set-up but change it around a little bit. Which slide transfers the most power, or are they all three the same? Explain why you think this.
  • Slide 1: 15 children go down the slide in 15 seconds.
  • Slide 2: 15 children go down the slide in 10 seconds.
  • Slide 3: 15 children go down the slide in 5 seconds.
• Do you think that power and energy are directly or inversely proportional? Explain why you think this.
• Do you think that power and time are directly or inversely proportional? Explain why you think this. 

In Class, Part 1: Personal Power Rating Lab (100 min):

Introduction (25 min): 

Use the Pre-Class assignment as a jumping off point for today's lab. Give your students several minutes to discuss their answers to the pre-class questions in small groups before asking groups to share out answers.

Discussion question: As you move up the stairs, what kinds of energy are involved and how are they transforming? Develop an energy transformation diagram and also derive the formula for Gravitational Potential Energy (GPE) based on physical intuition.

• This discussion connects back to previous units (developing an energy transformation diagram, Unit 2.3) and also foreshadows future units on heat engines and forces. Pointing these connections out to your students demonstrates that all elements of this class are vertically aligned to work together and are not separate, siloed topics.
• You are asking your students to derive the formula GPE = mass * height * g instead of giving it to them. They can easily understand that GPE and mass should be directly proportional, for example. This helps bridge the gap between physics and math.

Personal Power Rating Lab (60 min)

• In this lab, students repeatedly run up a set of stairs with varying levels of effort and develop the concept of power as the rate of energy transformation. They take measurements of the height of the staircase and the time to run up the stairs and use that information to calculate power output and GPE. Data are collected and organized in a data table.
• Organizational prompts given to the students before taking data include: 
  • How can you use the materials provided to determine the amount of GPE you gain by climbing the stairs? 
  • How can you use your GPE calculation to determine a runner's power output? What other information do you need?
  • What method(s) will you use to ensure that your measurements of time are both accurate and precise?
  • What method(s) will you use to ensure that your measurements of the height of the stairs are both accurate and precise?
• Follow-up Discussion Questions include:
  1. A typical LED light bulb uses 9W of power (i.e., 9 Joules of energy every second that it's on). More values for typical power ratings of common household objects are in the table shown. Could you power your own lifestyle if you needed to? 
  2. What type of proportionality describes the relationship between GPE and height? Explain how you know.
  3. What type of proportionality describes the relationship between power output and height? Explain how you know.
  4. Explain how the gain in GPE can be the same for all three trials, but power output varies.
  5. You assumed that the power output of the runner was constant for their run up the stairs. Why might that not be quite true?

Wrap up (15 min):

• Students share out their group measurements for the height of the stairs and power outputs. With this group data, ask the question: Any outliers, clever methods, or issues with accuracy? Discuss!
• We know that energy is never created or destroyed, but when you get back down to the bottom of the stairs you stop, so there's no KE or GPE. What happened to that energy? Do you think you could take that energy back and use it to get you back up the stairs?
  • This is prep for our next unit on Heat Engines. Heat is usually a dead end – once energy escapes a system via heat, it's very difficult to harness that energy or turn it into another kind of energy. Now is a great time to plant this seed so students are prepared for it. In a heat engine, we take heat and turn it back into KE or PE, which is pretty cool! Example: When you're at the top of the staircase, you've exhausted a bit of heat into the air and stairs to get up, which caused a tiny amount of warming in the air molecules. It's now very difficult to turn that heat/thermal energy into some other kind of energy. On the other hand, a ball at the top of a staircase has a much more useful kind of energy – gravitational potential energy – and it's easy to imagine harnessing that potential energy to turn into something else (ex: if you drop it, it'll turn into KE!). 

In Class, Part 2: Hand-Crank Generator Extension Lab (90 min):

• Build the DIY Hand-Crank Generators:
  • Step-by-step instructions are provided both in the slides and in the handout, courtesy of WikiHow. This is an engineering activity, and most of the active time will be spent just building the generators.
  • There may be trial and error (and failure!) in this lab. Emphasize to your students this learning objective for the unit: Experience failure as a critical part of the scientific process. As a science instructor, you are trying to un-train the brains of the students in this course that they must be perfect in order to learn science.
• Wrap-up whole-class discussion. Questions include:
  • This generator makes electricity by spinning. What else can you think of that gets its energy from spinning?
  • Eventually, we want to make connections back to our volcano. Can we do that with this electricity generator? Why/why not?
  • This lab activity required more "step-by-step" than others we have done. Discuss what you like/dislike about this lack of freedom and explain why.
  • Which SEP (Science and Engineering Practices) are used in this lab?
  • Which CCC (Cross-Cutting Concepts) could you apply to this lab? How would you integrate them?

If the Personal Power Rating lab isn't going to fit in your course due to time constraints, many of the same lessons can be learned (albeit without the data taking or quantitative analysis) in the following 10-minute activity: Use a Genecon Hand Crank Generator to power one, two, and then three light bulbs in parallel. You'll have to work harder to turn on more lights. We do not focus heavily on Power later in the course as a building block to other things, so if a topic needs to be shortened then this is a good candidate for that.

If you are completing the Hand-Crank Generator Extension lab, then make sure to order supplies in advance and test out. Here are materials that worked well for us:

LED Light bulbs/diodes:

• haraqi 500 Pcs 5mm LED Light Emitting Diode Assortment Kit, Low Voltage Diffused Diode for DIY PCB Circuit, Indicator Lights

Magnets:

• Powerful Neodymium Bar Magnets, Rare-Earth Metal Neodymium Magnet, N52, Incredibly Strong 33 LB Strength - 60 x 10 x 5 mm, Pack of 10

Copper wire:

• Magnet Wire 30 Gauge AWG Enameled Copper 1570 Feet Coil Winding and Crafts Red
• Note: The magnet wire needs to be spooled off into separate spools for each group and tangles easily.

• Two pre-class assignments (described above) are graded for completion only, not correctness. Administer these pre-class assignments using the same format throughout your course (through the LMS, turn in paper copies, guided discussion/participation in class, etc.). Consider setting the due date an hour or so before your class begins to give you time to summarize where your students sit with these concepts (this is a form of Just in Time Teaching).
• A student worksheet is provided as part of the resources for both of the labs in this unit. Instructors can choose to use this worksheet in lieu of the science journal format. Make a choice now: do you want to follow the science journal format, or do you want to continue with worksheets? If using the science journal format, then PowerPoint slides are all that is needed (no worksheet). As the course goes on, we will often but not always provide student worksheets. In classes with time constraints, worksheets can be faster, but they do not ask your students to engage as deeply with the practices of science.
• The Unit 2.4 Reflection asks students to connect the topics of Energy and Power back to this unit's motivating question about the Hunga Tonga volcanic eruption. A grading rubric is included in the assignment link.

For the Pre-Class assignment, students read the following documents:

• Power vs. Energy Explanation
• What is the Difference Between Power and Energy?

Images in the DIY Hand-Crank Generator Lab files are courtesy of wikiHow, a wiki building the world's largest, highest quality how-to manual. wikiHow Staff created these materials in the article "How to Make a Simple Electric Generator," available online: https://www.wikihow.com/Make-a-Simple-Electric-Generator. Content on wikiHow can be shared under a Creative Commons License: http://creativecommons.org/licenses/by-nc-sa/3.0/