Electric Energy: Evaluating the Cost

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Given the local generation service charge for electricity, students evaluate the cost of operating various appliances.

Big Idea

When evaluating the cost of an electronic device, the cost of operation, as well as the upfront cost are important to consider.

Context and Supplies

In this lesson, students evaluate the claim of cost savings that is advertised on an LED Light Bulb Package. The claim is that the consumer saves "$165 in energy costs" if they purchase and use this LED rather than an incandescent light bulb. To accomplish this,  students take what they learn from the previous lesson, Electric Energy: Calculating the Cost, and use that knowledge to calculate the cost to operate an incandescent and LED light bulb and evaluate the claim made by the manufacturer.

As students look to evaluate the costs, they use NGSS Science Practice 4: Analyzing and interpreting data, Science Practice 5: Using mathematics and computational thinking, Science Practice 6: Constructing explanations (for science) and designing solutions (for engineering) and Science Practice 7: Engaging in argument from evidence are used as well as HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts. Since voltage is an electric potential due to electric fields, the performance standard HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction, is relevant.

Also applied are CCSS Math Practice 1: Make sense of problems and persevere in solving them, Math Practice 2: Reason abstractly and quantitatively, Math Practice 3: Construct viable arguments and critique the reasoning of others and Math Practice 4: Model with mathematics.

Calculate the Cost Practice

25 minutes

Students work in groups of 4 and use the electronic devices they chose for homework assigned during the lesson Circuit Power. I hand out Home Appliances Group worksheet, 1 for each group. Students draw schematics of their circuits with their chosen appliances and then determine the current drawn when all four are operating. This is a review activity that requires knowledge from several previous lessons including Parallel and Series CircuitsHousehold Circuits and Circuit Power

Then students calculate the cost to run their devices over the course of a month. The back of the worksheet has students applying the same concepts but instead of calculating the total current, they are given a maximum current (20 A) that their circuit can handle and they figure out what appliances can run on that circuit.

This is a cooperative learning activity. As students work cooperatively, they have a chance to talk through this complicated, open-ended assignment and to help each other. While students are engaged in the activity, I walk around the room and monitor their progress. I also employ the Colored Cups strategy. This serves as an efficient way to inform me of when groups are in need of support.

Watts In a Lightbulb Activity

25 minutes

After the groups finish the Home Appliance worksheet, I give each student the Watts In a Lightbulb worksheet. I collect their Home Appliance solutions which I assess for understanding after class. Every student gets their own sheet, however they are free to work with others to complete this activity. The premise of this activity is that students are to evaluate a manufacturer's claim that users save $165 if they use their LED light bulb instead of an incandescent bulb.

This activity is a great example of how operating costs are a significant part of the cost of an appliance. Incandescent bulbs are sold in packages and cost roughly 25 cents per bulb. Compare that to the cost of an LED bulb, which just a few years ago cost upwards of $20 each, the incandescent bulb is much cheaper to buy. However, as students work through Watts In a Light Bulb - Solutions Front, they learn that they do receive significant savings with the LED. 

With 5 minutes left in the period, I use the document camera to show My Solution to the front of the sheet.  I take this opportunity to have at least one other group show their solution if it is different from mine. To close the class, we review the Watts In a Light Bulb - Solutions Back.