Sunday, March 22, 2015

Electrophorus Engineering

One of the scariest parts of the Next Generation Science Standards (NGSS) are the Science & Engineering Practices. An easy way to slowly align your curriculum to NGSS is to modify a current demo, lesson or lab so that it is aligned with one of the Science & Engineering Practices. In a (brief) nutshell the Science & Engineering Practices are a skill set students should have in order to explore science phenomenon and engineer solutions to problems or fullfill a human need. Bozeman Science has a nice set of videos on the eight practices. The first one on Asking Questions & Defining Problems is helpful in understanding what NGSS considers the difference between Science & Engineering.

In the past I've created an electrophorus from a styrofoam cup and aluminum pie pan and used it to light a small neon bulb as part of my electrostatics lecture. Among The Exploratorium's many "snacks" is one called Charge and Carry that explains the traditional demonstration. Usually a styrofoam sheet is rubbed vigorously with a cloth to separate charge through friction. The electrophorus is set on the styrofoam and the charges in the aluminum pan polarize; by then touching the top of the aluminum pan you charge it by induction. If you pick it up by the insulating handle, touch one lead of a neon bulb to the aluminum while holding the other you can light a small neon bulb. You form a complete path of conducting material to the ground allowing charges to flow.

This year I did not show my students the electrophorus but asked students to experiment with different materials to explore the best way to light the neon bulb. Specifically students were working on the sixth practice "Constructing Explanations and Designing Solutions." Students had access to the following: plastic cups, plastic plates, paper cups, paper plates, styrofoam cups, styrofoam plates, aluminum plates, aluminum cups (made from rolled aluminum foil).

Students were given this image to understand the arrangement of their materials and instructions on how to charge and ground the electrophorus. Students were told to try different designs to light the neon bulb; each time they changed materials they were to record their results and try something else.

All groups eventually realized they had to use an aluminum plate to conduct the charge to the neon bulb. Most groups used a  styrofoam cup as the handle although some experimented with multiple stacked paper cups and reported a longer and brighter light from the neon bulb. Some groups tried rubbing the aluminum pan directly, skipping the styrofoam sheet, and reported even brighter lights.

After they optimize their design students were asked to write a conclusion paragraph: 
In an age appropriate paragraph explain (1) how the bulb can be lit this way and (2) justify your design choices and how well it worked. Be sure to discuss each of your designs and how their results influences later designs.  

As you might expect, results varied. Some groups really dove into it, referencing their book, asking me clarifying questions and constructed thorough explanations of what they were seeing. From others I could tell that students did not understand how the charge was initially separated, why a conductor was used for one part and an insulator for the other or how the static charge lit the bulb. By discussing their results the next day most students were able to correct their misconceptions. In the end I think they ended up with a much better understanding about the electrophorus and begin to see how current works than if I had just done it as a demo. 

Did it take longer? Yes. Was it worth it? Yes!

Electrophorus Build It Activity (PDF) (Google Doc)

1 comment:

Paul Doherty said...

This sounds great , thanks for posting it.