Electric Fields

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Objective

Students will be able to identify and explore the qualitative and quantitative properties of electric fields.

Big Idea

After students define key components of electric fields, they virtually explore the properties of electric field with a computer simulation.

Context

Students already have a working knowledge of charges fundamentals and Coulomb's Law, so today's lesson focuses on the properties of electric fields (HS-PS1-3). The goal of the lesson is to give students an overview of how charged particles interact with each other and the force fields they create. The lesson starts with a misconception check before students actively take notes. These notes define electrical forces as field forces, provide students with the equation to calculate the magnitude of electric fields, and show the directions of electric fields. Students apply their newly learned knowledge using a simulation that models electric field lines around point charges (SP2 & SP4).

Misconception Check: Charge Interactions

5 minutes

When students walk into the classroom, I have a a misconception written on the board at the front of the room. I ask students to clear their desks and sit quietly for this activity because I really want them to think about what they know and how it applies to the statement on the board. Intentionally, there is no writing during this activity because I don't want to risk students copying down the misconception and then studying from it in the future.

Today's misconception reads "Charges must be touching in order for them to interact." Its purpose is to get students thinking about how electrical forces are field forces. After I read the statement aloud, I stay at the front of the room and just leave students to think for 2 or 3 minutes. I also remind students that in the world of AP Physics, an answer is not enough - we must justify everything. 

After time is up, I ask students to raise their hands if they agree with the statement written on the board. I take a mental note of these students before asking one of them to explain their reasoning. Then, I ask a student who does not have his hand up to share why he disagrees with the statement. This process continues until a several students from both sides share their justifications.

Before moving into our final activity, I ensure that students understand that this is a big misconception. Charges produce electric fields and do not need to touch in order to impact each other. Also, I remind students that objects did not need to touch in the static lab. As we move into the next part of the class, I make a special effort to check-in with students that agreed with the misconception to see how they are feeling about things.

Electric Fields PowerPoint

25 minutes

It's time for students to take out a sheet of paper and get ready to learn about the characteristics of electric fields. My students are operating under the expectation that they must write down key concepts from the presentation. This expectation of how to take notes has been outlined and ingrained in their learning since freshman year.    

I display the Electric Fields PowerPoint (also available as a PDF) to help the students understand what they need to write down. As I'm showing the slides on the front board, I have a hard copy of the PowerPoint which includes teacher notes (viewable when the file is downloaded). These notes help me to stay focused and ensure I mention the highlights as we progress through each slide.

The presentation starts with an explanation of how charges can interact without physical contact and reminds students that forces are vectors. Then, students are presented with the equation for calculating the strength of an electric field and the direction that field points. After a visual representation of field lines is shared, students work through two examples.

While I describe this section as "direct instruction," I usually have a lot of interaction with my students throughout the presentation and am constantly moving throughout the room to change my proximity. The students ask questions and connect to real-world examples to stay engaged the entire time. In this particular set of notes, a student asks if electric fields cause magnets to repel or attract when brought near each other. This question leads to a discussion about the similarities between magnetic and electric forces.

Electric Field Simulation Activity

20 minutes

After we've finished taking notes, the lesson ends when students use a computer simulation that allows them to be more active in their learning. The simulation allows students to visualize electric fields. Specifically, students start by observing the electric field around a single, positive point charge. Then, they add a second (and eventually a third) point charge and observe the field line strength and direction between the charges. The students repeat this process with negative point charges and continue to make observations about the strength and direction of the different fields.

Before we start the activity, I assign partners using the random student generator that already has my students' names loaded. By displaying the random generator on the front board, there is an element of suspense as the partners are assigned. Partners work best for this activity so that each student can be actively engaged throughout the work time. Because this activity does not need to be completed outside of class, I feel comfortable in choosing the pairings for my students.

We use MacBook Pro's in my district, so my students are familiar with how the computer and cart organization works. After each pair has been assigned, the students move to sit near each other, push their desks together, and grab a computer. As the computers are booting I pass out the charges & fields simulation so that each student receives a copy. This document directs them to PhET where they will be using the charges and fields simulation. The activity sheet is also meant to direct the students in their learning so that they are confident in what material needs to be understood.

While students are working, I walk around to ensure they are actively engaged in the learning process. This means that they are on the proper website, reading or discussing some component of the simulation, and writing down appropriate information from the simulation. When I walk around, I'm spot checking their written work and engaging students in questions such as "Were you able to figure out what the length of the vectors mean?" or "Where are the fields the strongest?" 

The students' written answers are the most important part of this activity, so I ensure they are thinking through how they will prove any claims they make. I do a lot of walking and questioning throughout their work time to ensure they can justify anything that has been written down. The AP Physics 1 exam places a lot of emphasis on justifying thought processes and writing, so my goal in our dialogue is to practice with students how to successfully justify their arguments.

When there is approximately five minutes prior to the end of class, I ask students to put the computers back on the cart and return to their seats. I also tell them at this point that the lab is due at the start of the next class meeting. While most students are able to come close to finishing the activity before class ends, about half of the students take their work home to make adjustments to their writing or complete the activity itself. The students do not have any additional homework tonight and are working alone, so the time needed to complete this assignment should be no more than half an hour.