Coulomb's Law

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Students will explore the applications of Coulomb's Law by taking notes and solving practice problems.

Big Idea

Today, students calculate electrical forces using Coulomb's Law.


Now that students are familiar with charge interactions, today's goal is to take students' conceptual understanding of charge interactions and expand it to include Coulomb's Law (HS-PS1-3). Students participate in a field hockey demo first, which activates their prior knowledge about charge interactions. Then, students engage in a reading exploration activity (SP8) the defines the components of Coulomb's Law and how it is applicable to interacting charges. The lesson closes with students using Coulomb's Law to calculate forces, charge distance, or charge magnitude with some collaborative problem solving (SP5).

Electric Field Hockey Demo

15 minutes

At the start of class, I have the Electric Field Hockey simulation loaded, running on my computer, and projected on the front board. I purposely have this simulation ready to go when students enter the room so it piques their interests. This way, when the bell rings I inevitably have some curious students who are ready to volunteer and participate in today's demonstration. The Electric Field Hockey has students arrange positive and negative charges to attract or repel a charged "puck." The hope is that students can get the puck into the goal by using their knowledge of how charges interact with each other. The demonstration increases in difficulty when different barriers are introduced between the starting point of the puck and the goal.

I ask a student to come to the front of the room and attempt to score a goal. When that student is successful, I then ask another student to come forward and we increase the difficulty level until about ten minutes of the class have elapsed. The goal of this introduction is to refresh students' thinking about oppositely charged objects attracting each other and like charged objects repelling. It's also a fun activity and students really enjoy cheering each other on as they attempt to score goals. We have a lot of laughs during this demo and students help each other by offering suggestions of where to put charges.

My students want to play this game for hours, so after ten minutes are over we sit down as a class and debrief on the simulation. I use a discussion guide to remind me of certain questions I want to ask. My goal is to have students apply what they already know about charges and begin to think about the impacts of a force between two charges. For example, if students contribute the word vector during our discussion, that shows me students remember the vector nature of a force.

Now that students have charge interactions (and forces in general) fresh in their minds, we move into the next section of the lesson. This second part of the lesson, gives students the opportunity to define the force between charges with Coulomb's Law.

Coulomb's Law Exploration Activity

25 minutes

I now allow students to choose a partner and take a computer from the cart at the front of the room. We use MacBook Pro's in my district and each department has a cart that can be shared. At this point in the year my students are familiar with the organization of the computers and cart. I decide to let students choose their partner since I want them to be comfortable and focus on internalizing today's material.

As the computers are booting and after students have pushed their desks together with their partner, I give each student a copy of the Coulomb's Law exploration. This document is a guideline that helps students identify the absolute minimum material they should record into their physics notebooks. Once they've opened the document, the students follow the link to the Coulomb's Law section of the Physics Classroom. At this website and still in their pairs, students read through the text and use the document to guide them. I let students know that they can read aloud or individually, but the point of them being in pairs is to discuss and determine the most important parts of the text.  

While students are working, I walk around with the answer key to ensure they are actively engaged in the learning process. To me, this means that they are on the proper website, reading or discussing some component of Coulomb's Law, and writing down a thorough set of notes. When I walk around, I spot check their written work and engage students in questions such as "So how does this remind you of Newton's Law of Gravitation?" or "Is Coulomb's Law directly proportional to distance or charge?"

Students won't all finish this activity at exactly the same time. However, the first and last students to finish are usually no more than ten minutes apart. Since the next portion of the lesson has some leeway, it's not crucial that everyone finishing at the same time. Instead, as students finish their reading they go right into applying Coulomb's Law in a collaborative problem solving activity.

Collaborative Problem Solving with Coulomb's Law

10 minutes

As closure and an informal assessment, students have the rest of the class to start tonight's homework. The problems are meant to ensure students can use and manipulate Coulomb's Law to solve for force, charge distance or charge magnitude. The first four problems are straight-forward applications of Coulomb's Law and have only two charges present in each situation. In the fifth problem, students must recall the vector nature of forces to properly solve for the force on a charge when it is the third charge within a system.

Students remain with their partners from the paired reading activity as I pass out a copy of the homework to each student. This is an assignment that needs to be completed by each individual, although they may use their partner from the previous activity as a resource while working in class. I encourage collaboration, as students apply their new knowledge for the first time. 

Also, I walk around (with a copy of the answer key) to offer help or problem solve with the students as they are working. You might hear me say "Well, did you properly convert all of your units?" in response to a student asking about an answer that is off a few decimal places from his or her partner. My style is to lead the students to the answer, not just provide it for them. That being said, if a student is continually struggling and in obvious need of being shown the answer, I accommodate him or her. Most students get through the first three problems before class is dismissed.

I call the homework informal because I don't want students to get nervous that it counts as a quiz or test grade. Since my goal is to assess their level of understanding, I collect and grade the assignment for accuracy at the start of the next class meeting. Not only do I want to give students personalized feedback on this homework assignment, I also want to check the pacing of the course and make sure my students are ready to move on to the next lesson.

This is our closure activity for today and it's meant to have students apply their newly learned knowledge from the paired reading activity. I am also trying to take a step towards a flipped classroom. I like students to have me as a resource when they work through problems, and I think it helps them build confidence. In the past I've attempted to do entire class periods of a full flipped classroom, but it's hard to hold the students accountable for digesting the needed material. I find that a combination of work time (that lasts right up until the bell rings) and in-class learning best fits the needs of my students.