Coulomb's Law

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Students use Coulomb's Law and compare the electric force to the gravitational force between two protons.

Big Idea

The electric force is many trillions of times stronger than the gravitational force.


In the previous few lessons (Atomic Charge and Charge Stations), students learn how charge behaves and transfers between objects. However, their understanding is entirely qualitative. This lesson, students learn about Coulomb's Law and perform calculations using it. CCSS Math Practice 2: Reason abstractly and quantitatively and Math Practice 4: Model with mathematics are applied as is NGSS Science Practice 5: Using mathematics and computational thinking. Students also compare the relative strength of Coulomb's Law and Newton's Universal Law of Gravity which applies Science Practice 4: Analyzing and interpreting data and performance standard HS-PS2-4: Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to describe and predict the gravitational and electrostatic forces between objects.

Socrative Quiz on Coulomb's Law

10 minutes

We start the lesson with students demonstrating their understanding of Coulomb's Law. This is a homework assignment given at the end of the last lesson. Students read College Physics text on Coulomb's Law and take notes on the material. They should understand the application of Coulomb's Law and should work through some of the sample problems. 

As students come in to class, displayed on the board is the instruction to take the Socrative quiz on Coulomb's Law. Most students use their smart phones, but there are laptops available for the few who don't have a smart phone. This is a five question quiz whose purpose is to give a snap-shot of what students know about Coulomb's Law and the application of the law to word problems. This quiz is set so that reveals student's names and their responses as they give them (only I can see the results). Looking at Quiz Data - Coulombs Law, I see that half the class is not comfortable doing calculations with Coulomb's Law. 

To help these students, I do Small Group Instruction. This is a three step strategy to help struggling learners. First, I collect data. In this case I give the Coulomb's Law Quiz formative assessment. Then I analyze that data to identify students who need additional help. While the rest of the class does an activity, I pull those students aside in groups of 3 or 4 and give them direct instruction on their area of need.


Coulomb's Law Review and Lecture

30 minutes

Once the quiz is done and I have selected the students that are to get small group instruction, I instruct the class to take out their notebooks. I go through the Coulomb's Law Power Point and tell students to write down any new information they see that is not already in their notebooks. The first few slides are review of material they read about for homework. When I get to the slide on the mathematical relationship between the variables of Coulomb's Law and the force, I instruct all students to copy and fill out the table in their notebook.  

The purpose of this slide is to have students develop their own understanding of what happens to the force if the charge is doubled or the distance is halved. After students have filled in the table I show the next slide with the answers. Then there is some information about the units of the Coulomb Constant and the experiment that Charles Coulomb performed to determine the Coulomb Constant.

The next series of slides has students doing calculations in their notebooks that show the relative strength of the electric force between two protons as compared to the gravitational force between the same to protons. This is good practice as well as an amazing result as students learn that the electric force is a thousand   x   million   x   million   x   million   x   million  times stronger than the gravitational force.

Venn Diagram on Coulomb's Law vs. Gravity

10 minutes

The thirteenth slide of the Coulomb's Law Power Point has students work in groups of four to create a Venn diagram that compares the differences and similarities between Coulomb's Law and the Universal Law of Gravity. Each group has 5 minutes to make their own diagrams and I expect every student to put their group's diagram in his or her own notebook. Then, each group gives me a unique contribution to a Venn diagram which I create and display with my document camera. It is a competition to see if each group can give me 2 unique contributions to the class diagram.

While groups work on the Venn diagram, I call up the students that struggle with the Coulomb's Law calculation. To each of these students, I hand a small sheet of paper from the Small Group Instruction sheet. This instructs them to watch a short video on YouTube that brings students through some sample Coulomb's Law practice problems. This video shows how I work through a practice problem and also gives pointers on common student mistakes that are made on these calculations. I only meet with each group for 10-15 seconds and this process is done after a few minutes. 

After the 5 minutes are up, I call on groups one at a time to give me one item to put into the Venn Diagram. Once all the groups have given me one item, I go back through the groups in reverse order asking for one more item. If a group is able to give me two unique items to put on the Venn Diagram, I award them points toward a class participation grade. While we do this, students are to add contributions they do not already have in their own notebook. 

For homework, students are to research the Inverse Square Law and provided a description of it. The information is on the last slide of the Coulomb's Law Power Point.