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# Coulomb's Law

Lesson 4 of 6

## Objective: Students will be able to identify relationships between force & distance and force & charge and solve problems using Coulomb's Law.

*70 minutes*

The goal of this lesson is to introduce students to Coulomb's Law. Students develop a model to represent Coulomb's Law (SP2) and use computational thinking to use Coulomb's Law to solve problems (SP5). Since we have previously discussed the ideas of charging and how charges interact qualitatively, we now look at these interactions quantitatively. This helps students to understand the relationships of Coulomb's law which ultimately helps them to understand standard HS-PS2-3.

Before we introduce Coulomb's Law, I start out class by going over the Triboelectric Series WS homework and the Charging Checkpoint from the previous lesson. We first look at the homework, so I choose one person at random from each table by rolling a die to determine the problem that they get to write on the answer key. Then I ask for a volunteer for the remaining problem. Each students writes his or her answer on the master answer sheet I have at the front of the room. While students are writing their answers on the front sheet, I ask the other students to compare their answers with the students at their table to see if they agree.

After all of the answers are on the sheet, I show them on the projector screen at the front. I show one problem at a time and I ask students to give me a thumbs up if they got it right and thumbs down if they got it wrong. This helps me see how students are doing without checking each student's sheet. As you can see below, #6-8 are the most difficult for my students; I go over those problems with the students and explain why each of them attract (due to polarization of a neutral object by a charged object).

After reviewing the homework, I go over the checkpoint making sure that students understand how objects become charged. I think that it is important to review so that students can see their mistakes and can correct them.

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#### Coulomb's Law Guided Notes

*25 min*

After going over the homework and the checkpoint, students take out their Guided Notes and turn to the Coulomb's Law notes (Page 7). The video below goes through the notes that I provide to my students.

As stated in the video, I provide to pictorial examples that students need to compare and figure out which one they think will have more force; the positive and negative charge closer together or the positive and negative charge further apart. They turn to a partner and discuss and then I have them raise their hand for the first situation or the second situation. We discuss that the relationship between force and distance are inversely proportional. Then, I do the same thing with a picture of the positive and negative charge with one charge each and the positive and negative charge with three charges each at the same distance apart. They turn to their partner and discuss and then I have them raise their hand for the first situation or the second situation. We discuss that the relationship between force and charge are directly proportional.

After discussing the relationships, I ask students to help me determine the equation for Coulomb's Law based on the relationships. Once we determine the equations, we go through the first problem together. I emphasize that parentheses are crucial when entering values into a calculator. Then I ask students to work on two problems on their own. After about 5 minutes, I show them the answers for the two problems and I ask them to give me a thumbs up if they got both right, thumbs to the side if they got 1 right and thumbs down if they got both wrong. I see that almost all students have their thumbs up or to the side, so we can move onto the group practice problems.

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#### Coulomb's Law Problems

*25 min*

After we do some example problems, I have students complete some Coulomb's Law practice problems from the physics classroom. I have them go to the link and complete #3,4,5 and 7,8,9. To complete these problems, I have them work with their table groups or just the person sitting next to them. I want them to make sure they are checking with their partner or group as they work and I walk around the room to make sure students are on task and to answer any calculator questions since calculator errors up a lot with these problems. Below you can see some student work the Coulomb's Law problems. If they do not finish the problems, I ask them to complete them for homework and that we will go over them during the next lesson.

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#### Stoplight Exit Slip

*5 min*

After the Coulomb's Law problems, each students takes a post-it note from the resource bin at the center of the table and gets to choose from the red, yellow or green light. They must complete the sentence starter on the light they choose.

The Red Light: Today my learning stopped because...

The Yellow Light: Today I considered a question, new idea or perspective...

The Green Light: Today I learned __________ because...

Additionally, I ask students to rate themselves on a scale of 1-4, 1 being novice and 4 being expert, on the Coulomb's Law learning target. When they complete their sentence, they stick the post-it note on whatever light they choose. The stoplight is at the front of the room whenever they finish writing their response to a light.

I do this to get an idea of where students are at in learning the material in a way that they can anonymously express exactly what they are learning or exactly what they are struggling with. With their rating I can also see how students feel about the learning target. The students work shown below are categorized by how they rated themselves.

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- LESSON 1: Introduction to Electrostatics, Day 1
- LESSON 2: Introduction to Electrostatics, Day 2
- LESSON 3: How Do Objects Become Positive Or Negative?
- LESSON 4: Coulomb's Law
- LESSON 5: Coulomb's Law and Newton's Law of Gravitation
- LESSON 6: Electrostatics & Newton's Law of Gravitation Quiz