# Electric Field Lines

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## Objective

Students use electric field lines to determine the electric force on charged particles .

#### Big Idea

Electric field lines provide a visual representation of the direction and magnitude of forces that would act on a test charge.

## Context

Electric field lines are the bridge between electrostatics and circuits. Students learned about Coulomb's Law in the previous lesson and now apply that understanding to the concept of potential difference as seen by electric field lines. During the course of this lesson, student read about electric field lines and look for the rules that govern their formation. They practice drawing the field lines and determine the relative direction magnitude of electric force that acts on a test charge placed in an electric field.

CCSS Math Practice 7: Look for and make use of structure is applied in this lesson. Students also develop and use models (electric field lines) which is NGSS Science Practice 2Science Practice 6: Constructing explanations and Science Practice 8: Obtaining, evaluating, and communicating information are also used and this is in the context of performance standards 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 and HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.

## Do Now - Coulomb's Law Practice

5 minutes

The lesson opens with a quick review of Coulomb's Law which is the first slide of Electric Field Lines Power Point. Students write down the problem in their notebooks and complete it. They can use their notes as a resource, as they need the value for an elementary charge and the value for Coulomb's constant to perform the calculation. I give the class a few minutes to work through the problem. Then I call on a random student to provide the answer, which is 4.35x10^(-21) newtons and the force causes the electrons to repel. If a student is not able to get that answer, I hand them a sheet of paper with the link to a YouTube video that does sample Coulomb's Law practice problems I tell them to practice the calculations until they get it correct.

The reason for this review is to prepare students for the concept of electric field lines. Coulomb's Law informs one of the strength of an electric force, but the direction is difficult to determine except for the most simple cases. Electric field lines give us a powerful visual to helps with this.

## Read for Information - Electric Field Lines

30 minutes

I give to all students the Electric Field Lines Worksheet. With this worksheet, students define contact force and field force. They also read through the text to determine the rules for electric field lines. The sheet also contains practice problems for field line drawings; students apply the rules of field lines to draw the field lines around different collections of charge. Students work alone and I walk around the room to make sure students are on task and to give support where needed.

After 20 minutes, I use my document camera to show exemplar student work. First I review the Electric Field Line Answer Key - Front and answer student questions as they come up. Electric Field Line Answer Key - Back is next and I indicate to students that if they have a different answer that either they should ask about their interpretation or fix what they have to match the sample shown.

## Test Charge and Forces

15 minutes

Now that students have been introduced to the concept of field lines and practiced drawing them, I display slides 2-5 on the Electric Field Lines Power Point. This power point contains animation that do not work in preview mode. The purpose of these notes is to give students a formal definition of electric field lines and test charge as a resource in their notebooks. The next several slides show field lines for various charge configurations. I want to expose students to several different field line formations, so there is no need to write these samples in their notebooks as this would take too much time.

I then hand out the Test Charge Worksheet to all students. Again, this is a solo activity where students determine the relative magnitude and direction of electric forces on the charges shown in the picture. This leads to the concept of electric potential and circuits which begin the next lesson.

## Closure

5 minutes

The class finishes with a review of the Test Charge sheet. Again, I collect some student samples to display with my document camera. I ask students to explain their answers to the rest of the class.

The final departing concept has to do with electric potential. We can arrange charge objects to create an electric potential that causes charged objects, such as electrons, to accelerate. This is seen on the Electric Potential sample. Two charge plates change the path of an electron, as seen on the final slides of Electric Field Lines Power Point. The applications are many, such as a cathode ray tube in older TVs and particle detectors. Electric potential is a key idea for the next lesson, Turning On Simple Circuits

For homework, students are to find and take a picture of the breaker panel that is in their house. I ask them to do a 3-2-1: to record three observations, two things they know about it and one question they have.