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# How Do Objects Become Positive Or Negative?

Lesson 3 of 6

## Objective: Given a specific situation, students will be able to identify if an object will be positive, negative, or neutral.

The goal of this class is to introduce students to how to determine what charge an object has after experiencing one type of charging. Students will be using the models that we created for how objects become charged from the previous class (SP2) to explain how and why an object has a certain charge. This is an application of the types of charging that students learned and read about in the previous lessons.

To start out class, I ask students to do some quick review so I ask them to stand up and walk 10 steps to find a partner to work with to answer the quick review questions. I have them stay with the same partner for about 3-4 questions before moving to find a new partner. I do this because it is good for students to be moving around and working with a partner as well as to recall the ideas they learned in the previous two lessons that we will build on in this lesson. I ask them the following questions:

- What is electrostatics?

- What is electric charge?

- In what type of situation would particles attract? Repel?

- Compare and contrast a proton and electron.

- Given an example where I rub a plastic rod with fur. Before I rub them together what is the charge of each object? If the fur becomes positive, what is the charge of the plastic rod? What about the amount of charge that transfers?

- Compare and contrast a conductor and insulator.

- Explain what an electrical grounding is.

- Compare and contrast charging by conduction, induction and polarization.

After I ask each question, I give students 15-30 seconds to discuss with their partner. I give more time depending on the question to discuss the answer. Then I ask the students to raise their hand to volunteer an answer to go over it. I try to get as many students participating as possible so I do not call on the same student twice to provide an answer to the whole class. After all of the questions, I ask the students to return to their seats for the next activity.

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#### Charging Examples

*15 min*

After the partner review, I have students open their Guided Notes to the electrostatics charging example problems. These problems help students to understand how charge is transferred and how to determine what the overall charge of an object is. I skip the first one for time's sake at the beginning of these notes and focus on the second and third because they parallel what students practice in the worksheet later in the lesson.

In the second problem, I read the problem aloud and then model drawing pictures for each situation. I start with A and B and ask the class to tell me what happens to the charges in that situation. A student volunteer tells me that the negative charges attract and the positive charges repel; so the overall charge on A is negative and B is positive. I repeat the same process with the C/D situation and the E/F situation getting a new student volunteer each time. Finally, we go through how to rank these based on charge but the most important part of the second example.

In the third example, the goal is to help students see how charging by conduction and charging by polarization works. So I start out with Ball B in the first example and I show them that with charging by conduction, the charge transfers because it is a neutral ball and a charged rod and the rod will want to take the excess electrons. Then if you have a charged object with a neutral object, they attract because the charged object polarizes the neutral object. We then do the second example and see that they are both charged by conduction and are the same charge so they repel.

#### Resources

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#### Charging Worksheet

*15 min*

After the charging notes, I have students complete the Electrostatics Charging Practice with their table groups. I do the first problem with them so they get the idea of what I am expecting them to do. I ask them to draw a picture and show where the charge goes with arrows, as well as providing the answer. After the example, I give them about 8-10 minutes to work on it and I tell groups that they are assigned one problem to put up on the board. The goal of this activity is to give students some practice with determining the charge of an object and how the charge transfers.

When all groups have sent a representative up to put the answer on the master answer sheet (as shown below), I go through the answers that the students put by asking if students agree or disagree by raising their hands. If most of the class agrees, and it is correct, I move on. If it is incorrect or most of the class disagrees, I ask the table that had written the answer to explain or I explain why the answer is correct.

#### Resources

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After the charging worksheet, my students typically want more practice. So I project the Electrostatics Practice up on the board and I go through about 10 questions with them. I have them get up out of their seats and walk to find a partner across the room from them. Then I show the problem on the screen, I read the problem to them and give them about 30 seconds to talk with their partner. When I bring them back together as a class, I ask them to vote on the right answer so I can see what each group came up with. We continue until most groups are getting correct answers and they feel comfortable with these types of problems. If a majority of the class votes for the incorrect answers, I ask students who voted correctly to explain it to the rest of the class or I explain it to the class. I do this to help students feel more comfortable with different charged objects.

#### Resources

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#### Triboelectric Series Notes

*10 min*

After the partner quick practice, I have students turn to the Triboelectric Series notes in the Guided Notes packet. I introduce students to the triboelectric series by projecting it on the screen which tells what the charge of each material will be when two materials are rubbed together. I tell them that this is like a continuum where you only need to focus on the two objects you are concerned about. Whichever object is higher up on the series becomes positive and the lower object becomes negative. I ask students why we would not have a neutral object. They respond that when two objects touch and charge is transferred, one object gains electrons and becomes negative and the other object loses electrons and becomes positive so charge is conserved.

We then complete a few examples as a class. I go through the first example and I show them how to answer. For the other two examples, I ask students to try to complete them on their own first. Then we discuss the answers as a class and I ask for student volunteers to share their answers.

#### Resources

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#### Charging Checkpoint

*5 min*

After the triboelectric series notes, I have students complete the Charging Checkpoint. This asks them three different questions ways to find the charge of an object. There are two versions of question 3 depending on how well the triboelectric series notes went. I want to get an idea of what the students learn during this lesson and to give them a formative grade for it. Students work individually on this checkpoint and I show each slide for about 1 minute before moving to the next slide. As you can see below, this checkpoint gives me an idea of which students I needed to work with.

To end class, I tell students to complete the Triboelectric Series WS for homework. Since we completed the notes, the worksheet should be pretty straight forward. I tell my students to spend no more than 15 minutes on the worksheet at home. I give them this homework so that my students can get some practice using the triboelectric series.

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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