All Charged Up

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Objective

Students make charge diagrams that show how charge flows through a system.

Big Idea

Charge stays in place on an insulator and charge is free to move around on a conductor.

Context and Supplies

The ability to picture the invisible in your mind is an important physics skill. Today, students create charge diagrams that show they understand how charge moves through a system. They have to picture in their mind and then draw pictures that show how charge builds up based on what they see happen in a series of demonstrations. This is based on a few basic ideas about how like charges repel, which was covered in the last lesson.

CCSS Math Practice 8: Look for and express regularity in repeated reasoning is applied here as well as NGSS Science Practice 1: Asking questions (for science) and defining problems (for engineering),  Science Practice 2: Developing and using models. Students also use Science Practice 8: Obtaining, evaluating, and communicating information as they create their charge diagrams. This lesson builds a foundation for further study of 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.

Make a Charge Chain

10 minutes

In the previous class, one or two students experienced the Van de Graaff generator (VDG). Now the whole class gets to experience it as they make a human chain around the room. I tell them they get shocked and that this activity is optional. Also, if they have a pace makers or any kind of heart ailment, they should not participate.

I instruct students to make a big circle around the room and hold hands. The student closest to the VDG places there hands on it. I say to everyone, "don't let go!" and flip on the generator. As the generator runs, the student chain is slowly charged up. The student at the other end discharges the whole chain by touching the sink faucet. We do a count down, "3-2-1" and they touch it and the whole class screams and they breaks out in laughter, as is seen in the Student Charge Chain video.

This activity is always fun and exciting and leaves students with big smiles. I do this activity because it leads to a good discussion on what happened as we move on to a brief lecture on charge diagrams.

Lecture on Charge Diagrams

10 minutes

The VDG student chain activity has students experience what it feels like to have over 100,000 volts suddenly leave their bodies. I remind them that the high voltage is not dangerous because the amount of charge was small. Now they learn how to draw a picture that represents this situation. I display the Charge Diagrams Power Point and students take notes in their notebooks. 

Charge diagrams are a useful model to represent net charges on an object or person and how net charge transfers between objects. A “+” is used to display positive charge and a "-" to display negative charge. If an object has no net charge, one can either draw nothing or have an equal number of positive and negative signs. Because like charges repel, and because a conductor allows charge to freely flow through it, when charge is place on a conductor it spreads out. That is what happens with the human chain. A net charge slowly enters their bodies and then it suddenly leaves when the student on the end connects the chain to the ground (via the sink faucet).  On an insulator, a net charge remains where it was placed. Charge diagrams are an effective way to represent both conductors and insulators.

After students write down the  rules of writing charge diagrams, I show them a charge diagram of the the human chain activity they just experienced. Based on what they see, I ask why the person closest to the sink has the biggest shock. I have students do a 30 second "turn and talk" to discuss with their neighbors and create an explanation. Then I call on a random student to answer the question. Students recognize that the person closest to the sink has ALL of the charge of the whole group flow through him or her, whereas the people closest to the VDG only discharge themselves, which is why they barely feel the shock.

Demonstrations and 4 Panel Charge Diagrams

20 minutes

I do the four VDG Demonstrations which are detailed on the Charge Diagrams Power Point. I do the demonstrations as described because students are to  make a series of charge diagrams that represent each demonstration.

  1. Aluminum pie pans: The VDG (van de Graaff generator) is off with the pie pans placed on top of it. Then the VDF is turned on. Charge builds up on the pie pans until the like charges cause the pans fly off the VDG.
  2. Person with charged hair: The VDG is off with a person touching the metal sphere of the VDG. The person should be standing on a rubber mat so that when the the VDF is turned on, they build up charge. The VDG is turned on and charge builds up in the person until the like charges cause their hair to stand up on end.
  3. VDG and one person: The VDG is off with a person touching the metal sphere of the VDG. The person should be standing on a rubber mat so that when the the VDF is turned on, they build up charge. The VDG is turned on and charge builds up on them. They let go of the VDG and give another student a fist bump, which causes a charge to transfer to the other student.
  4. VDG confetti: The VDG is off. I place a plastic bottle that has a few hundred paper dots (collected from a hole-punch) in it. The VDG is turned on. The bottom of the plastic bottle builds up charge and transfers that charge to the paper dots. When the paper dots become charged they begin to fly up the bottle causing a confetti volcano. 

After the demonstration is done, I hand out a blank piece of paper to each student. I show them how to fold the paper in half and then in half again so that they end up with four rectanges of equal size. The students use these four panels to create a series of charge diagrams for one of the four demonstrations done. My students are seated in groups of four, so each student in each group picks one of the demonstrations so that each group has all of the demonstrations done as a 4-panel charge diagram. While students create their diagrams, I walk around the room to monitor and provide support. They have 15 minutes to create their diagrams.

My Favorite "No" with Charge Diagrams

10 minutes

With 10 minutes left in the period, some students have started to finish their charge diagrams. I choose a few to be my "favorite no". This is when I display student work that contains errors that are very common. I congratulate that student on positive points of their work (very important to do this). Then I point out their one mistake. I tell the student that this is a common error and ask their permission to share it with the class. They often agree. If not, I say "no problem" and I move on.

I take the student work and place it under my document camera for the whole class to see. I ask the class, "What do you like about this work?" or "What is good about this work?" After students supply several answers, I ask, "How can we improve it?"

For this piece, the only issue I see is that the charge is not shown in the hair. While this is a minor point, the goal of the exercise is to draw the charge to explain what we see. So this is an important part of the charge diagram.

For this Student Work Sample 1, the student neglectes to include the charge on the last panel. 

On Student Work Sample 2, again charge should be on the forth panel since the VDG is still on.

For Student Work Sample 3, they put the wrong charge on the pie plates. The charge should be the same charge as the VDG which is why the pie pans fly off. The like charge repels.

With each of these samples, we start off with what is good about them, such as, the charge is evenly distributed or clearly drawn. Then we move to improvements and changes.

When I am done showing these samples, students still have a few minutes to make corrections to their own work which is the purpose of "my favorite no".  Right before the students leave, I show Student Work Sample 4 which is an exemplar.