Electricity and MagnetismTeacher Lesson Plan

Intro: Emphasize the fact that they (the students) need to be really focused these next 5 blocks since we are doing a quick, sweeping over-view of E&M. They cannot BS on their HW or do anything last minute because everyday they will have an assessment at the beginning of class that will be a summary of what they did the previous block and the reinforcement we asked them to do for HW. We will not have time to go over all the HW questions, just a few at the beginning of class. Be prepared. Come in and see us during tutorial if you feel things are going too fast or you get confused and need extra help. The bottom line is that we are upping the expectations over the next 5 blocks because we need to finish before spring break. After spring break we will do a week of review for our final and the STAR test. We take the physics STAR test, one week after we return from spring break, on Tuesday, May 1st. After that we take our final. We end the year with some fun lessons on light and optics and the physics projects.

Todayʼs Lesson: ElectrostaticsHave them try to figure out the meaning of the word.Remind the kids to stay focused so they donʼt miss the fun stuff at the end of class!

Sticky Tape Lab (~ 30 minutes)Have lab set up around the room to speed things up. Materials Needed:•Ring Stand with cross bar for suspending tape, paper, and aluminum•Tape•Aluminum Foil•Paper•Scissors•Sharpie•Sticky Tape Lab Set-uphandout

Pre-Lab Introduction•Pass out Sticky Tape Lab•Show Video of Jason setting up the tape (~ 2 minutes)•Remind kids that at each lab station is a “Sticky Tape Lab Set-up” handout that has visual instructions about how to set up the tape (similar to Jasonʼs video)•Have an example of how to hang the tape, paper, and Al foil. Remind kids to keep them far apart from one another.

Lab DebriefGo over the lab to make sure everyone has the correct observations. Key Ideas to gain from the lab:Like objects repelOpposite object attractNeutral objects attract both

Lecture/DemoReview Hewitt Lecture on Electrostatics for ideas (I transcribed it before the tape got eaten by the VCR. Whew!). Use freely as needed. Here is what I did.•Discuss idea of charge and where it ultimately comes from- atoms- discuss electrons(-) and protons(+), and how the electrical force is what

causes all chemical bonding and it is the force that holds us together. Cool idea!Neutral object can be attracted to charged objects•Briefly compare the mysterious invisible electric force to the mysterious invisible gravitational force. Mentioned that the electric force is caused by charge (not mass) and charge comes in two types (positive and negative) and, unlike the gravitational force, it is not only attractive, it is also repulsive.• If you didnʼt call it charge at the end of the Sticky Tape Lab you can now re-state the obvious ruleLike charges repel Opposite charges attract

• Ask students if they think objects hold onto electrons equally. They should say “no” since they just did the Sticky Tape Lab! Demo: Rub Black Rubber Tube with egg crate material.

As you rub the black rubber tube with the egg crate material electrons are pulled off the egg crate material, onto the black rubber tube. Since it has extra electrons it is no longer neutral, it is negatively charge. To demonstrate that the rubber rod is charged use a small aluminum covered pith ball. At first it will be attracted to the rod and then, quickly, it will be repelled. Have kids try to figure out what is going on.

To mix it up a little do the same demo with a piece of packing material. It acts differently. It is attracted to the black rubber rod but is never repelled. Have kids try to figure out what is going on.

Do the same pith ball and packing piece demo with the clear plastic rod. Before you do this, ask the kids that they expect to happen. The results are identical to what happened with the black rubber rod! Note: the effect with the plastic rod is not as strong as with the black rubber rod and so you might want to use a new piece of egg crate that has not been rubbed yet to get better results. Experiment around with this before class to see what works best for you.

What is going on?Attraction, in both cases (pith ball and packing pieces), is due to charge polarization ( for a great explanation of this see the Conceptual Physics text section 32.7 pg 490). The black rubber rod has extra electrons (is negatively charged). When it is brought near the pith ball the negative charges in the pith ball are pushed away since “like charges repel.” The positives really canʼt move since they are in the nucleus of atoms, but, as a result of the movement of electrons away from the surface of the pith ball facing the black rubber rod there is a slightly more positive charge on the surface of the pith ball. Since “opposite charges attract,” the pith ball is attracted to the black rubber rod. But, as soon as the pith ball touches the black rubber rod, the electrons flow off it, onto the aluminum covered pith ball. Now the pith ball is negatively charged since it has extra electrons on it. Since the black rubber rod is also still negative (because not all the electrons have flown off it) it repels the negatively charged pith ball. This happens very fast. At this point you might want to introduce the idea of conductors and insulators.

Conductor: A material that allows the free flow of electrons (or charge)Examples: metals

Insulators: A material that resists the free flow of electrons (or charge)Examples: rubber, plastic, packing pieces, etc.

To explain why the packing piece, an insulator, attracts, but never repels, you use the same charge polarization idea. At first, things are the sames as with the pith ball. The black rubber rod has extra electrons (is negatively charged). When it is brought near the packing piece the negative charges in the packing piece are pushed away since “like charges repel.” The positives really canʼt move since they are in the nucleus of atoms, but, as a result of the movement of electrons away from the surface of the packing piece facing the black rubber rod there is a slightly more positive charge on the surface of the packing piece. Since “opposite charges attract,” the packing piece is attracted to the black rubber rod. However, it is never repelled. Why? Because the the extra electrons on the black rubber rod cannot flow onto the packing piece because it is an insulator!

You can redo the charge polarization demo using the clear plastic rod and get identical results, so, the kids can see that it is not unique to negative charge.

A great way to visualize charge polarization is to show the PhET “Balloons and Static Electricity” simulation. Play around with this simulation, it is very simple and elegant, and gets the idea of charge polarization across very easily and quickly for the visual learners in our classes. http://phet.colorado.edu/en/simulation/balloons

Other great examples of charge polarization:•This is what happened in the Sticky Tape lab with the paper and the aluminum foil- it is why, even though they were both neutral, they were both attracted to the negatively charged tape and the positively charged tape. •It explains why you can pick up little pieces of paper using the black rubber rod.

•It is why you can bend water with the black rubber rod- or a comb you have just used on your hair (because water is polar).

Demo: Van De Graaff GeneratorThe simple thing to say about this machine is that it is doing something similar to what we were doing when we rubbed the black rubber rod with the egg crate. The belt inside is moving charge up to the sphere and, because the sphere is a conductor, the electrons are spreading out evenly distributing themselves all over the surface (because like charges repel). Since there are billions and billions and billions of electrons on the surface of the sphere (i.e. it is highly charged) we can have some fun with it and see how different things respond to this high negative charge.

Pie Tins (conductor): Like charges repel

Packing Pieces (insulator): Like charges repel

Bubbles: Attracted at first due to charge polarization, bubbles pop, negative charges are sprayed onto other bubbles and they are pushed away because “like charges repel.”

Crazy Hair: Like charges repel•Student needs to be standing on an insulated box.

Shock Value: Have at least 5-10 students hold hands and have the crazy hair person grab the hand of the person at the end. See how far the travels.

Cathode Ray TubeHold the tube parallel to the VDG Generator surface and perpendicular to the surface. The tube lights when you hold it perpendicular because of the greater potential. This is a lead in to tomorrowʼs lesson. . .

Main Ideas From Day One•All matter is made from charges since the atom is made of positive protons and negative electrons.•There is a force between these charges called the electric force.•The electric force is similar to the gravitational force. It is invisible and is carried mysteriously through space between charges. Unlike gravity it is not only attractive it is also repulsive. •The simple rule is:Like charges repelUnlike charges attract•Charge is conserved (just like mass, energy, and momentum). I didnʼt mention this in my lecture but I should have.•Most materials can be classified as either conductors or insulators. Conductors allow the flow of chargeInsulators donʼt allow the flow of chargeThe concept of charge polarization is important to explain many things that were observed in lab and lecture today.

What Was Not CoveredCoulombs Law: They will read about this tonight and we follow up with this next time.Charging by Induction: Too tricky and complicated to include today and get through all the material (not covered on the STAR test)