Opposites Attract

Author: Mark Roland (Partially adapted from a lesson plan by John Miller, applets for second day created by Wolfgang Christian)
Edited by Rachel Hughes and Stephanie Nardei


2 Class Periods

Preparation Time:

20 Minutes

Materials: Plastic Tube (like PVC pipe or golf club tubes)
Tiny scraps of paper
Scotch Magic tape
A piece of cloth like wool or fur
Styrofoam square 1' x 1'
A pie-tin with a Styrofoam cup glued to the center (hot glued works best)
Internet access

This lesson introduces students to the forces involved with like and unlike electrical charges. The first day is an inquiry-based activity allowing students to see these forces through hands-on experimentation. The second day students further explore these forces using quantitative problem solving with Coulomb’s Law.

This lesson engages students as they experiment with static electricity, and explore electrical charge forces..

Students will be able to:
1. Charge an object with static electricity using the following methods:

  • induction,
  • conduction, and
  • friction

2. Understand like charges repel, and unlike charges attract.

3. Use Coulomb’s Law to quantitatively determine the force between two charges..

National Science Education Standard:
CONTENT STANDARD A: Science as Inquiry
- FORMULATE AND REVISE SCIENTIFIC EXPLANATIONS AND MODELS USING LOGIC AND EVIDENCE. Student inquiries should culminate in formulating an explanation or model. Models should be physical, conceptual, and mathematical. In the process of answering the questions, the students should engage in discussions and arguments that result in the revision of their explanations. These discussions should be based on scientific knowledge, the use of logic, and evidence from their investigation.

CONTENT STANDARD C: Physical Science
- The electric force is a universal force that exists between any two charged objects. Opposite charges attract while like charges repel. The strength of the force is proportional to the charges, and, as with gravitation, inversely proportional to the square of the distance between them.

- Between any two charged particles, electric force is vastly greater than the gravitational force. Most observable forces such as those exerted by a coiled spring or friction may be traced to electric forces acting between atoms and molecules.

Teacher Background

Related and Resource Websites
http://www.iit.edu/~smile/ph95p5.html (Lesson Plan for day 1 from John Miller)
http://webphysics.ph.msstate.edu/javamirror/explrsci/dswmedia/coulomb.htm (Fun web-based simulation of Coulomb’s Law with point charges)
Note--The following websites are all from this index page: (Index page, look under Electro-Statics)

http:// qbx6.ltu.edu /~s_schneider/physlets/main/efield.shtml (Electric field)
http:// qbx6.ltu.edu /~s_schneider/physlets/main/eforcex3.shtml (Points on an axis)
http:// qbx6.ltu.edu /~s_schneider/physlets/main/es_03.shtml (Moving charge)
http:// qbx6.ltu.edu /~s_schneider/physlets/main/electroscope.shtml (Electroscope)
http:// qbx6.ltu.edu /~s_schneider/physlets/main/es_02.shtml (Charges puzzle)
http:// qbx6.ltu.edu /~s_schneider/physlets/main/es_01.shtml (Electric field puzzle)


Day 1
(A possible introductory activity--a demonstration of a Van de Graff generator)
1. Have students bring the end of the plastic tube rod near the bits of paper. (The small bits of paper should not attract to the rod).

2. Have students rub the plastic rod with the cloth and bring it near the scraps of paper and observe what happens. (The bits of paper should attract to the tube where it was rubbed.) Introduce the term "neutral" and "charged". We will say that something is neutral if it does not attract tiny bits of paper. We will call something charged if it attracts tiny bits of paper.

3. Have students bring the end of the plastic tube rod they did not rub near the pieces of plastic. (The small bits of paper should not attract to the rod where it was not rubbed.) Note: This is because the plastic is an insulator, which does not let the rubbed on charge freely flow.

4. Have students try rubbing other objects around the room (a comb or pen) to see which ones can hold a charge. (Answers will vary)

5. Have students take about 15 cm (6 inches) of scotch tape and fold the first few cm of each end sticky side together. Have them place the sticky end down on a tabletop and peel the tape up quickly. Bring the tape near the small bits of paper. Is the tape charged? (It should be.)

6. Have students make another tape just like before and bring it near the other tape where both tapes freely swing. What happens? (They should repel)

7. Have students make two new pieces of tape like before, but this time, have them place the first one sticky side down. Have them place the second tape on top of the first and press down. Peel the two tapes up together and separate them. Bring the two tapes near each other hanging freely. What happens? (The tapes should attract)

8. Have students hang the two tapes freely from the side of the desk. Re-rub the plastic tube and bring it near each tape. What happens? (The tube should repel the tape that was on the bottom and attract the tape that was on top.)

NOTE: This could be an opportunity where student inquiry is encouraged. Give students sometime to make other observations on their own.Now is a good time to re-group and provide the students with some content. There are two types of charges, some attract and some repel. Benjamin Franklin set the standard that when glass is rubbed with silk the glass becomes "positive" and when amber (a semi-precious stone) is rubbed with fur it becomes "negative." Ask students to accept that the rod became "negative." The rule for charges is opposites attract and likes repel so positive and positive repel, negative and negative repel and positive and negative attract. The idea of the atom and positive protons and negative electrons may be also introduced along with the idea of electrically polarized. (That explains why neutral objects attract to charged objects.)

9. Ask the students if the rod is negative and this tape repels, what must the charge of the tape be? (Negative)

10. Ask the students if the rod is negative and this other tape attracts, what must the charge of the tape be? (Positive)

11. Take the Styrofoam and rub it with the cloth. Place the pie-tin on the Styrofoam while holding it by the insulated handle. Touch the metal (you should get a small shock). Lift the pie-tin by the insulated handle. Now touch the metal. You should again get a shock. This method of charging is called "induction." (When the pie-tin was near the negatively charged Styrofoam and you touched the metal with your hand, the negative charges were repelled leaving the pie-tin positive. When you picked up the pie-tin by the handle, it was positively charged. When you touched it, negatives from your hand were attracted and entered the pie-tin making it neutral again.

Day 2 (Computer lab needed)
1. As students enter, have these questions posted for them to respond to on paper:

  1. What did you learn about attraction and repulsion of charges yesterday?
  2. What is one way in which you “charged” an object during yesterday’s class?
  3. What is another word for when these objects are pushed together or pulled apart? In other words, what must be present in order for motion to occur? (FORCE!)
  4. If motion is occurring, then we must be performing ______ on the object. (WORK!)
    There are a series of applets that examine electric force and electric fields on various point charges in different scenarios. The amount of direction you give students is up to you. They could “play” quite a bit, and although they wouldn’t get a great quantitative grasp on the concept, it would still be educational and fun.

You can direct the students to the following applets:

These are some great applets. Here is a suggestion for more structured activities:

2. Now that the ideas of force and work have come up in the discussion, we can begin to see exactly how strong the forces are under different circumstances. The first applet that should be used is labeled Electric Field. This shows the electric field generated by moving charges around. While you are moving the charges, their coordinate point and current force are shown in the lower-left hand corner. Have your students move the two charges closer together in measurable amounts (keep it on either axis so that the Pythagorean Theorem won’t be necessary to calculate distance) and create a table of distance and force. They should be able to see the force varies inversely with the square of the distance. Let each group/set of partners discover this.

3. Have students go to the link labeled Points on an Axis. This demonstration is for multiple charges, so before beginning, the values for Q1 and Q3 should be set to zero. Move the green charge exactly one unit away from Q2, and just like the last applet, while dragging the point, the coordinates and force are listed in the lower-left hand corner. With the green charge exactly one unit away, the force should be one unit. Now have students experiment with different values for Q2, keeping Q1 and Q3 zero. See what effect this has on the force. Students should see this is a direct relationship. If you double the charge, the force doubles.

4. Use the results from steps 2 and 3 to help student arrive at Coulomb’s Law which is:
k is a constant, q1 and q2 are charges, and r is the distance
Your student’s math experience with formulas will show how much leading to give, but they should know what effect inverse and direct relationships will have on the formula.

5. Explain briefly when there is more than two charges present, the forces generated by multiple charges add together as vectors (assuming some previous knowledge here), and just let them play! All of the applets are cool simulations, so you can change variables.

6. Use a summary discussion at your discretion. Monitor students’ experimentation; ensuring that they’re on task, and see what questions or conclusions come up. Present them to the class if you choose.

Quantitative problems on Coulomb’s Law could be provided, if desired.

Embedded Assessment
Day 1--Have the students make a negative charged tape by following procedures in number 5. Rub the Styrofoam with the cloth. Bring the Styrofoam near the tape that is free to hang. The tape should repel. Ask the students, "What can we say about the two charges?" (They are the same. Some students may say they are both negative, this is a great response if they can back the statement with facts, not opinions.)

Day 2--Two of the applets are puzzles for learning about unknown charges based on the force generated on a known charge. Challenge students to solve these puzzles.














PULSE is a project of the Community Outreach and Education Program of the Southwest Environmental Health Sciences Center and is funded by:

NIH/NCRR award #16260-01A1
The Community Outreach and Education Program is part of the Southwest Environmental Health Sciences Center: an NIEHS Award


Supported by NIEHS grant # ES06694

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Last update: November 10, 2009
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