An electrostatics puzzler

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: September 13, 2010

Summary

A simple demonstration will tests students' understanding of electrostatics. This puzzler stumps even experienced physics educators and is sure to entertain a physics class. A plastic rod is charged using a piece of animal fur. The rod is touched to an aluminum pop can, creating a faint popping sound indicating that charge is transferred to the can. Next, the rod is held next to the can. Will the can be attracted to the rod, repelled by the rod, or neither of these? Before they see the outcome, students must make a prediction and work to convince other students that their prediction is correct. Only after all arguments are exhausted is the answer revealed. A video provided here describes the demonstration.

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Learning Goals

Students will learn to apply basic electrostatics concepts such as charge transfer, charge by induction, electrostatic attraction and repulsion. In addition, students practice using physics reasoning to make a prediction and also to either defend or abandon their prediction when presented with their peers' reasoning.

Context for Use

This activity is an example of an interactive lecture activity. It is appropriate for any introductory physics class that includes electricity and magnetism.

Description and Teaching Materials

A video is included that shows the demonstration. Instructors can either use the video, or (better) conduct the demonstration in class. Most people think that since charge is transferred from the rod to the can, there will be a repulsive force between the can and the rod because they have the same charge. However, the positive charge in the rod attracts electrons within the can so that the side of the can nearest the rod is actually negatively charged, even though the can has a net positive charge. The charge induced in the side of the can near the rod is greater than the charge transferred to the can when the can and rod touch.

It is interesting to consider the motion of charge within the can while it is rolling. Electrons in the metal can continuously move as they are attracted to the rod, creating a current in the metal surface of the can.

Instructors can download the QuickTime version for use in class movie of electrostatics puzzler (Quicktime Video 9MB Aug20 10)

Teaching Notes and Tips

It pays to spend time carefully explaining the puzzler. Encourage students to make predictions of the outcome and defend their predictions verbally using sound physics reasoning. Give positive feedback for any sound reasoning, even if it leads to the wrong answer.

To ensure that students actually make a prediction, consider the following technique. Designate one side of the room as the "the can will be attracted" side, the other side of the room as the "the can will be repeled" side, and the back of the room as the "the can will not move" side. Require that students move to one of these places, discuss their reasoning with students around them and be prepared to defend their reason for being in that location. Students are free to change their location (and their prediction if they are convinced by another student's reasoning.

If students respond well to this activity, it is easy to spend 15-30 minutes. Students can engage in lively debate, trying to persuade peers to adopt their position. If possible the instructor should resist student requests (demands!) to be shown the demonstration until students have engaged thoughtfully in making a prediction.

Assessment

If an instructor wanted to assess what students gained from this demonstration, students could be asked to predict the outcome of a slightly different experiment. For example, what if the can were an insulator? In this case, the situation is the same as the classic balloon-sticks-to-the-ceiling. The induced charge at the surface of the object will still be opposite of the charged rod, causing an attractive force.

References and Resources

This balloons animated simulation from PhET demonstrates the same concepts as this demonstration.