An electrostatics puzzler

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.

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

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)

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.

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.

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