Robust triboelectric charging of identical balloons of different radii

Electrification by rubbing different materials is a well known phenomenon with a history that begun more than five centuries BC. However, simple experiments can lead to contradictory or inconsistent results and the history of this phenomena is plagued with non-intuitive results. For example, triboelectric charging by rubbing identical materials is possible. In this work we want to highlight some historical aspects of triboelectricity that could enrich the discussion of electrostatics in an undergraduate physics course. We will focus on the effect of strain on the triboelectric properties of a sample, which we think is not well known to physics teachers. We will show that it is possible to obtain robust polarities by rubbing identical rubber balloons of different radii and we will also show that this charging method can be very useful in introductory physics courses.