A practical lab on composite materials and sensors, enhanced with electrical percolation threshold theory

Abstract

Composites based on candle wax and pencil graphite were fabricated with different concentrations of graphite in the range 0% w/w to 70% w/w. These composites were fabricated (‘ironed’) into a sandwich structure with kitchen aluminum foil serving as electrodes. The composites’ electrical resistance was measured and it was found that the composite with 15% w/w of graphite was the best choice for further investigation according to the percolation theory of electrical conductivity thresholds. The wax–graphite composites were subjected to impact forces from a basketball, measured by the school’s PASCO equipment. A calibration curve of the specific electrical resistance versus the force of impact was constructed. Following the standards of the International Basketball Federation (FIBA), the quality of the basketball was evaluated in terms of the bounce height of a free-falling ball and the force acting on the ball during the impact with the floor. The wax–graphite composite with a threshold concentration of 15% w/w graphite proved to be a sensitive sensor for measuring the impact force, even when small forces were under investigation. The project as presented here could be used as a laboratory topic for advanced level high school physics or undergraduate lab work in materials science or applied physics.