Concession Stand Notes: Why Does Popcorn Cling?
1On opening night of the Lakeview Community Theater's fall play, eighth graders Ines Valdivia and Kofi Brandt volunteered at the concession stand. The lobby air was dry, and whenever Kofi moved the stand's plastic scoop toward the popcorn, small flakes jumped up from about a centimeter away and clung to it. Ines noticed that the flakes jumped more often right after the scoop had been wiped with the stand's wool cleaning cloth. The stand manager, Ms. Marguerite Oduya, teaches physical science on weekdays. She said that the rubbed scoop was probably carrying an electric charge, and she challenged the pair to turn what they saw into questions they could test.
2The next week, in Ms. Oduya's classroom, Ines and Kofi built a test stand. A small foam ball wrapped in aluminum foil sat on a paper cup on a digital scale that reads to 0.01 g. They rubbed a plastic ruler with a wool cloth and clamped it in a holder right above the ball. When the charged ruler pulls up on the ball, the scale reading drops, so the size of the drop shows how strong the pull is. A drop of 0.20 g means the ruler pulls up on the ball with a force equal to the weight of 0.20 g. Before every trial, Kofi touched the ball with a finger to remove any leftover charge, and the pair used a fresh ruler from a set of identical rulers.
3In Test 1, they changed the number of rubs and always held the ruler 2 cm above the ball. In Test 2, they rubbed every ruler 10 times and changed the distance from the ruler to the top of the ball. They ran three trials for each setting and averaged the drops.
4They also wrote down two extra observations. First, a metal spoon held by its handle and rubbed 10 times with the wool cloth made no drop on the scale at all. Second, when Kofi hung a rubbed ruler from a thread and brought a second rubbed ruler near it, the hanging ruler swung away. It barely moved when the second ruler was 10 cm away, but it swung away sharply when the rulers were 3 cm apart.